update

Use typ() instead of [] throughout DeltaChannel so reducers over dict
(and other non-list types) work correctly. fromCheckpoint(MISSING) now
leaves value as typ() from __init__ instead of overwriting with MISSING.
copy() uses value.copy() to handle dicts. update() initialises base from
typ() when value is MISSING. Add four tests covering the deepagents-style
dict-merge / file-deletion reducer pattern.

.gitignore

@@ -100,3 +100,4 @@ dmypy.json
 .turbo
 .editorconfig
 .scratch
+.worktrees/

docs/superpowers/specs/2026-04-17-diff-channel-design.md

@@ -0,0 +1,405 @@
+# DiffChannel: Incremental Checkpoint Storage for Append-Style Reducers
+
+**Date:** 2026-04-17  
+**Status:** Approved for implementation  
+**Scope:** `libs/checkpoint`, `libs/langgraph`, `libs/checkpoint-postgres`
+
+---
+
+## Motivation
+
+LangGraph checkpoints today store the **full accumulated value** of every channel on every step. For a `messages` channel backed by `add_messages`, this means each checkpoint blob contains the entire conversation history. Storage cost grows O(N²) in the number of turns: step 1 stores 1 message, step 100 stores 100 messages, step 1000 stores 1000 messages. For long-running agentic conversations with high-token messages this is untenable.
+
+The fix is to store only the **delta** (new writes) per step, reconstructing the full accumulated value at load time by replaying the chain. This is an opt-in mechanism — existing graphs are unaffected.
+
+---
+
+## Non-Goals
+
+- **Compaction / materialized snapshots**: deferred. Load cost stays O(N) blob fetches but those fetches are batched into a single query — acceptable for now.
+- **SQLite saver support**: SQLite stores all channel values inline in one row (no per-channel blob table). Deferred to a follow-up.
+- **Automatic migration** of existing `BinaryOperatorAggregate` channels: users opt in explicitly. Old checkpoints load correctly via the backwards-compatibility path in `from_checkpoint`.
+
+---
+
+## Architecture Overview
+
+```
+User state definition
+  └── Annotated[list[AnyMessage], DiffChannel(add_messages)]
+
+Write path (per superstep)
+  DiffChannel.update()       — apply operator, accumulate writes in _pending
+  DiffChannel.checkpoint()   — return DiffDelta(delta=_pending, prev_version=_base_version)
+  serde.dumps_typed()        — serialize DiffDelta as ("diff", msgpack_bytes)
+  saver.put()                — store blob at (thread_id, ns, "messages", version_N)
+  DiffChannel.after_checkpoint(version_N) — advance _base_version, clear _pending
+
+Read path (on graph load or time-travel)
+  saver.get_tuple()          — fetch current-version blob per channel
+  saver._load_blobs()        — detect "diff" type → follow chain to reconstruct DiffChainValue
+  DiffChannel.from_checkpoint(DiffChainValue) — replay deltas with operator → full list
+  DiffChannel.after_checkpoint(version_N)     — set _base_version for next write
+```
+
+The pregel layer (`_checkpoint.py`, `_loop.py`) is unchanged except for two small additions to call the new `after_checkpoint` hook. The saver public interface (`BaseCheckpointSaver`) gains no new methods. All chain-following logic lives inside each saver's private `_load_blobs`.
+
+---
+
+## New Protocol Types
+
+**Location:** `libs/checkpoint/langgraph/checkpoint/base/__init__.py`
+
+Two dataclasses form the contract between `DiffChannel` and savers:
+
+```python
+@dataclass
+class DiffDelta:
+    """Returned by DiffChannel.checkpoint(). Written to the blob store."""
+    delta: list[Any]           # raw writes passed to update() this step
+    prev_version: str | None   # version of the previous diff blob; None = chain root
+```
+
+```python
+@dataclass
+class DiffChainValue:
+    """Passed to DiffChannel.from_checkpoint(). Assembled by _load_blobs()."""
+    base: list[Any] | None     # starting accumulated value (None = empty start)
+    deltas: list[list[Any]]    # write-sets ordered oldest → newest
+```
+
+`DiffDelta` lives in the checkpoint base package (not the channel module) so savers can import it without creating a circular dependency. `DiffChainValue` is there for the same reason.
+
+---
+
+## `BaseChannel.after_checkpoint()` Hook
+
+**Location:** `libs/langgraph/langgraph/channels/base.py`
+
+```python
+def after_checkpoint(self, version: Any) -> None:
+    """Called after checkpoint() (with the new version) and after from_checkpoint()
+    (with the current version). No-op by default; DiffChannel overrides."""
+    pass
+```
+
+This is a **non-abstract, no-op default** — fully backwards compatible. All existing channels inherit it silently. It is NOT in the abstract interface.
+
+---
+
+## `DiffChannel[V]`
+
+**Location:** `libs/langgraph/langgraph/channels/diff.py` (new file)
+
+### Internal state
+
+| Attribute | Type | Description |
+|---|---|---|
+| `value` | `list[V]` | Full accumulated value (the reconstructed list) |
+| `operator` | `Callable` | The binary reducer (e.g. `add_messages`) |
+| `_pending` | `list[Any]` | Raw writes accumulated since last `after_checkpoint` call |
+| `_base_version` | `str \| None` | Version this channel was last checkpointed at (= `prev_version` for next delta) |
+| `_overwritten` | `bool` | True if an `Overwrite` was applied since last `after_checkpoint`; makes next blob a chain root |
+
+### `update(values)`
+
+Mirrors `BinaryOperatorAggregate.update()` with two additions:
+
+1. For each non-Overwrite value: apply `self.operator(self.value, value)` as before; **also append the raw incoming value to `self._pending`**.
+2. For an `Overwrite(v)` value: set `self.value = v`; set `self._pending = list(v)` (full value becomes the new delta); set `self._overwritten = True`.
+
+The key: `_pending` stores the **incoming writes** (what was passed to `update()`), not the diff of `self.value`. This is important because `add_messages` handles removal and update-by-ID — replaying the writes with `operator` during reconstruction applies that logic correctly.
+
+### `checkpoint()`
+
+```python
+def checkpoint(self) -> DiffDelta:
+    return DiffDelta(
+        delta=self._pending[:],
+        prev_version=None if self._overwritten else self._base_version,
+    )
+```
+
+- Normal step: `prev_version = self._base_version` → chain link
+- After Overwrite: `prev_version = None` → chain root (reconstruction stops here and uses `delta` as the full base value)
+
+Returns `DiffDelta`, never the raw accumulated list. The serde handles serialization.
+
+### `from_checkpoint(checkpoint)`
+
+```python
+def from_checkpoint(self, checkpoint) -> Self:
+    new = DiffChannel(self.typ, self.operator)
+    new.key = self.key
+    if checkpoint is MISSING:
+        new.value = []
+    elif isinstance(checkpoint, DiffChainValue):
+        accumulated = checkpoint.base or []
+        for step_writes in checkpoint.deltas:
+            # Mirror update() exactly: apply each write individually so operator
+            # semantics (e.g. add_messages ID-based removal) are respected.
+            for write in step_writes:
+                accumulated = new.operator(accumulated, write)
+        new.value = accumulated
+    elif isinstance(checkpoint, DiffDelta):
+        # Unsupported saver: _load_blobs returned a raw DiffDelta instead of
+        # assembling a DiffChainValue. Raise rather than silently losing history.
+        raise ValueError(
+            "DiffChannel received a raw DiffDelta from the checkpoint saver. "
+            "Your saver does not support incremental channel storage. "
+            "Use InMemorySaver or PostgresSaver."
+        )
+    else:
+        # Backwards compat: plain list from old BinaryOperatorAggregate checkpoint.
+        new.value = checkpoint
+    new._pending = []
+    new._base_version = None  # set by the subsequent after_checkpoint() call
+    return new
+```
+
+The operator is available on `self` (the channel spec) so reconstruction is correct for any reducer — the saver never needs to know about `add_messages`.
+
+`_pending` stores **individual writes** (each `value` from `update()`'s `values` sequence), so each `step_writes` list in `DiffChainValue.deltas` is replayed write-by-write — identical to the `update()` loop.
+
+### `after_checkpoint(version)`
+
+```python
+def after_checkpoint(self, version: Any) -> None:
+    if version != self._base_version:
+        self._base_version = version
+        self._pending = []
+        self._overwritten = False
+```
+
+No-op when `version == self._base_version` (channel wasn't updated this step — blob was not written). Clears `_pending` and advances `_base_version` when the channel was actually checkpointed.
+
+### Opt-in API
+
+```python
+from langgraph.channels.diff import DiffChannel
+
+class State(TypedDict):
+    messages: Annotated[list[AnyMessage], DiffChannel(add_messages)]
+```
+
+`StateGraph` already handles `BaseChannel` instances as annotation metadata — `DiffChannel` inherits this without any changes to `StateGraph`.
+
+---
+
+## Serde Extension
+
+**Location:** `libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py`
+
+Add one branch to `dumps_typed` (before the `else` msgpack fallback), using the existing module-level `_msgpack_enc` so message ext-types (Pydantic v2, etc.) are handled correctly:
+
+```python
+elif isinstance(obj, DiffDelta):
+    return "diff", _msgpack_enc({"d": obj.delta, "p": obj.prev_version})
+```
+
+Add one branch to `loads_typed` so savers can decode diff blobs without importing `ormsgpack` directly:
+
+```python
+elif type_ == "diff":
+    return ormsgpack.unpackb(
+        data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
+    )
+    # returns {"d": [writes...], "p": prev_version_str_or_none}
+```
+
+Savers call `serde.loads_typed(("diff", raw_bytes))` to decode a diff blob into `{"d": ..., "p": ...}`, then check `type_tag == "diff"` to trigger chain traversal. The serde layer is the only place that knows about `ormsgpack`.
+
+---
+
+## Saver Changes
+
+### InMemorySaver
+
+**`put()` — `libs/checkpoint/langgraph/checkpoint/memory/__init__.py`**
+
+No change needed. The existing `self.serde.dumps_typed(values[k])` call already handles `DiffDelta` via the new serde branch above, storing it as `("diff", bytes)`.
+
+**`_load_blobs()` — same file**
+
+After checking `vv[0] != "empty"`, add a branch for `"diff"` before calling `serde.loads_typed`:
+
+```python
+def _load_blobs(self, thread_id, checkpoint_ns, versions):
+    channel_values = {}
+    diff_channels = {}  # channel_name -> current_version for diff channels
+
+    for k, v in versions.items():
+        kk = (thread_id, checkpoint_ns, k, v)
+        if kk not in self.blobs:
+            continue
+        type_tag, blob_bytes = self.blobs[kk]
+        if type_tag == "diff":
+            diff_channels[k] = v  # handle below
+        elif type_tag != "empty":
+            channel_values[k] = self.serde.loads_typed((type_tag, blob_bytes))
+
+    for k, current_version in diff_channels.items():
+        # Follow chain: newest → oldest, then reverse
+        chain_deltas = []
+        base = None
+        version = current_version
+        while version is not None:
+            kk = (thread_id, checkpoint_ns, k, version)
+            if kk not in self.blobs:
+                break
+            type_tag, blob_bytes = self.blobs[kk]
+            if type_tag == "diff":
+                # Use serde so we don't need to import ormsgpack directly
+                payload = self.serde.loads_typed((type_tag, blob_bytes))
+                chain_deltas.append(payload["d"])
+                version = payload["p"]  # prev_version; None = root
+            else:
+                # Old non-diff blob encountered: treat as base accumulated value
+                base = self.serde.loads_typed((type_tag, blob_bytes))
+                break
+        chain_deltas.reverse()
+        channel_values[k] = DiffChainValue(base=base, deltas=chain_deltas)
+
+    return channel_values
+```
+
+Each blob lookup is O(1) on the dict. Total: N dict lookups for a chain of depth N. Memory usage is identical to loading a single full-list blob (same total bytes, split across N entries).
+
+### PostgresSaver
+
+**`_load_blobs()` — `libs/checkpoint-postgres/langgraph/checkpoint/postgres/base.py`**
+
+The existing `SELECT_SQL` fetches one blob per channel via a JOIN. After running that query, detect any `"diff"` channels in the result and issue one additional range query:
+
+```python
+def _load_blobs(self, blob_values):
+    if not blob_values:
+        return {}
+
+    result = {}
+    diff_channels = {}  # channel_name -> current_version (as str)
+
+    for k, t, v in blob_values:
+        channel = k.decode()
+        type_tag = t.decode()
+        if type_tag == "diff":
+            # Decode via serde — no direct ormsgpack import needed
+            payload = self.serde.loads_typed((type_tag, v))
+            diff_channels[channel] = payload  # store for chain fetch
+        elif type_tag != "empty":
+            result[channel] = self.serde.loads_typed((type_tag, v))
+
+    if diff_channels:
+        result.update(self._load_diff_chains(diff_channels))
+
+    return result
+```
+
+`_load_diff_chains` issues one SQL query per diff channel (typically just `messages`):
+
+```sql
+SELECT version, type, blob
+FROM checkpoint_blobs
+WHERE thread_id = %s
+  AND checkpoint_ns = %s
+  AND channel = %s
+  AND version <= %s
+ORDER BY version ASC
+```
+
+In Python, iterate rows in ascending version order: if `type = "diff"`, accumulate the delta; if any other type is encountered, treat it as the base accumulated value and stop. Return `DiffChainValue(base=..., deltas=[...])`.
+
+This results in **at most 2 queries total** for a graph with one `DiffChannel` — existing behaviour for all other channels is unchanged.
+
+**`put()` / `_dump_blobs()`**
+
+No change needed. `_dump_blobs` calls `self.serde.dumps_typed(v)` for each channel value in `new_versions`. When `v` is a `DiffDelta`, the serde produces `("diff", bytes)` which is stored as `type = "diff"` in `checkpoint_blobs`. The `ON CONFLICT DO NOTHING` semantics are preserved.
+
+### SQLite
+
+Deferred. `SqliteSaver` stores the entire checkpoint as a single serialized row — it has no per-channel blob table. Supporting `DiffChannel` on SQLite would require adding a new blobs table, which is a separate migration tracked separately.
+
+---
+
+## Pregel Layer Changes
+
+### `channels_from_checkpoint` — `libs/langgraph/langgraph/pregel/_checkpoint.py`
+
+After constructing each channel from its checkpoint value, call `after_checkpoint` so the channel records its current version:
+
+```python
+channels = {}
+for k, v in channel_specs.items():
+    ch = v.from_checkpoint(checkpoint["channel_values"].get(k, MISSING))
+    ch.after_checkpoint(checkpoint["channel_versions"].get(k))
+    channels[k] = ch
+return channels, managed_specs
+```
+
+Existing channels get the no-op `after_checkpoint`. `DiffChannel` uses it to set `_base_version`.
+
+### `PregelLoop._put_checkpoint` — `libs/langgraph/langgraph/pregel/_loop.py`
+
+After `create_checkpoint(self.checkpoint, self.channels, self.step, ...)` returns and `do_checkpoint is True` and `self.channels is not None`, iterate channels and notify:
+
+```python
+if do_checkpoint and self.channels:
+    for k, ch in self.channels.items():
+        ch.after_checkpoint(self.checkpoint["channel_versions"].get(k))
+```
+
+This is called after `create_checkpoint` updates `self.checkpoint["channel_versions"]`, so `get(k)` returns the new version for updated channels and the old version for unchanged ones. `DiffChannel.after_checkpoint` only clears `_pending` when `version != _base_version`, so unchanged channels are no-ops.
+
+---
+
+## Backwards Compatibility
+
+| Scenario | Behaviour |
+|---|---|
+| Existing graph using `add_messages` (BinaryOperatorAggregate) | Unaffected — no code changes, no data migration |
+| New graph with `DiffChannel`, loading old checkpoint blobs | `from_checkpoint` receives a plain `list` → used directly as accumulated value |
+| `DiffChannel` with `InMemorySaver` or `PostgresSaver` | Fully supported |
+| `DiffChannel` with `SqliteSaver` | `from_checkpoint` receives a raw `DiffDelta` (SqliteSaver stores channel_values inline), raises `ValueError` with a clear message pointing to supported savers |
+| Time-travel / fork to past checkpoint | Chain traversal uses the version at that checkpoint → reconstruction is correct |
+| `update_state` | Treated as a normal step: writes are deltas chained to history |
+| `Overwrite` value | Resets chain: next blob has `prev_version=None`; reconstruction starts fresh |
+
+---
+
+## Testing Strategy
+
+1. **Unit tests for `DiffChannel`** (`libs/langgraph/tests/`):
+   - `update` → `checkpoint` → `after_checkpoint` → `checkpoint` lifecycle (2 steps, verify delta isolation)
+   - `from_checkpoint(DiffChainValue)` correctly replays multi-step chains using the operator
+   - `from_checkpoint(plain_list)` backwards-compat path
+   - `Overwrite` creates a root blob (`prev_version=None`) and reconstruction ignores prior chain
+   - `after_checkpoint` no-ops when version is unchanged
+
+2. **Integration tests with `InMemorySaver`** (`libs/langgraph/tests/`):
+   - 10-step conversation: verify final loaded state equals full accumulated messages
+   - Time-travel: fork to step 5, verify only messages 1–5 are present
+   - Mixed graph: some channels `BinaryOperatorAggregate`, one `DiffChannel` — both reconstruct correctly
+
+3. **Serde tests** (`libs/checkpoint/tests/`):
+   - `DiffDelta` round-trips through `dumps_typed` / saver storage
+   - Old `"msgpack"` blob for a channel → `DiffChannel.from_checkpoint` handles it
+
+4. **Postgres integration tests** (`libs/checkpoint-postgres/tests/`):
+   - Range query reconstructs correct full list after N steps
+   - Time-travel to checkpoint M reconstructs correct list of M messages
+
+---
+
+## Files Changed
+
+| File | Change |
+|---|---|
+| `libs/checkpoint/langgraph/checkpoint/base/__init__.py` | Add `DiffDelta`, `DiffChainValue` dataclasses |
+| `libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py` | Add `"diff"` branch in `dumps_typed` |
+| `libs/checkpoint/langgraph/checkpoint/memory/__init__.py` | Chain traversal in `_load_blobs` |
+| `libs/langgraph/langgraph/channels/base.py` | Add no-op `after_checkpoint` method |
+| `libs/langgraph/langgraph/channels/diff.py` | **New file** — `DiffChannel` implementation |
+| `libs/langgraph/langgraph/channels/__init__.py` | Export `DiffChannel` |
+| `libs/langgraph/langgraph/pregel/_checkpoint.py` | Call `after_checkpoint` in `channels_from_checkpoint` |
+| `libs/langgraph/langgraph/pregel/_loop.py` | Call `after_checkpoint` after `create_checkpoint` |
+| `libs/checkpoint-postgres/langgraph/checkpoint/postgres/base.py` | Range-query chain reconstruction in `_load_blobs` |

libs/checkpoint-conformance/uv.lock

@@ -306,7 +306,7 @@ test = [
 
 [[package]]
 name = "langsmith"
-version = "0.7.3"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -319,9 +319,9 @@ dependencies = [
     { name = "xxhash" },
     { name = "zstandard" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/8d/bc/8172fefad4f2da888a6d564a27d1fb7d4dbf3c640899c2b40c46235cbe98/langsmith-0.7.3.tar.gz", hash = "sha256:0223b97021af62d2cf53c8a378a27bd22e90a7327e45b353e0069ae60d5d6f9e", size = 988575, upload-time = "2026-02-13T23:25:32.916Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/e6/11/696019490992db5c87774dc20515529ef42a01e1d770fb754ed6d9b12fb0/langsmith-0.7.31.tar.gz", hash = "sha256:331ee4f7c26bb5be4022b9859b7d7b122cbf8c9d01d9f530114c1914b0349ffb", size = 1178480, upload-time = "2026-04-14T17:55:41.242Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/f4/9d/5a68b6b5e313ffabbb9725d18a71edb48177fd6d3ad329c07801d2a8e862/langsmith-0.7.3-py3-none-any.whl", hash = "sha256:03659bf9274e6efcead361c9c31a7849ea565ae0d6c0d73e1d8b239029eff3be", size = 325718, upload-time = "2026-02-13T23:25:31.52Z" },
+    { url = "https://files.pythonhosted.org/packages/1d/a1/a013cf458c301cda86a213dd153ce0a01c93f1ab5833f951e6a44c9763ce/langsmith-0.7.31-py3-none-any.whl", hash = "sha256:0291d49203f6e80dda011af1afda61eb0595a4d697adb684590a8805e1d61fb6", size = 373276, upload-time = "2026-04-14T17:55:39.677Z" },
 ]
 
 [[package]]

libs/checkpoint-postgres/langgraph/checkpoint/postgres/__init__.py

@@ -430,6 +430,43 @@ class PostgresSaver(BasePostgresSaver):
                 with conn.cursor(binary=True, row_factory=dict_row) as cur:
                     yield cur
 
+    def get_channel_blob(
+        self,
+        thread_id: str,
+        checkpoint_ns: str,
+        checkpoint_id: str,
+        channel: str,
+    ) -> Any:
+        """Look up a channel blob by checkpoint ID + channel via checkpoint_blobs."""
+        with self._cursor() as cur:
+            cur.execute(
+                """
+                SELECT cb.type, cb.blob
+                FROM checkpoint_blobs cb
+                WHERE cb.thread_id = %s
+                  AND cb.checkpoint_ns = %s
+                  AND cb.channel = %s
+                  AND cb.version = (
+                      SELECT checkpoint->'channel_versions'->>%s
+                      FROM checkpoints
+                      WHERE thread_id = %s AND checkpoint_ns = %s AND checkpoint_id = %s
+                  )
+                """,
+                (
+                    thread_id,
+                    checkpoint_ns,
+                    channel,
+                    channel,
+                    thread_id,
+                    checkpoint_ns,
+                    checkpoint_id,
+                ),
+            )
+            row = cur.fetchone()
+        if row is None:
+            return NotImplemented
+        return self.serde.loads_typed((row["type"], row["blob"]))
+
     def _load_checkpoint_tuple(self, value: DictRow) -> CheckpointTuple:
         """
         Convert a database row into a CheckpointTuple object.
@@ -442,6 +479,13 @@ class PostgresSaver(BasePostgresSaver):
             including its configuration, metadata, parent checkpoint (if any),
             and pending writes.
         """
+        with self._cursor() as cur:
+            channel_values = self._load_blobs(
+                value["channel_values"],
+                thread_id=value["thread_id"],
+                checkpoint_ns=value["checkpoint_ns"],
+                cur=cur,
+            )
         return CheckpointTuple(
             {
                 "configurable": {
@@ -454,7 +498,7 @@ class PostgresSaver(BasePostgresSaver):
                 **value["checkpoint"],
                 "channel_values": {
                     **(value["checkpoint"].get("channel_values") or {}),
-                    **self._load_blobs(value["channel_values"]),
+                    **channel_values,
                 },
             },
             value["metadata"],

libs/checkpoint-postgres/langgraph/checkpoint/postgres/aio.py

@@ -391,6 +391,43 @@ class AsyncPostgresSaver(BasePostgresSaver):
                 async with conn.cursor(binary=True, row_factory=dict_row) as cur:
                     yield cur
 
+    async def aget_channel_blob(
+        self,
+        thread_id: str,
+        checkpoint_ns: str,
+        checkpoint_id: str,
+        channel: str,
+    ) -> Any:
+        """Async look up of a channel blob by checkpoint ID + channel name."""
+        async with self._cursor() as cur:
+            await cur.execute(
+                """
+                SELECT cb.type, cb.blob
+                FROM checkpoint_blobs cb
+                WHERE cb.thread_id = %s
+                  AND cb.checkpoint_ns = %s
+                  AND cb.channel = %s
+                  AND cb.version = (
+                      SELECT checkpoint->'channel_versions'->>%s
+                      FROM checkpoints
+                      WHERE thread_id = %s AND checkpoint_ns = %s AND checkpoint_id = %s
+                  )
+                """,
+                (
+                    thread_id,
+                    checkpoint_ns,
+                    channel,
+                    channel,
+                    thread_id,
+                    checkpoint_ns,
+                    checkpoint_id,
+                ),
+            )
+            row = await cur.fetchone()
+        if row is None:
+            return NotImplemented
+        return self.serde.loads_typed((row["type"], row["blob"]))
+
     async def _load_checkpoint_tuple(self, value: DictRow) -> CheckpointTuple:
         """
         Convert a database row into a CheckpointTuple object.
@@ -403,11 +440,19 @@ class AsyncPostgresSaver(BasePostgresSaver):
             including its configuration, metadata, parent checkpoint (if any),
             and pending writes.
         """
+        thread_id = value["thread_id"]
+        checkpoint_ns = value["checkpoint_ns"]
+        blob_values = value["channel_values"]
+
+        channel_values: dict[str, Any] = {}
+        if blob_values:
+            channel_values = self._load_blobs(blob_values)
+
         return CheckpointTuple(
             {
                 "configurable": {
-                    "thread_id": value["thread_id"],
-                    "checkpoint_ns": value["checkpoint_ns"],
+                    "thread_id": thread_id,
+                    "checkpoint_ns": checkpoint_ns,
                     "checkpoint_id": value["checkpoint_id"],
                 }
             },
@@ -415,15 +460,15 @@ class AsyncPostgresSaver(BasePostgresSaver):
                 **value["checkpoint"],
                 "channel_values": {
                     **(value["checkpoint"].get("channel_values") or {}),
-                    **self._load_blobs(value["channel_values"]),
+                    **channel_values,
                 },
             },
             value["metadata"],
             (
                 {
                     "configurable": {
-                        "thread_id": value["thread_id"],
-                        "checkpoint_ns": value["checkpoint_ns"],
+                        "thread_id": thread_id,
+                        "checkpoint_ns": checkpoint_ns,
                         "checkpoint_id": value["parent_checkpoint_id"],
                     }
                 }

libs/checkpoint-postgres/langgraph/checkpoint/postgres/base.py

@@ -185,15 +185,22 @@ class BasePostgresSaver(BaseCheckpointSaver[str]):
         )
 
     def _load_blobs(
-        self, blob_values: list[tuple[bytes, bytes, bytes]]
+        self,
+        blob_values: list[tuple[bytes, bytes, bytes]],
+        *,
+        thread_id: str = "",
+        checkpoint_ns: str = "",
+        cur: Any = None,
     ) -> dict[str, Any]:
         if not blob_values:
             return {}
-        return {
-            k.decode(): self.serde.loads_typed((t.decode(), v))
-            for k, t, v in blob_values
-            if t.decode() != "empty"
-        }
+        result: dict[str, Any] = {}
+        for k, t, v in blob_values:
+            channel = k.decode()
+            type_tag = t.decode()
+            if type_tag != "empty":
+                result[channel] = self.serde.loads_typed((type_tag, v))
+        return result
 
     def _dump_blobs(
         self,

libs/checkpoint-postgres/tests/test_async.py

@@ -371,3 +371,47 @@ async def test_get_checkpoint_no_channel_values(
 
         checkpoint = await saver.aget_tuple(config)
         assert checkpoint.checkpoint["channel_values"] == {}
+
+
+@pytest.mark.parametrize("saver_name", ["base", "pool", "pipe"])
+async def test_delta_channel_chain_reconstruction(saver_name: str) -> None:
+    """AsyncPostgresSaver reconstructs DeltaChannel chain via point-lookup traversal."""
+    pytest.importorskip(
+        "langgraph.channels.delta", reason="langgraph core not installed"
+    )
+
+    from typing import Annotated
+
+    from langchain_core.messages import AIMessage, HumanMessage
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph import START, StateGraph
+    from langgraph.graph.message import add_messages
+    from typing_extensions import TypedDict
+
+    class State(TypedDict):
+        messages: Annotated[list, DeltaChannel(add_messages)]
+
+    def respond(state: State) -> dict:
+        n = len(state["messages"])
+        return {"messages": [AIMessage(content=f"reply-{n}", id=f"ai-{n}")]}
+
+    builder = StateGraph(State)
+    builder.add_node("respond", respond)
+    builder.add_edge(START, "respond")
+
+    async with _saver(saver_name) as saver:
+        graph = builder.compile(checkpointer=saver)
+        config = {"configurable": {"thread_id": "diff-channel-test-1"}}
+
+        await graph.ainvoke({"messages": [HumanMessage(content="hi", id="h1")]}, config)
+        await graph.ainvoke(
+            {"messages": [HumanMessage(content="there", id="h2")]}, config
+        )
+
+        state = await graph.aget_state(config)
+        msgs = state.values["messages"]
+        assert len(msgs) == 4, f"expected 4, got {len(msgs)}: {msgs}"
+        assert msgs[0].content == "hi"
+        assert msgs[1].content == "reply-1"
+        assert msgs[2].content == "there"
+        assert msgs[3].content == "reply-3"

libs/checkpoint-postgres/uv.lock

@@ -259,7 +259,7 @@ wheels = [
 
 [[package]]
 name = "langgraph-checkpoint"
-version = "4.0.1"
+version = "4.0.2"
 source = { editable = "../checkpoint" }
 dependencies = [
     { name = "langchain-core" },
@@ -382,7 +382,7 @@ test = [
 
 [[package]]
 name = "langsmith"
-version = "0.6.4"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -392,11 +392,12 @@ dependencies = [
     { name = "requests" },
     { name = "requests-toolbelt" },
     { name = "uuid-utils" },
+    { name = "xxhash" },
     { name = "zstandard" },
 ]
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+sdist = { url = "https://files.pythonhosted.org/packages/e6/11/696019490992db5c87774dc20515529ef42a01e1d770fb754ed6d9b12fb0/langsmith-0.7.31.tar.gz", hash = "sha256:331ee4f7c26bb5be4022b9859b7d7b122cbf8c9d01d9f530114c1914b0349ffb", size = 1178480, upload-time = "2026-04-14T17:55:41.242Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/1d/a1/a013cf458c301cda86a213dd153ce0a01c93f1ab5833f951e6a44c9763ce/langsmith-0.7.31-py3-none-any.whl", hash = "sha256:0291d49203f6e80dda011af1afda61eb0595a4d697adb684590a8805e1d61fb6", size = 373276, upload-time = "2026-04-14T17:55:39.677Z" },
 ]
 
 [[package]]
@@ -1284,6 +1285,124 @@ wheels = [
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 ]
 
+[[package]]
+name = "xxhash"
+version = "3.6.0"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/02/84/30869e01909fb37a6cc7e18688ee8bf1e42d57e7e0777636bd47524c43c7/xxhash-3.6.0.tar.gz", hash = "sha256:f0162a78b13a0d7617b2845b90c763339d1f1d82bb04a4b07f4ab535cc5e05d6", size = 85160, upload-time = "2025-10-02T14:37:08.097Z" }
+wheels = [
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+]
+
 [[package]]
 name = "zstandard"
 version = "0.25.0"

libs/checkpoint/langgraph/checkpoint/base/__init__.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
+import asyncio
 import copy
+import dataclasses
 import logging
 from collections.abc import AsyncIterator, Collection, Iterator, Mapping, Sequence
 from typing import (  # noqa: UP035
@@ -28,7 +30,46 @@ from langgraph.checkpoint.serde.types import (
 
 V = TypeVar("V", int, float, str)
 PendingWrite = tuple[str, str, Any]
+
+
+@dataclasses.dataclass
+class DeltaValue:
+    """Returned by DeltaChannel.checkpoint(). Represents one step's writes."""
+
+    delta: list[Any]
+    prev_checkpoint_id: (
+        str | None
+    )  # ID of checkpoint containing previous blob; None = chain root
+
+
+@dataclasses.dataclass
+class DeltaChainValue:
+    """Passed to DeltaChannel.from_checkpoint(). Assembled during checkpoint hydration."""
+
+    base: list[Any] | None  # starting accumulated value; None = start from empty
+    deltas: list[list[Any]]  # per-step write-sets, ordered oldest → newest
+
+
+CheckpointHydrationKind = Literal["delta"]
+
+
+@dataclasses.dataclass(frozen=True)
+class IncrementalChannelSpec:
+    """Describes a checkpoint field that needs saver-side materialization."""
+
+    name: str
+    kind: CheckpointHydrationKind
+
+
+@dataclasses.dataclass(frozen=True)
+class CheckpointHydrationPlan:
+    """Lists the checkpoint fields eligible for saver-side materialization."""
+
+    channels: tuple[IncrementalChannelSpec, ...]
+
+
 logger = logging.getLogger(__name__)
+_MISSING_SENTINEL = object()
 
 
 # Marked as total=False to allow for future expansion.
@@ -457,6 +498,172 @@ class BaseCheckpointSaver(Generic[V]):
         """
         raise NotImplementedError
 
+    def materialize_checkpoint(
+        self,
+        config: RunnableConfig,
+        checkpoint: Checkpoint,
+        plan: CheckpointHydrationPlan | None = None,
+    ) -> Checkpoint:
+        """Materialize any saver-managed incremental values in a checkpoint."""
+        if plan is None or not plan.channels:
+            return checkpoint
+
+        thread_id = str(config["configurable"]["thread_id"])
+        checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
+        current_checkpoint_id = checkpoint.get("id")
+        assembled: dict[str, Any] = {}
+
+        for spec in plan.channels:
+            value = checkpoint["channel_values"].get(spec.name)
+            if spec.kind != "delta" or not isinstance(value, DeltaValue):
+                continue
+
+            assembled_value = self._materialize_delta_channel(
+                thread_id=thread_id,
+                checkpoint_ns=checkpoint_ns,
+                current_checkpoint_id=current_checkpoint_id,
+                channel=spec.name,
+                value=value,
+            )
+            if assembled_value is not None:
+                assembled[spec.name] = assembled_value
+
+        if not assembled:
+            return checkpoint
+
+        return {
+            **checkpoint,
+            "channel_values": {**checkpoint["channel_values"], **assembled},
+        }
+
+    def materialize_checkpoint_tuple(
+        self,
+        value: CheckpointTuple,
+        plan: CheckpointHydrationPlan | None = None,
+    ) -> CheckpointTuple:
+        """Materialize incremental values for a single checkpoint tuple."""
+        checkpoint = self.materialize_checkpoint(value.config, value.checkpoint, plan)
+        if checkpoint is value.checkpoint:
+            return value
+        return value._replace(checkpoint=checkpoint)
+
+    def materialize_checkpoint_tuples(
+        self,
+        values: Sequence[CheckpointTuple],
+        plan: CheckpointHydrationPlan | None = None,
+    ) -> Sequence[CheckpointTuple]:
+        """Materialize incremental values for a batch of checkpoint tuples."""
+        return [self.materialize_checkpoint_tuple(value, plan) for value in values]
+
+    async def amaterialize_checkpoint(
+        self,
+        config: RunnableConfig,
+        checkpoint: Checkpoint,
+        plan: CheckpointHydrationPlan | None = None,
+    ) -> Checkpoint:
+        """Async materialization hook for saver-managed incremental values."""
+        if plan is None or not plan.channels:
+            return checkpoint
+
+        thread_id = str(config["configurable"]["thread_id"])
+        checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
+        current_checkpoint_id = checkpoint.get("id")
+
+        targets = [
+            (spec, checkpoint["channel_values"][spec.name])
+            for spec in plan.channels
+            if spec.kind == "delta"
+            and isinstance(checkpoint["channel_values"].get(spec.name), DeltaValue)
+        ]
+        if not targets:
+            return checkpoint
+
+        # Walks for independent channels can run concurrently — each has its own chain.
+        results = await asyncio.gather(
+            *(
+                self._amaterialize_delta_channel(
+                    thread_id=thread_id,
+                    checkpoint_ns=checkpoint_ns,
+                    current_checkpoint_id=current_checkpoint_id,
+                    channel=spec.name,
+                    value=value,
+                )
+                for spec, value in targets
+            )
+        )
+        assembled = {
+            spec.name: result
+            for (spec, _), result in zip(targets, results, strict=True)
+            if result is not None
+        }
+
+        if not assembled:
+            return checkpoint
+
+        return {
+            **checkpoint,
+            "channel_values": {**checkpoint["channel_values"], **assembled},
+        }
+
+    async def amaterialize_checkpoint_tuple(
+        self,
+        value: CheckpointTuple,
+        plan: CheckpointHydrationPlan | None = None,
+    ) -> CheckpointTuple:
+        """Async materialization hook for a single checkpoint tuple."""
+        checkpoint = await self.amaterialize_checkpoint(
+            value.config, value.checkpoint, plan
+        )
+        if checkpoint is value.checkpoint:
+            return value
+        return value._replace(checkpoint=checkpoint)
+
+    async def amaterialize_checkpoint_tuples(
+        self,
+        values: Sequence[CheckpointTuple],
+        plan: CheckpointHydrationPlan | None = None,
+    ) -> Sequence[CheckpointTuple]:
+        """Async materialization hook for a batch of checkpoint tuples."""
+        return [
+            await self.amaterialize_checkpoint_tuple(value, plan) for value in values
+        ]
+
+    def get_channel_blob(
+        self,
+        thread_id: str,
+        checkpoint_ns: str,
+        checkpoint_id: str,
+        channel: str,
+    ) -> Any:
+        """Look up a single channel blob by checkpoint ID + channel name.
+
+        Returns NotImplemented if this saver does not support efficient
+        per-channel-version blob lookup. The pregel layer will fall back to
+        get_tuple() traversal in that case.
+
+        Savers with a dedicated blob store (InMemorySaver, PostgresSaver)
+        should override this for O(1) performance.
+        """
+        return NotImplemented
+
+    async def aget_channel_blob(
+        self,
+        thread_id: str,
+        checkpoint_ns: str,
+        checkpoint_id: str,
+        channel: str,
+    ) -> Any:
+        """Look up a single channel blob by checkpoint ID + channel name (async).
+
+        Returns NotImplemented if this saver does not support efficient
+        per-channel-version blob lookup. The pregel layer will fall back to
+        aget_tuple() traversal in that case.
+
+        Savers with a dedicated blob store (InMemorySaver, PostgresSaver)
+        should override this for O(1) performance.
+        """
+        return NotImplemented
+
     def get_next_version(self, current: V | None, channel: None) -> V:
         """Generate the next version ID for a channel.
 
@@ -489,6 +696,138 @@ class BaseCheckpointSaver(Generic[V]):
         clone.serde = maybe_add_typed_methods(serde)
         return clone
 
+    def _materialize_delta_channel(
+        self,
+        *,
+        thread_id: str,
+        checkpoint_ns: str,
+        current_checkpoint_id: str | None,
+        channel: str,
+        value: DeltaValue,
+    ) -> DeltaChainValue | None:
+        chain_deltas: list[list[Any]] = []
+        base: list[Any] | None = None
+        cursor: DeltaValue = value
+        visited: set[str] = {current_checkpoint_id} if current_checkpoint_id else set()
+
+        while True:
+            chain_deltas.append(cursor.delta)
+            prev_id = cursor.prev_checkpoint_id
+            if prev_id is None:
+                break
+            if prev_id in visited:
+                logger.warning(
+                    "DeltaChannel chain cycle at checkpoint %r for channel %r; breaking",
+                    prev_id,
+                    channel,
+                )
+                break
+            visited.add(prev_id)
+
+            blob = self.get_channel_blob(thread_id, checkpoint_ns, prev_id, channel)
+            if blob is not NotImplemented:
+                if isinstance(blob, DeltaValue):
+                    cursor = blob
+                    continue
+                base = blob
+                break
+
+            parent_config: RunnableConfig = {
+                "configurable": {
+                    "thread_id": thread_id,
+                    "checkpoint_ns": checkpoint_ns,
+                    "checkpoint_id": prev_id,
+                }
+            }
+            parent_tuple = self.get_tuple(parent_config)
+            if parent_tuple is None:
+                logger.warning(
+                    "DeltaChannel chain broken: checkpoint %r not found for channel %r",
+                    prev_id,
+                    channel,
+                )
+                break
+            prev_val = parent_tuple.checkpoint["channel_values"].get(
+                channel, _MISSING_SENTINEL
+            )
+            if prev_val is _MISSING_SENTINEL:
+                break
+            if isinstance(prev_val, DeltaValue):
+                cursor = prev_val
+            else:
+                base = prev_val
+                break
+
+        chain_deltas.reverse()
+        return DeltaChainValue(base=base, deltas=chain_deltas)
+
+    async def _amaterialize_delta_channel(
+        self,
+        *,
+        thread_id: str,
+        checkpoint_ns: str,
+        current_checkpoint_id: str | None,
+        channel: str,
+        value: DeltaValue,
+    ) -> DeltaChainValue | None:
+        chain_deltas: list[list[Any]] = []
+        base: list[Any] | None = None
+        cursor: DeltaValue = value
+        visited: set[str] = {current_checkpoint_id} if current_checkpoint_id else set()
+
+        while True:
+            chain_deltas.append(cursor.delta)
+            prev_id = cursor.prev_checkpoint_id
+            if prev_id is None:
+                break
+            if prev_id in visited:
+                logger.warning(
+                    "DeltaChannel chain cycle at checkpoint %r for channel %r; breaking",
+                    prev_id,
+                    channel,
+                )
+                break
+            visited.add(prev_id)
+
+            blob = await self.aget_channel_blob(
+                thread_id, checkpoint_ns, prev_id, channel
+            )
+            if blob is not NotImplemented:
+                if isinstance(blob, DeltaValue):
+                    cursor = blob
+                    continue
+                base = blob
+                break
+
+            parent_config: RunnableConfig = {
+                "configurable": {
+                    "thread_id": thread_id,
+                    "checkpoint_ns": checkpoint_ns,
+                    "checkpoint_id": prev_id,
+                }
+            }
+            parent_tuple = await self.aget_tuple(parent_config)
+            if parent_tuple is None:
+                logger.warning(
+                    "DeltaChannel chain broken: checkpoint %r not found for channel %r",
+                    prev_id,
+                    channel,
+                )
+                break
+            prev_val = parent_tuple.checkpoint["channel_values"].get(
+                channel, _MISSING_SENTINEL
+            )
+            if prev_val is _MISSING_SENTINEL:
+                break
+            if isinstance(prev_val, DeltaValue):
+                cursor = prev_val
+            else:
+                base = prev_val
+                break
+
+        chain_deltas.reverse()
+        return DeltaChainValue(base=base, deltas=chain_deltas)
+
 
 def _with_msgpack_allowlist(
     serde: SerializerProtocol, extra_allowlist: Collection[tuple[str, ...]]

libs/checkpoint/langgraph/checkpoint/memory/__init__.py

@@ -126,12 +126,46 @@ class InMemorySaver(
         channel_values: dict[str, Any] = {}
         for k, v in versions.items():
             kk = (thread_id, checkpoint_ns, k, v)
-            if kk in self.blobs:
-                vv = self.blobs[kk]
-                if vv[0] != "empty":
-                    channel_values[k] = self.serde.loads_typed(vv)
+            if kk not in self.blobs:
+                continue
+            vv = self.blobs[kk]
+            if vv[0] != "empty":
+                channel_values[k] = self.serde.loads_typed(vv)
         return channel_values
 
+    def get_channel_blob(
+        self,
+        thread_id: str,
+        checkpoint_ns: str,
+        checkpoint_id: str,
+        channel: str,
+    ) -> Any:
+        """Fast-path blob lookup: checkpoint → channel version → blob."""
+        ns_storage = self.storage.get(thread_id, {}).get(checkpoint_ns, {})
+        entry = ns_storage.get(checkpoint_id)
+        if entry is None:
+            return NotImplemented
+        checkpoint = self.serde.loads_typed(entry[0])
+        version = checkpoint["channel_versions"].get(channel)
+        if version is None:
+            return NotImplemented
+        kk = (thread_id, checkpoint_ns, channel, version)
+        if kk not in self.blobs:
+            return NotImplemented
+        vv = self.blobs[kk]
+        if vv[0] == "empty":
+            return NotImplemented
+        return self.serde.loads_typed(vv)
+
+    async def aget_channel_blob(
+        self,
+        thread_id: str,
+        checkpoint_ns: str,
+        checkpoint_id: str,
+        channel: str,
+    ) -> Any:
+        return self.get_channel_blob(thread_id, checkpoint_ns, checkpoint_id, channel)
+
     def get_tuple(self, config: RunnableConfig) -> CheckpointTuple | None:
         """Get a checkpoint tuple from the in-memory storage.
 

libs/checkpoint/langgraph/checkpoint/serde/_msgpack.py

@@ -80,6 +80,8 @@ SAFE_MSGPACK_TYPES: frozenset[tuple[str, ...]] = frozenset(
         ("langgraph.types", "Overwrite"),
         ("langgraph.store.base", "Item"),
         ("langgraph.store.base", "GetOp"),
+        # DeltaChannel checkpoint value type
+        ("langgraph.checkpoint.base", "DeltaValue"),
     }
 )
 

libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py

@@ -47,6 +47,12 @@ EMPTY_BYTES = b""
 logger = logging.getLogger(__name__)
 
 
+def _is_delta_value(obj: Any) -> bool:
+    from langgraph.checkpoint.base import DeltaValue  # lazy import avoids circular dep
+
+    return isinstance(obj, DeltaValue)
+
+
 class JsonPlusSerializer(SerializerProtocol):
     """Serializer that uses ormsgpack, with optional fallbacks.
 
@@ -239,6 +245,8 @@ class JsonPlusSerializer(SerializerProtocol):
             return "bytes", obj
         elif isinstance(obj, bytearray):
             return "bytearray", obj
+        elif _is_delta_value(obj):
+            return "delta", _msgpack_enc({"d": obj.delta, "c": obj.prev_checkpoint_id})
         else:
             try:
                 return "msgpack", _msgpack_enc(obj)
@@ -261,6 +269,13 @@ class JsonPlusSerializer(SerializerProtocol):
             return ormsgpack.unpackb(
                 data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
             )
+        elif type_ == "delta":
+            from langgraph.checkpoint.base import DeltaValue  # lazy import
+
+            raw = ormsgpack.unpackb(
+                data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
+            )
+            return DeltaValue(delta=raw["d"], prev_checkpoint_id=raw.get("c"))
         elif self.pickle_fallback and type_ == "pickle":
             return pickle.loads(data_)
         else:

libs/checkpoint/pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "langgraph-checkpoint"
-version = "4.0.1"
+version = "4.0.2"
 description = "Library with base interfaces for LangGraph checkpoint savers."
 authors = []
 requires-python = ">=3.10"

libs/checkpoint/tests/test_jsonplus.py

@@ -983,3 +983,31 @@ def test_msgpack_nested_pydantic_serializes_as_dict(
     # No blocking should occur - inner is serialized as dict, not ext
     assert "blocked" not in caplog.text.lower()
     assert result == obj
+
+
+def test_delta_value_serde_round_trip() -> None:
+    from langgraph.checkpoint.base import DeltaValue
+    from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
+
+    serde = JsonPlusSerializer()
+    original = DeltaValue(
+        delta=[{"type": "human", "content": "hi"}], prev_checkpoint_id="abc-123"
+    )
+    type_tag, blob = serde.dumps_typed(original)
+    assert type_tag == "delta"
+    loaded = serde.loads_typed((type_tag, blob))
+    assert isinstance(loaded, DeltaValue)
+    assert loaded.delta == original.delta
+    assert loaded.prev_checkpoint_id == "abc-123"
+
+
+def test_delta_value_serde_chain_root() -> None:
+    from langgraph.checkpoint.base import DeltaValue
+    from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
+
+    serde = JsonPlusSerializer()
+    original = DeltaValue(delta=[], prev_checkpoint_id=None)
+    type_tag, blob = serde.dumps_typed(original)
+    loaded = serde.loads_typed((type_tag, blob))
+    assert isinstance(loaded, DeltaValue)
+    assert loaded.prev_checkpoint_id is None

libs/checkpoint/tests/test_memory.py

@@ -308,3 +308,36 @@ def test_memory_saver_with_allowlist_proxy_isolated() -> None:
     assert direct is not None
     expected = obj.model_dump() if hasattr(obj, "model_dump") else obj.dict()
     assert direct.checkpoint["channel_values"]["foo"] == expected
+
+
+class TestInMemorySaverDeltaChannel:
+    def test_get_channel_blob(self) -> None:
+        """get_channel_blob returns the deserialized blob for a checkpoint+channel."""
+        from langgraph.checkpoint.base import DeltaValue, empty_checkpoint
+
+        saver = InMemorySaver()
+        serde = JsonPlusSerializer()
+
+        thread_id, ns, channel = "t1", "", "messages"
+        version = "00000000000000000000000000000001.0000000000000000"
+        delta = DeltaValue(delta=[{"content": "hi"}], prev_checkpoint_id=None)
+        saver.blobs[(thread_id, ns, channel, version)] = serde.dumps_typed(delta)
+
+        cp = empty_checkpoint()
+        cp["id"] = "cp1"
+        cp["channel_versions"][channel] = version
+        saver.storage[thread_id][ns] = {
+            "cp1": (serde.dumps_typed(cp), serde.dumps_typed({}), None)
+        }
+
+        result = saver.get_channel_blob(thread_id, ns, "cp1", channel)
+        assert isinstance(result, DeltaValue)
+        assert result.delta == [{"content": "hi"}]
+        assert result.prev_checkpoint_id is None
+
+    def test_get_channel_blob_missing(self) -> None:
+        """get_channel_blob returns NotImplemented when checkpoint or channel not found."""
+        saver = InMemorySaver()
+        assert (
+            saver.get_channel_blob("t1", "", "no-such-cp", "messages") is NotImplemented
+        )

libs/checkpoint/uv.lock

@@ -286,7 +286,7 @@ wheels = [
 
 [[package]]
 name = "langgraph-checkpoint"
-version = "4.0.1"
+version = "4.0.2"
 source = { editable = "." }
 dependencies = [
     { name = "langchain-core" },
@@ -369,7 +369,7 @@ test = [
 
 [[package]]
 name = "langsmith"
-version = "0.6.4"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -379,11 +379,12 @@ dependencies = [
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     { name = "requests-toolbelt" },
     { name = "uuid-utils" },
+    { name = "xxhash" },
     { name = "zstandard" },
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+]
+
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 name = "zstandard"
 version = "0.25.0"

libs/cli/js-examples/yarn.lock

@@ -3676,9 +3676,9 @@ keyv@^4.5.4:
     json-buffer "3.0.1"
 
 "langsmith@>=0.5.0 <1.0.0":
-  version "0.5.18"
-  resolved "https://registry.yarnpkg.com/langsmith/-/langsmith-0.5.18.tgz#c691ad23614f0b46eaf07d982e0ac988e1f43880"
-  integrity sha512-3zuZUWffTHQ+73EAwnodADtf534VNEZUpXr9jC12qyG8/IQuJET7PRsCpTb9wX2lmBspakwLUpqpj3tNm/0bVA==
+  version "0.5.20"
+  resolved "https://registry.yarnpkg.com/langsmith/-/langsmith-0.5.20.tgz#4021847d2ccd5a86c5eb96060f9bb5f19f80eca5"
+  integrity sha512-ULhLM8RswvQDXufLtNtvclHrWCBx8Cb5UPI6lAZC+8Dq59iHsVPz/3Ac9khWNm1VIvChRsuykixD/WrmzuuA3Q==
   dependencies:
     p-queue "6.6.2"
     uuid "10.0.0"

libs/cli/js-monorepo-example/yarn.lock

@@ -1328,9 +1328,9 @@ keyv@^4.5.4:
     json-buffer "3.0.1"
 
 "langsmith@>=0.5.0 <1.0.0":
-  version "0.5.18"
-  resolved "https://registry.yarnpkg.com/langsmith/-/langsmith-0.5.18.tgz#c691ad23614f0b46eaf07d982e0ac988e1f43880"
-  integrity sha512-3zuZUWffTHQ+73EAwnodADtf534VNEZUpXr9jC12qyG8/IQuJET7PRsCpTb9wX2lmBspakwLUpqpj3tNm/0bVA==
+  version "0.5.20"
+  resolved "https://registry.yarnpkg.com/langsmith/-/langsmith-0.5.20.tgz#4021847d2ccd5a86c5eb96060f9bb5f19f80eca5"
+  integrity sha512-ULhLM8RswvQDXufLtNtvclHrWCBx8Cb5UPI6lAZC+8Dq59iHsVPz/3Ac9khWNm1VIvChRsuykixD/WrmzuuA3Q==
   dependencies:
     p-queue "6.6.2"
     uuid "10.0.0"

libs/cli/langgraph_cli/__init__.py

@@ -1 +1 @@
-__version__ = "0.4.21"
+__version__ = "0.4.22"

libs/cli/langgraph_cli/analytics.py

@@ -26,8 +26,15 @@ class LogData(TypedDict):
     params: dict[str, Any]
 
 
-def get_anonymized_params(kwargs: dict[str, Any]) -> dict[str, bool]:
-    params = {}
+def get_anonymized_params(
+    kwargs: dict[str, Any], *, cli_command: str
+) -> dict[str, bool | str]:
+    params: dict[str, bool | str] = {}
+
+    if cli_command == "deploy" and (
+        analytics_source := os.getenv("LANGGRAPH_CLI_ANALYTICS_SOURCE")
+    ):
+        params["source"] = analytics_source
 
     # anonymize params with values
     if config := kwargs.get("config"):
@@ -88,7 +95,7 @@ def log_command(func):
             "python_version": platform.python_version(),
             "cli_version": __version__,
             "cli_command": func.__name__,
-            "params": get_anonymized_params(kwargs),
+            "params": get_anonymized_params(kwargs, cli_command=func.__name__),
         }
 
         background_thread = threading.Thread(target=log_data, args=(data,))

libs/cli/uv-examples/monorepo/uv.lock

@@ -290,7 +290,7 @@ wheels = [
 
 [[package]]
 name = "langsmith"
-version = "0.7.26"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -303,9 +303,9 @@ dependencies = [
     { name = "xxhash" },
     { name = "zstandard" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/76/86/6de4f6f0451a9658f26f633e0bb090552a4dafd7df3f1ae7f0d40558e67e/langsmith-0.7.26.tar.gz", hash = "sha256:a3e06f3d689ce7195717aa6b8f91082319819ec7ea9b9a62cdcd3d9dc25bfc7b", size = 1146118, upload-time = "2026-04-06T15:01:03.336Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/e6/11/696019490992db5c87774dc20515529ef42a01e1d770fb754ed6d9b12fb0/langsmith-0.7.31.tar.gz", hash = "sha256:331ee4f7c26bb5be4022b9859b7d7b122cbf8c9d01d9f530114c1914b0349ffb", size = 1178480, upload-time = "2026-04-14T17:55:41.242Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/81/8e/7eb7d65ce62e98e74b9f18f193ea7ac3996d4fbd71fffcc67d0f7ba3103e/langsmith-0.7.26-py3-none-any.whl", hash = "sha256:fe5c877972cea450c1c48251c8fae0f18543c8d19dfdb9ff9a9c4263763dde4e", size = 360160, upload-time = "2026-04-06T15:01:01.516Z" },
+    { url = "https://files.pythonhosted.org/packages/1d/a1/a013cf458c301cda86a213dd153ce0a01c93f1ab5833f951e6a44c9763ce/langsmith-0.7.31-py3-none-any.whl", hash = "sha256:0291d49203f6e80dda011af1afda61eb0595a4d697adb684590a8805e1d61fb6", size = 373276, upload-time = "2026-04-14T17:55:39.677Z" },
 ]
 
 [[package]]

libs/cli/uv-examples/simple/uv.lock

@@ -266,7 +266,7 @@ wheels = [
 
 [[package]]
 name = "langsmith"
-version = "0.7.26"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -279,9 +279,9 @@ dependencies = [
     { name = "xxhash" },
     { name = "zstandard" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/76/86/6de4f6f0451a9658f26f633e0bb090552a4dafd7df3f1ae7f0d40558e67e/langsmith-0.7.26.tar.gz", hash = "sha256:a3e06f3d689ce7195717aa6b8f91082319819ec7ea9b9a62cdcd3d9dc25bfc7b", size = 1146118, upload-time = "2026-04-06T15:01:03.336Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/e6/11/696019490992db5c87774dc20515529ef42a01e1d770fb754ed6d9b12fb0/langsmith-0.7.31.tar.gz", hash = "sha256:331ee4f7c26bb5be4022b9859b7d7b122cbf8c9d01d9f530114c1914b0349ffb", size = 1178480, upload-time = "2026-04-14T17:55:41.242Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/1d/a1/a013cf458c301cda86a213dd153ce0a01c93f1ab5833f951e6a44c9763ce/langsmith-0.7.31-py3-none-any.whl", hash = "sha256:0291d49203f6e80dda011af1afda61eb0595a4d697adb684590a8805e1d61fb6", size = 373276, upload-time = "2026-04-14T17:55:39.677Z" },
 ]
 
 [[package]]

libs/cli/uv.lock

@@ -1120,7 +1120,7 @@ wheels = [
 
 [[package]]
 name = "langsmith"
-version = "0.7.26"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx", marker = "python_full_version >= '3.11'" },
@@ -1133,9 +1133,9 @@ dependencies = [
     { name = "xxhash", marker = "python_full_version >= '3.11'" },
     { name = "zstandard", marker = "python_full_version >= '3.11'" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/76/86/6de4f6f0451a9658f26f633e0bb090552a4dafd7df3f1ae7f0d40558e67e/langsmith-0.7.26.tar.gz", hash = "sha256:a3e06f3d689ce7195717aa6b8f91082319819ec7ea9b9a62cdcd3d9dc25bfc7b", size = 1146118, upload-time = "2026-04-06T15:01:03.336Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/e6/11/696019490992db5c87774dc20515529ef42a01e1d770fb754ed6d9b12fb0/langsmith-0.7.31.tar.gz", hash = "sha256:331ee4f7c26bb5be4022b9859b7d7b122cbf8c9d01d9f530114c1914b0349ffb", size = 1178480, upload-time = "2026-04-14T17:55:41.242Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/1d/a1/a013cf458c301cda86a213dd153ce0a01c93f1ab5833f951e6a44c9763ce/langsmith-0.7.31-py3-none-any.whl", hash = "sha256:0291d49203f6e80dda011af1afda61eb0595a4d697adb684590a8805e1d61fb6", size = 373276, upload-time = "2026-04-14T17:55:39.677Z" },
 ]
 
 [package.optional-dependencies]

libs/langgraph/langgraph/_internal/_constants.py

@@ -66,9 +66,6 @@ CONFIG_KEY_RUNTIME = sys.intern("__pregel_runtime")
 # holds a `Runtime` instance with context, store, stream writer, etc.
 CONFIG_KEY_RESUME_MAP = sys.intern("__pregel_resume_map")
 # holds a mapping of task ns -> resume value for resuming tasks
-CONFIG_KEY_STREAM_MESSAGES_V2 = sys.intern("__pregel_stream_messages_v2")
-# when True, attach StreamMessagesHandlerV2 so content-block (v2) events
-# flow through stream_mode="messages"; set by GraphStreamer only.
 
 # --- Other constants ---
 PUSH = sys.intern("__pregel_push")
@@ -110,7 +107,6 @@ RESERVED = {
     CONFIG_KEY_CHECKPOINT_ID,
     CONFIG_KEY_CHECKPOINT_NS,
     CONFIG_KEY_RESUME_MAP,
-    CONFIG_KEY_STREAM_MESSAGES_V2,
     # other constants
     PUSH,
     PULL,

libs/langgraph/langgraph/channels/__init__.py

@@ -1,6 +1,7 @@
 from langgraph.channels.any_value import AnyValue
 from langgraph.channels.base import BaseChannel
 from langgraph.channels.binop import BinaryOperatorAggregate
+from langgraph.channels.delta import DeltaChannel
 from langgraph.channels.ephemeral_value import EphemeralValue
 from langgraph.channels.last_value import LastValue, LastValueAfterFinish
 from langgraph.channels.named_barrier_value import (
@@ -20,6 +21,7 @@ __all__ = (
     "UntrackedValue",
     "EphemeralValue",
     "BinaryOperatorAggregate",
+    "DeltaChannel",
     "NamedBarrierValue",
     "NamedBarrierValueAfterFinish",
     # topics

libs/langgraph/langgraph/channels/base.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from abc import ABC, abstractmethod
 from collections.abc import Sequence
-from typing import Any, Generic, TypeVar
+from typing import Any, Generic, Literal, TypeVar
 
 from typing_extensions import Self
 
@@ -119,3 +119,17 @@ class BaseChannel(Generic[Value, Update, Checkpoint], ABC):
         Returns `True` if the channel was updated, `False` otherwise.
         """
         return False
+
+    def after_checkpoint(self, version: Any, checkpoint_id: str | None = None) -> None:
+        """Called after checkpoint() with the assigned version, and after
+        from_checkpoint() with the current channel version.
+
+        No-op by default. Override in channels that track their own version
+        for incremental checkpointing (e.g. DeltaChannel).
+        """
+        pass
+
+    @property
+    def checkpoint_hydration_kind(self) -> Literal["delta"] | None:
+        """Return the saver hydration kind for this channel, if any."""
+        return None

libs/langgraph/langgraph/channels/delta.py

@@ -0,0 +1,228 @@
+from __future__ import annotations
+
+import collections.abc
+from collections.abc import Callable, Sequence
+from copy import copy
+from typing import Any, Generic, Literal
+
+from langgraph.checkpoint.base import DeltaChainValue, DeltaValue
+from typing_extensions import Self
+
+from langgraph._internal._typing import MISSING
+from langgraph.channels.base import BaseChannel, Value
+from langgraph.channels.binop import _get_overwrite, _strip_extras
+from langgraph.errors import EmptyChannelError
+
+__all__ = ("DeltaChannel",)
+
+
+def _copy_value(value: Any) -> Any:
+    if value is MISSING:
+        return value
+    try:
+        return value.copy()
+    except AttributeError:
+        return copy(value)
+
+
+class DeltaChannel(Generic[Value], BaseChannel[list[Value], Value, DeltaValue]):
+    """A channel that stores only per-step write deltas in checkpoints.
+
+    Reconstructs the full accumulated list at load time by replaying the
+    chain of deltas through the operator. Use with append-style reducers
+    (e.g. `add_messages`) on long-running threads to reduce checkpoint
+    storage from O(N²) to O(N).
+
+    Works with all checkpointers. Savers with a dedicated blob store
+    (InMemorySaver, PostgresSaver) use an O(1) fast-path per chain step;
+    all others (SQLite, MongoDB, etc.) fall back to get_tuple traversal.
+
+    Use `snapshot_every=N` to cap chain traversal depth at N steps. Every N
+    steps a full snapshot is written as the chain root; subsequent deltas
+    chain back to it, so `get_state` / reload never traverses more than N
+    checkpoints regardless of thread length. Recommended for savers without
+    a dedicated blob store.
+
+    Usage::
+
+        class State(TypedDict):
+            messages: Annotated[list[AnyMessage], DeltaChannel(add_messages)]
+            # Cap reconstruction depth (recommended for SQLite / MongoDB savers):
+            messages: Annotated[list[AnyMessage], DeltaChannel(add_messages, snapshot_every=50)]
+    """
+
+    __slots__ = (
+        "value",
+        "operator",
+        "snapshot_every",
+        "_pending",
+        "_base_version",
+        "_last_checkpoint_id",
+        "_overwritten",
+        "_steps_since_snapshot",
+    )
+
+    def __init__(
+        self,
+        operator: Callable[[list[Value], Any], list[Value]],
+        typ: type = list,
+        *,
+        snapshot_every: int | None = None,
+    ) -> None:
+        typ = _strip_extras(typ)
+        if typ in (
+            collections.abc.Sequence,
+            collections.abc.MutableSequence,
+        ):
+            typ = list
+        super().__init__(typ)
+        self.operator = operator
+        self.snapshot_every = snapshot_every
+        try:
+            self.value: list[Value] = typ()
+        except Exception:
+            self.value = []
+        self._pending: list[Any] = []
+        self._base_version: str | None = None
+        self._last_checkpoint_id: str | None = None
+        self._overwritten: bool = False
+        self._steps_since_snapshot: int = 0
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, DeltaChannel):
+            return False
+        if self.snapshot_every != other.snapshot_every:
+            return False
+        if (
+            self.operator.__name__ != "<lambda>"
+            and other.operator.__name__ != "<lambda>"
+        ):
+            return self.operator is other.operator
+        return True
+
+    @property
+    def ValueType(self) -> Any:
+        return list[self.typ]  # type: ignore[name-defined]
+
+    @property
+    def UpdateType(self) -> Any:
+        return self.typ | list[self.typ]  # type: ignore[name-defined]
+
+    @property
+    def checkpoint_hydration_kind(self) -> Literal["delta"]:
+        return "delta"
+
+    def copy(self) -> Self:
+        new = DeltaChannel(self.operator, self.typ, snapshot_every=self.snapshot_every)
+        new.key = self.key
+        new.value = _copy_value(self.value)
+        new._pending = self._pending[:]
+        new._base_version = self._base_version
+        new._last_checkpoint_id = self._last_checkpoint_id
+        new._overwritten = self._overwritten
+        new._steps_since_snapshot = self._steps_since_snapshot
+        return new
+
+    def from_checkpoint(self, checkpoint: Any) -> Self:
+        new = DeltaChannel(self.operator, self.typ, snapshot_every=self.snapshot_every)
+        new.key = self.key
+        if checkpoint is MISSING:
+            pass
+        elif isinstance(checkpoint, DeltaChainValue):
+            accumulated: list[Value] = (
+                checkpoint.base if checkpoint.base is not None else new.typ()
+            )
+            for step_writes in checkpoint.deltas:
+                for write in step_writes:
+                    accumulated = new.operator(accumulated, write)
+            new.value = accumulated
+            # Seed the counter from actual chain depth so rehydration fires at
+            # the right time regardless of how many prior invocations there were.
+            new._steps_since_snapshot = len(checkpoint.deltas)
+        elif isinstance(checkpoint, DeltaValue):
+            # Should never reach here — checkpoint hydration should assemble
+            # DeltaValues into DeltaChainValue before calling from_checkpoint.
+            raise AssertionError(
+                "DeltaChannel.from_checkpoint received a raw DeltaValue. "
+                "This is a bug in checkpoint hydration — chain assembly should "
+                "have occurred before from_checkpoint was called."
+            )
+        else:
+            # Backwards compat: plain value from old BinaryOperatorAggregate checkpoint
+            # or a full snapshot emitted by DeltaChannel.
+            new.value = _copy_value(checkpoint)
+        new._pending = []
+        new._base_version = None  # set by the subsequent after_checkpoint() call
+        new._overwritten = False
+        return new
+
+    def update(self, values: Sequence[Any]) -> bool:
+        if not values:
+            return False
+        seen_overwrite = False
+        for value in values:
+            is_overwrite, overwrite_value = _get_overwrite(value)
+            if is_overwrite:
+                if seen_overwrite:
+                    from langgraph.errors import (
+                        ErrorCode,
+                        InvalidUpdateError,
+                        create_error_message,
+                    )
+
+                    msg = create_error_message(
+                        message="Can receive only one Overwrite value per super-step.",
+                        error_code=ErrorCode.INVALID_CONCURRENT_GRAPH_UPDATE,
+                    )
+                    raise InvalidUpdateError(msg)
+                self.value = (
+                    _copy_value(overwrite_value)
+                    if overwrite_value is not None
+                    else self.typ()
+                )
+                self._pending = (
+                    [] if overwrite_value is None else [_copy_value(self.value)]
+                )
+                self._overwritten = True
+                seen_overwrite = True
+            elif not seen_overwrite:
+                base = self.typ() if self.value is MISSING else self.value
+                self.value = self.operator(base, value)
+                self._pending.append(value)
+        return True
+
+    def get(self) -> list[Value]:
+        if self.value is MISSING:
+            raise EmptyChannelError()
+        return self.value
+
+    def is_available(self) -> bool:
+        return self.value is not MISSING
+
+    def checkpoint(self) -> Any:
+        if (
+            self.snapshot_every is not None
+            and self._steps_since_snapshot >= self.snapshot_every
+        ):
+            # Emit a full snapshot to cap chain depth at snapshot_every.
+            # The saver stores this as a plain (non-diff) blob, so future
+            # deltas will chain back to it and traversal depth resets to 1.
+            return _copy_value(self.value)
+        return DeltaValue(
+            delta=self._pending[:],
+            prev_checkpoint_id=None if self._overwritten else self._last_checkpoint_id,
+        )
+
+    def after_checkpoint(self, version: Any, checkpoint_id: str | None = None) -> None:
+        if version != self._base_version:
+            if self._base_version is None:
+                pass  # First call after from_checkpoint — anchor without counting a step.
+            elif self.snapshot_every is not None:
+                if self._steps_since_snapshot >= self.snapshot_every:
+                    self._steps_since_snapshot = 0
+                else:
+                    self._steps_since_snapshot += 1
+            self._base_version = version
+            self._last_checkpoint_id = checkpoint_id
+            self._pending = []
+            self._overwritten = False

libs/langgraph/langgraph/config.py

@@ -1,7 +1,5 @@
 import asyncio
 import sys
-from collections.abc import Callable
-from contextvars import ContextVar
 from typing import Any
 
 from langchain_core.runnables import RunnableConfig
@@ -11,18 +9,6 @@ from langgraph.store.base import BaseStore
 from langgraph._internal._constants import CONF, CONFIG_KEY_RUNTIME
 from langgraph.types import StreamWriter
 
-_tool_call_writer: ContextVar[Callable[[Any], None] | None] = ContextVar(
-    "langgraph_tool_call_writer", default=None
-)
-"""ContextVar holding the writer for the currently-executing tool call.
-
-Set by `StreamToolCallHandler.on_tool_start` and reset on end/error.
-Defined here (rather than alongside the handler in `pregel/_tools.py`)
-so `emit_tool_output_delta` can import it without triggering the
-pregel import chain — user tool code does
-`from langgraph.config import emit_tool_output_delta` at import time.
-"""
-
 
 def _no_op_stream_writer(c: Any) -> None:
     pass
@@ -208,30 +194,3 @@ def get_stream_writer() -> StreamWriter:
     """
     runtime = get_config()[CONF][CONFIG_KEY_RUNTIME]
     return runtime.stream_writer
-
-
-def emit_tool_output_delta(delta: Any) -> None:
-    """Emit a `tool-output-delta` event onto the `tools` stream mode.
-
-    Must be called from inside a tool's execution scope (sync or async).
-    While a tool is running, `StreamToolCallHandler.on_tool_start` sets a
-    writer closure on a ContextVar keyed to that call's `tool_call_id`
-    and namespace; this helper reads the ContextVar and forwards `delta`
-    through it.
-
-    When called outside any tool call, or when the graph was not
-    streamed with `"tools"` in `stream_mode`, this is a silent no-op —
-    tool authors can leave `emit_tool_output_delta` calls in place
-    without gating them on stream mode.
-
-    Args:
-        delta: The partial output chunk to stream. Shape is up to the
-            caller — strings are the common case, but any JSON-
-            serializable value is accepted and surfaced as-is on the
-            `tools` channel's `tool-output-delta` payload under
-            `"delta"`.
-    """
-    writer = _tool_call_writer.get()
-    if writer is None:
-        return
-    writer(delta)

libs/langgraph/langgraph/graph/state.py

@@ -1045,7 +1045,6 @@ class StateGraph(Generic[StateT, ContextT, InputT, OutputT]):
         interrupt_after: All | list[str] | None = None,
         debug: bool = False,
         name: str | None = None,
-        transformers: Sequence[Callable[..., Any]] | None = None,
     ) -> CompiledStateGraph[StateT, ContextT, InputT, OutputT]:
         """Compiles the `StateGraph` into a `CompiledStateGraph` object.
 
@@ -1078,11 +1077,6 @@ class StateGraph(Generic[StateT, ContextT, InputT, OutputT]):
             interrupt_after: An optional list of node names to interrupt after.
             debug: A flag indicating whether to enable debug mode.
             name: The name to use for the compiled graph.
-            transformers: Optional sequence of zero-arg factories returning
-                `StreamTransformer` instances. Registered on the compiled
-                graph and instantiated per-run whenever `stream_v2` /
-                `astream_v2` is called. Appended after the built-in
-                `ValuesTransformer` and `MessagesTransformer`.
 
         Returns:
             CompiledStateGraph: The compiled `StateGraph`.
@@ -1165,7 +1159,6 @@ class StateGraph(Generic[StateT, ContextT, InputT, OutputT]):
             store=store,
             cache=cache,
             name=name or "LangGraph",
-            stream_transformers=transformers,
         )
         compiled._serde_allowlist = serde_allowlist
 

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -3,7 +3,11 @@ from __future__ import annotations
 from collections.abc import Mapping
 from datetime import datetime, timezone
 
-from langgraph.checkpoint.base import Checkpoint
+from langgraph.checkpoint.base import (
+    Checkpoint,
+    CheckpointHydrationPlan,
+    IncrementalChannelSpec,
+)
 from langgraph.checkpoint.base.id import uuid6
 
 from langgraph._internal._typing import MISSING
@@ -13,6 +17,19 @@ from langgraph.managed.base import ManagedValueMapping, ManagedValueSpec
 LATEST_VERSION = 4
 
 
+def checkpoint_hydration_plan(
+    specs: Mapping[str, BaseChannel | ManagedValueSpec],
+) -> CheckpointHydrationPlan | None:
+    """Build a saver hydration plan from channel specs."""
+    channels = tuple(
+        IncrementalChannelSpec(name=name, kind=channel.checkpoint_hydration_kind)
+        for name, channel in specs.items()
+        if isinstance(channel, BaseChannel)
+        and channel.checkpoint_hydration_kind is not None
+    )
+    return CheckpointHydrationPlan(channels=channels) if channels else None
+
+
 def empty_checkpoint() -> Checkpoint:
     return Checkpoint(
         v=LATEST_VERSION,
@@ -67,13 +84,12 @@ def channels_from_checkpoint(
             channel_specs[k] = v
         else:
             managed_specs[k] = v
-    return (
-        {
-            k: v.from_checkpoint(checkpoint["channel_values"].get(k, MISSING))
-            for k, v in channel_specs.items()
-        },
-        managed_specs,
-    )
+    channels: dict[str, BaseChannel] = {}
+    for k, v in channel_specs.items():
+        ch = v.from_checkpoint(checkpoint["channel_values"].get(k, MISSING))
+        ch.after_checkpoint(checkpoint["channel_versions"].get(k), checkpoint.get("id"))
+        channels[k] = ch
+    return channels, managed_specs
 
 
 def copy_checkpoint(checkpoint: Checkpoint) -> Checkpoint:

libs/langgraph/langgraph/pregel/_lifecycle.py

@@ -1,274 +0,0 @@
-from __future__ import annotations
-
-from collections.abc import AsyncIterator, Callable, Iterator
-from typing import Any, TypeVar, cast
-from uuid import UUID
-
-from langchain_core.callbacks import BaseCallbackHandler
-
-from langgraph._internal._constants import NS_SEP
-from langgraph.errors import GraphInterrupt
-from langgraph.pregel.protocol import StreamChunk
-
-try:
-    from langchain_core.tracers._streaming import _StreamingCallbackHandler
-except ImportError:
-    _StreamingCallbackHandler = object  # type: ignore[assignment,misc]
-
-
-T = TypeVar("T")
-
-
-_LANGGRAPH_SENTINEL_NODES = frozenset({"__start__", "__end__"})
-
-
-def _is_nested_pregel_start(
-    name: str | None,
-    metadata: dict[str, Any] | None,
-    parent_run_id: UUID | None,
-    task_run_ids: set[UUID],
-) -> bool:
-    """Recognize a nested `Pregel` invocation from its `on_chain_start` metadata.
-
-    When a compiled graph is added as a node, pregel fires two
-    `on_chain_start` callbacks at that task: first for the node chain
-    (whose `name` matches `metadata["langgraph_node"]`) and second for
-    the inner `Pregel` chain (whose `name` is the graph's `name`, not
-    the node name). Both share the same `langgraph_checkpoint_ns`.
-
-    Primary signal: a `langgraph_checkpoint_ns` is set AND `name`
-    differs from the owning task's `langgraph_node`. This covers the
-    common case where the compiled subgraph's name differs from the
-    node name it was registered under.
-
-    Fallback for name collisions (subgraph compiled with
-    `name == node_name`): the inner `Pregel` start's `parent_run_id`
-    is the run_id of the node chain's start event, which the handler
-    records in `task_run_ids` on the first start. Matching
-    `parent_run_id` to that set identifies the second start as the
-    nested `Pregel` even when names coincide.
-
-    Regular node chains are skipped; the root `Pregel` (which has no
-    `langgraph_node` metadata) isn't observed by this handler because
-    the root's start fires before the handler is attached.
-
-    Metadata-based detection is used because `on_chain_start`'s
-    `serialized` argument is `None` for compiled graphs in this
-    version of langchain-core, so class-based detection via
-    `serialized["id"]` isn't available.
-
-    Sentinel nodes (`__start__` / `__end__`) are excluded: conditional
-    edges from `START` fire an `on_chain_start` with `lg_node=__start__`
-    and the router function's name as `name`, which would otherwise
-    match the discriminator without representing an actual nested
-    `Pregel`.
-
-    Args:
-        name: The `name` kwarg from `on_chain_start`.
-        metadata: The `metadata` kwarg from `on_chain_start`.
-        parent_run_id: The `parent_run_id` kwarg from `on_chain_start`.
-        task_run_ids: The set of run_ids the handler has already seen
-            as node-chain starts (i.e. `name == langgraph_node`).
-    """
-    if not metadata:
-        return False
-    if not metadata.get("langgraph_checkpoint_ns"):
-        return False
-    lg_node = metadata.get("langgraph_node")
-    if lg_node is None or lg_node in _LANGGRAPH_SENTINEL_NODES:
-        return False
-    if name != lg_node:
-        return True
-    # Name collision fallback: the inner Pregel's parent_run_id is
-    # the node chain's run_id, which we recorded when that node
-    # chain's start fired.
-    return parent_run_id is not None and parent_run_id in task_run_ids
-
-
-class StreamLifecycleHandler(BaseCallbackHandler, _StreamingCallbackHandler):
-    """Callback handler that emits subgraph lifecycle events on the stream.
-
-    Pushes `LifecycleData`-shaped payloads onto the pregel stream under
-    the `"lifecycle"` mode, keyed by the subgraph's namespace tuple.
-    Drives the `started` → `running` → `completed` / `failed` /
-    `interrupted` state machine.
-
-    The handler is attached to `run_manager.inheritable_handlers` inside
-    a `Pregel.stream` / `astream` call, so it sees callbacks for every
-    descendant chain (nodes, nested `Pregel` subgraphs) but *not* for
-    the root `Pregel` whose start event has already fired. The root's
-    `started` event is emitted eagerly at construction; its terminal
-    state is emitted by `SubgraphTransformer.finalize` / `fail`.
-
-    `run_inline = True` keeps event ordering deterministic.
-    """
-
-    run_inline = True
-
-    def __init__(
-        self,
-        stream: Callable[[StreamChunk], None],
-        *,
-        root_graph_name: str | None = None,
-    ) -> None:
-        """Initialize the handler and emit the root graph's `started` event.
-
-        Args:
-            stream: Callable that accepts a `StreamChunk` tuple
-                `(namespace, mode, payload)` and enqueues it.
-            root_graph_name: The root `Pregel` instance's `name`, emitted
-                with the root's `started` lifecycle payload.
-        """
-        self.stream = stream
-        # Namespaces awaiting the started→running transition.
-        self._pending_running: set[tuple[str, ...]] = set()
-        # run_id → subgraph namespace; populated only for Pregel chains.
-        self._run_to_ns: dict[UUID, tuple[str, ...]] = {}
-        # run_ids of node-chain starts (name == langgraph_node); used
-        # as the parent_run_id fallback when a subgraph's name equals
-        # its node name. Cleared as each chain ends.
-        self._task_run_ids: set[UUID] = set()
-
-        root_payload: dict[str, Any] = {"event": "started"}
-        if root_graph_name is not None:
-            root_payload["graph_name"] = root_graph_name
-        self.stream(((), "lifecycle", root_payload))
-        self._pending_running.add(())
-
-    @staticmethod
-    def _subgraph_ns_from_metadata(metadata: dict[str, Any] | None) -> tuple[str, ...]:
-        """Return the running subgraph's own namespace from task metadata.
-
-        For a nested `Pregel` invoked as a node, `langgraph_checkpoint_ns`
-        ends at the node segment (no inner task appended yet), so
-        splitting on `NS_SEP` gives the subgraph's own namespace.
-        """
-        if not metadata:
-            return ()
-        nskey = metadata.get("langgraph_checkpoint_ns")
-        if not nskey:
-            return ()
-        return tuple(cast(str, nskey).split(NS_SEP))
-
-    @staticmethod
-    def _containing_ns_from_metadata(
-        metadata: dict[str, Any] | None,
-    ) -> tuple[str, ...]:
-        """Return the namespace of the subgraph that contains this task.
-
-        For an inner task with `langgraph_checkpoint_ns`
-        `"seg_a|seg_b"`, the containing subgraph is `("seg_a",)`.
-        """
-        if not metadata:
-            return ()
-        nskey = metadata.get("langgraph_checkpoint_ns")
-        if not nskey:
-            return ()
-        return tuple(cast(str, nskey).split(NS_SEP))[:-1]
-
-    def _emit(self, ns: tuple[str, ...], payload: dict[str, Any]) -> None:
-        self.stream((ns, "lifecycle", payload))
-
-    def tap_output_aiter(
-        self, run_id: UUID, output: AsyncIterator[T]
-    ) -> AsyncIterator[T]:
-        """Pass-through — required by the `_StreamingCallbackHandler` protocol.
-
-        Returns the iterator unchanged. A missing implementation lets
-        langchain's default `Protocol` body return `None`, which breaks
-        the `_consume_aiter` code path in `_runnable.py:900`.
-        """
-        return output
-
-    def tap_output_iter(self, run_id: UUID, output: Iterator[T]) -> Iterator[T]:
-        """Pass-through — sync counterpart to `tap_output_aiter`."""
-        return output
-
-    def _fire_running_if_pending(self, ns: tuple[str, ...]) -> None:
-        if ns in self._pending_running:
-            self._pending_running.discard(ns)
-            self._emit(ns, {"event": "running"})
-
-    def on_chain_start(
-        self,
-        serialized: dict[str, Any],
-        inputs: dict[str, Any],
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        tags: list[str] | None = None,
-        metadata: dict[str, Any] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        # Any descendant activity transitions the containing subgraph to running.
-        containing = self._containing_ns_from_metadata(metadata)
-        self._fire_running_if_pending(containing)
-
-        name = cast(str | None, kwargs.get("name"))
-        lg_node = (metadata or {}).get("langgraph_node")
-
-        # Record node-chain starts so the name-collision fallback in
-        # `_is_nested_pregel_start` can match the inner Pregel's
-        # parent_run_id to them.
-        if (
-            lg_node is not None
-            and lg_node not in _LANGGRAPH_SENTINEL_NODES
-            and name == lg_node
-        ):
-            self._task_run_ids.add(run_id)
-
-        if not _is_nested_pregel_start(
-            name, metadata, parent_run_id, self._task_run_ids
-        ):
-            return
-
-        ns = self._subgraph_ns_from_metadata(metadata)
-        if not ns:
-            return
-
-        self._run_to_ns[run_id] = ns
-        payload: dict[str, Any] = {"event": "started"}
-        if name:
-            payload["graph_name"] = name
-        # `cause` is intentionally not populated here: pregel does not know
-        # what on the parent namespace triggered this subgraph. Product-
-        # specific stream transformers populate `cause` before events
-        # reach the wire. See LifecycleCause in the protocol definition.
-        self._emit(ns, payload)
-        self._pending_running.add(ns)
-
-    def on_chain_end(
-        self,
-        response: Any,
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        self._task_run_ids.discard(run_id)
-        ns = self._run_to_ns.pop(run_id, None)
-        if ns is None:
-            return
-        # Ensure started→running fired even for empty subgraphs.
-        if ns in self._pending_running:
-            self._pending_running.discard(ns)
-            self._emit(ns, {"event": "running"})
-        self._emit(ns, {"event": "completed"})
-
-    def on_chain_error(
-        self,
-        error: BaseException,
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        self._task_run_ids.discard(run_id)
-        ns = self._run_to_ns.pop(run_id, None)
-        if ns is None:
-            return
-        self._pending_running.discard(ns)
-        if isinstance(error, GraphInterrupt):
-            self._emit(ns, {"event": "interrupted"})
-        else:
-            self._emit(ns, {"event": "failed", "error": str(error)})

libs/langgraph/langgraph/pregel/_loop.py

@@ -12,6 +12,7 @@ from contextlib import (
     ExitStack,
 )
 from datetime import datetime, timezone
+from functools import cached_property
 from inspect import signature
 from types import TracebackType
 from typing import (
@@ -29,6 +30,7 @@ from langgraph.checkpoint.base import (
     BaseCheckpointSaver,
     ChannelVersions,
     Checkpoint,
+    CheckpointHydrationPlan,
     CheckpointMetadata,
     CheckpointTuple,
     PendingWrite,
@@ -93,6 +95,7 @@ from langgraph.pregel._algo import (
 )
 from langgraph.pregel._checkpoint import (
     channels_from_checkpoint,
+    checkpoint_hydration_plan,
     copy_checkpoint,
     create_checkpoint,
     empty_checkpoint,
@@ -314,6 +317,29 @@ class PregelLoop:
         )
         self.prev_checkpoint_config = None
 
+    @cached_property
+    def _checkpoint_hydration_plan(self) -> CheckpointHydrationPlan | None:
+        """Build the saver hydration plan from this loop's channel specs."""
+        return checkpoint_hydration_plan(self.specs)
+
+    def _materialize_saved_checkpoint(
+        self, saved: CheckpointTuple | None
+    ) -> CheckpointTuple | None:
+        if saved is None or self.checkpointer is None:
+            return saved
+        return self.checkpointer.materialize_checkpoint_tuple(
+            saved, self._checkpoint_hydration_plan
+        )
+
+    async def _amaterialize_saved_checkpoint(
+        self, saved: CheckpointTuple | None
+    ) -> CheckpointTuple | None:
+        if saved is None or self.checkpointer is None:
+            return saved
+        return await self.checkpointer.amaterialize_checkpoint_tuple(
+            saved, self._checkpoint_hydration_plan
+        )
+
     def _push_graph_lifecycle_event(
         self,
         kind: Literal["resume", "interrupt"],
@@ -692,7 +718,7 @@ class PregelLoop:
         # writes so that interrupt() calls re-fire instead of returning
         # stale values. But if we're actively resuming, keep them —
         # multi-interrupt scenarios need previously resolved values preserved.
-        if self.is_replaying and (
+        is_time_traveling = self.is_replaying and (
             # Time-travel to a subgraph checkpoint: the parent sets
             # RESUMING=True (it can't distinguish time-travel from resume),
             # so we check if this subgraph's own ns is in checkpoint_map.
@@ -710,7 +736,8 @@ class PregelLoop:
                 # (subgraph input is a Send arg, not a Command)
                 or configurable.get(CONFIG_KEY_RESUMING, False)
             )
-        ):
+        )
+        if is_time_traveling:
             self.checkpoint_pending_writes = [
                 w for w in self.checkpoint_pending_writes if w[1] != RESUME
             ]
@@ -765,6 +792,26 @@ class PregelLoop:
                 if k in self.checkpoint["channel_versions"]:
                     version = self.checkpoint["channel_versions"][k]
                     self.checkpoint["versions_seen"][INTERRUPT][k] = version
+            # When time-traveling (replaying from a specific checkpoint),
+            # save a fork checkpoint so the replayed execution creates a
+            # new branch. Without this, if the execution hits an interrupt
+            # before after_tick() runs, no new checkpoint is created —
+            # the parent's latest checkpoint remains the old one and
+            # subsequent resumes load the wrong state.
+            # Skip for update_state forks (source=update/fork) since they
+            # already have their own fork checkpoint.
+            if is_time_traveling and self.checkpoint_metadata.get("source") not in (
+                "update",
+                "fork",
+            ):
+                # Clear old INTERRUPT writes from the loaded checkpoint.
+                # The fork will have a new checkpoint_id which changes
+                # task IDs — stale interrupt writes would accumulate and
+                # confuse the multiple-interrupt check in future resumes.
+                self.checkpoint_pending_writes = [
+                    w for w in self.checkpoint_pending_writes if w[1] != INTERRUPT
+                ]
+                self._put_checkpoint({"source": "fork"})
             # produce values output
             self._emit(
                 "values", map_output_values, self.output_keys, True, self.channels
@@ -807,14 +854,18 @@ class PregelLoop:
         if not self.is_nested:
             # Pass the resolved before-bound checkpoint ID so subgraphs can
             # find their corresponding checkpoint without re-fetching the
-            # parent. For forks (source=update), use the fork's parent
+            # parent. For forks (source=update/fork), use the fork's parent
             # checkpoint ID since the fork was created after the subgraph's
             # checkpoints from the original execution.
             replay_state: ReplayState | None = None
             if self.is_replaying:
                 replay_checkpoint_id = self.checkpoint["id"]
                 if (
-                    self.checkpoint_metadata.get("source") == "update"
+                    self.checkpoint_metadata.get("source")
+                    in (
+                        "update",
+                        "fork",
+                    )
                     and self.prev_checkpoint_config
                 ):
                     replay_checkpoint_id = self.prev_checkpoint_config[CONF].get(
@@ -856,6 +907,12 @@ class PregelLoop:
             id=self.checkpoint["id"] if exiting else None,
             updated_channels=self.updated_channels,
         )
+        if do_checkpoint and self.channels:
+            for k, ch in self.channels.items():
+                ch.after_checkpoint(
+                    self.checkpoint["channel_versions"].get(k),
+                    self.checkpoint.get("id"),
+                )
         # sanitize TASK channel in the checkpoint before saving (durability=="exit")
         if TASKS in self.checkpoint["channel_values"] and any(
             isinstance(channel, UntrackedValue) for channel in self.channels.values()
@@ -1212,6 +1269,8 @@ class SyncPregelLoop(PregelLoop, AbstractContextManager):
             # graph/thread. Returns None on first invocation.
             saved = self.checkpointer.get_tuple(self.checkpoint_config)
 
+        saved = self._materialize_saved_checkpoint(saved)
+
         if saved is None:
             saved = CheckpointTuple(
                 self.checkpoint_config, empty_checkpoint(), {"step": -2}, None, []
@@ -1414,6 +1473,8 @@ class AsyncPregelLoop(PregelLoop, AbstractAsyncContextManager):
             # graph/thread. Returns None on first invocation.
             saved = await self.checkpointer.aget_tuple(self.checkpoint_config)
 
+        saved = await self._amaterialize_saved_checkpoint(saved)
+
         if saved is None:
             saved = CheckpointTuple(
                 self.checkpoint_config, empty_checkpoint(), {"step": -2}, None, []

libs/langgraph/langgraph/pregel/_messages.py

@@ -24,11 +24,6 @@ try:
 except ImportError:
     _StreamingCallbackHandler = object  # type: ignore
 
-try:
-    from langchain_core.tracers._streaming import _V2StreamingCallbackHandler
-except ImportError:
-    _V2StreamingCallbackHandler = object  # type: ignore
-
 T = TypeVar("T")
 Meta = tuple[tuple[str, ...], dict[str, Any]]
 
@@ -261,185 +256,3 @@ class StreamMessagesHandler(BaseCallbackHandler, _StreamingCallbackHandler):
         **kwargs: Any,
     ) -> Any:
         self.metadata.pop(run_id, None)
-
-
-class StreamMessagesHandlerV2(StreamMessagesHandler, _V2StreamingCallbackHandler):
-    """v2 variant of `StreamMessagesHandler`.
-
-    Declaring `_V2StreamingCallbackHandler` as a base flips
-    `BaseChatModel.invoke` to route through `_stream_chat_model_events`
-    (firing `on_stream_event`) instead of `_stream` (firing
-    `on_llm_new_token`). Inherits `on_stream_event` from the parent,
-    which forwards protocol events onto the messages stream channel.
-
-    Pregel attaches this class instead of the v1 handler only when
-    `GraphStreamer` opts in via the internal
-    `CONFIG_KEY_STREAM_MESSAGES_V2` config key; direct
-    `graph.stream(stream_mode="messages")` callers keep the v1
-    AIMessageChunk shape.
-    """
-
-    def on_chat_model_start(
-        self,
-        serialized: dict[str, Any],
-        messages: list[list[BaseMessage]],
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        tags: list[str] | None = None,
-        metadata: dict[str, Any] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        """Record metadata with the FULL checkpoint namespace for v2.
-
-        v1's ``on_chat_model_start`` (inherited) slices the ns tuple
-        with ``[:-1]`` to re-position chat model tokens onto the
-        *containing pregel's* namespace — historically convenient for
-        consumers of ``stream_mode="messages"`` who want "where did
-        this node produce its output" rather than the chat-model's
-        own task ns.
-
-        For the protocol-v2 wire shape that is wrong: the client
-        subscribes the root feed at ``namespaces=[[]]`` with
-        ``depth=1``, and any message emitted at depth ``>=1`` that
-        still carries the containing node's ns must appear at the
-        *full* path from root so that depth filtering cleanly isolates
-        subgraph chatter from the root conversation. JS's
-        ``StreamMessagesHandlerV2`` already does
-        ``metadata.langgraph_checkpoint_ns.split("|")`` (no slice); this
-        override brings the Python v2 handler to the same shape.
-
-        Without this, a chat model invoked inside a nested subgraph
-        (e.g. ``research -> researcher``, ``research`` being a root
-        node that ``.ainvoke()``s a ``researcher`` subgraph) emits at
-        ``["research:<task>"]`` — a single level deep — which slips
-        through the root-feed depth-1 filter and pollutes the main
-        conversation with subgraph tokens. With this override we emit
-        at ``["research:<task>", "researcher:<task>"]`` so the client
-        routes those tokens to the subgraph card instead.
-        """
-        if metadata and (not tags or (TAG_NOSTREAM not in tags)):
-            task_checkpoint_ns = cast(str, metadata["langgraph_checkpoint_ns"])
-            # Keep the trailing ``:<task_id>`` segment (unlike the v1
-            # handler which strips it via ``[:-1]``). The client's
-            # lifecycle events land on the same ns, so message deltas
-            # now correlate 1:1 with a ``lifecycle: started`` event —
-            # ``useMessages(stream, subgraph)`` picks them up without
-            # needing to collapse sibling namespaces.
-            ns = tuple(task_checkpoint_ns.split(NS_SEP))
-            if not self.subgraphs and len(ns) > 1 and ns != self.parent_ns:
-                return
-            stream_metadata = dict(metadata)
-            # Preserve the v1-shaped ``langgraph_checkpoint_ns`` (task
-            # id stripped, trailing ``NS_END`` retained) so downstream
-            # consumers reading checkpoint metadata off a streamed
-            # message see the same shape they did pre-v2. Only the ns
-            # tuple emitted on the wire changes.
-            checkpoint_ns = (
-                f"{task_checkpoint_ns.rsplit(NS_END, 1)[0]}{NS_END}"
-                if NS_END in task_checkpoint_ns
-                else task_checkpoint_ns
-            )
-            stream_metadata["langgraph_checkpoint_ns"] = checkpoint_ns
-            stream_metadata["checkpoint_ns"] = checkpoint_ns
-            if tags:
-                if filtered_tags := [t for t in tags if not t.startswith("seq:step")]:
-                    stream_metadata["tags"] = filtered_tags
-            self.metadata[run_id] = (ns, stream_metadata)
-
-    def on_chain_start(
-        self,
-        serialized: dict[str, Any],
-        inputs: dict[str, Any],
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        tags: list[str] | None = None,
-        metadata: dict[str, Any] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        """Record chain (node) metadata with the FULL checkpoint ns.
-
-        Mirror of :meth:`on_chat_model_start` for the node-start path,
-        so messages returned by ``on_chain_end`` (``Command`` updates
-        and plain state dict outputs) land at the same full-path ns as
-        any chat-model deltas from within that node. See the
-        :meth:`on_chat_model_start` docstring for why the v1 ``[:-1]``
-        slice is dropped here.
-        """
-        if (
-            metadata
-            and kwargs.get("name") == metadata.get("langgraph_node")
-            and (not tags or TAG_HIDDEN not in tags)
-        ):
-            ns = tuple(cast(str, metadata["langgraph_checkpoint_ns"]).split(NS_SEP))
-            if not self.subgraphs and len(ns) > 1:
-                return
-            self.metadata[run_id] = (ns, metadata)
-            for value in _state_values(inputs):
-                if isinstance(value, BaseMessage):
-                    if value.id is not None:
-                        self.seen.add(value.id)
-                elif isinstance(value, Sequence) and not isinstance(value, str):
-                    for item in value:
-                        if isinstance(item, BaseMessage):
-                            if item.id is not None:
-                                self.seen.add(item.id)
-
-    def on_llm_new_token(
-        self,
-        token: str,
-        *,
-        chunk: ChatGenerationChunk | None = None,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        tags: list[str] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        """Intentional no-op — v1 chunks are not used on v2-flagged runs.
-
-        The v2 marker already steers `invoke` to the event generator, so
-        `on_llm_new_token` should not fire under normal routing. This
-        override stays a pass-through (no call to `super()`) to make
-        the intent explicit and to guard against any caller (e.g. a
-        node that calls `model.stream()` directly, which still fires
-        the v1 callback) leaking AIMessageChunks onto a v2-flagged
-        messages stream.
-        """
-        # Intentionally empty: v2 handler does not forward v1 chunks.
-
-    def on_stream_event(
-        self,
-        event: dict[str, Any],
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        tags: list[str] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        """Forward a protocol event from `stream_v2` as a messages stream part.
-
-        Fires once per `MessagesData` event (`message-start`, per-block
-        `content-block-*`, `message-finish`). The transformer layer
-        correlates events back to a single `ChatModelStream` via
-        `metadata["run_id"]` — attached here so the v1
-        `stream_mode="messages"` output (which emits
-        `(AIMessageChunk, metadata)` via `on_llm_new_token`) keeps its
-        original metadata shape.
-
-        Lives on the v2 handler rather than the v1 base: content-block
-        events are a v2-only concept, and forwarding them only when the
-        v2 handler is attached keeps the message channel's shape
-        predictable for v1 callers.
-        """
-        if meta := self.metadata.get(run_id):
-            # Record message_id on message-start so on_chain_end's
-            # dedupe skips the finalized AIMessage the node returns
-            # (otherwise the messages projection double-counts: once
-            # from streaming, once from the chain output).
-            if event.get("event") == "message-start":
-                msg_id = event.get("message_id")
-                if msg_id:
-                    self.seen.add(msg_id)
-            v2_meta = {**meta[1], "run_id": str(run_id)}
-            self.stream((meta[0], "messages", (event, v2_meta)))

libs/langgraph/langgraph/pregel/_tools.py

@@ -1,223 +0,0 @@
-from __future__ import annotations
-
-from collections.abc import AsyncIterator, Callable, Iterator
-from contextvars import Token
-from typing import Any, TypeVar, cast
-from uuid import UUID
-
-from langchain_core.callbacks import BaseCallbackHandler
-
-from langgraph._internal._constants import NS_SEP
-from langgraph.config import _tool_call_writer
-from langgraph.pregel.protocol import StreamChunk
-
-try:
-    from langchain_core.tracers._streaming import _StreamingCallbackHandler
-except ImportError:
-    _StreamingCallbackHandler = object  # type: ignore[assignment,misc]
-
-
-T = TypeVar("T")
-
-ToolCallWriter = Callable[[Any], None]
-"""A closure bound to a single tool call that emits `tool-output-delta` events."""
-
-
-class StreamToolCallHandler(BaseCallbackHandler, _StreamingCallbackHandler):
-    """Callback handler that emits tool-call lifecycle events on the stream.
-
-    Fires on LangChain's `on_tool_*` callbacks and pushes to the `tools`
-    stream mode. Emits `tool-started` / `tool-output-delta` /
-    `tool-finished` / `tool-error` payloads keyed by `tool_call_id`.
-
-    While a tool is executing, this handler sets `_tool_call_writer` to a
-    closure bound to that call's namespace and `tool_call_id`. The
-    `emit_tool_output_delta` helper in `langgraph.config` reads that
-    ContextVar so tool bodies can stream partial output without threading
-    the writer through their own signature.
-
-    Attached by `Pregel.stream` / `astream` when `"tools"` is in
-    `stream_modes`. `run_inline = True` keeps event ordering
-    deterministic.
-    """
-
-    run_inline = True
-
-    def __init__(self, stream: Callable[[StreamChunk], None]) -> None:
-        """Initialize the handler.
-
-        Args:
-            stream: Callable that accepts a `StreamChunk` tuple
-                `(namespace, mode, payload)` and enqueues it.
-        """
-        self.stream = stream
-        # run_id → (namespace, tool_call_id, ContextVar token)
-        # `on_tool_end` does not receive `tool_call_id` in kwargs, so
-        # we correlate by `run_id` which is present on every callback.
-        self._run_to_call: dict[
-            UUID, tuple[tuple[str, ...], str, Token[ToolCallWriter | None]]
-        ] = {}
-
-    @staticmethod
-    def _containing_ns_from_metadata(
-        metadata: dict[str, Any] | None,
-    ) -> tuple[str, ...]:
-        """Return the namespace of the subgraph that contains this tool call.
-
-        `langgraph_checkpoint_ns` on a tool's callback metadata ends with
-        the `node_name:task_id` segment of the node that invoked the
-        tool. Dropping that segment gives the subgraph's own namespace,
-        which matches what other `tools` / `lifecycle` / `messages`
-        emitters use.
-        """
-        if not metadata:
-            return ()
-        nskey = metadata.get("langgraph_checkpoint_ns")
-        if not nskey:
-            return ()
-        return tuple(cast(str, nskey).split(NS_SEP))[:-1]
-
-    def _start(
-        self,
-        serialized: dict[str, Any] | None,
-        input_str: str,
-        *,
-        run_id: UUID,
-        metadata: dict[str, Any] | None,
-        inputs: dict[str, Any] | None,
-        kwargs: dict[str, Any],
-    ) -> None:
-        tool_call_id = cast("str | None", kwargs.get("tool_call_id")) or str(run_id)
-        tool_name = (
-            (serialized or {}).get("name")
-            or cast("str | None", kwargs.get("name"))
-            or ""
-        )
-        ns = self._containing_ns_from_metadata(metadata)
-
-        def writer(delta: Any) -> None:
-            self.stream(
-                (
-                    ns,
-                    "tools",
-                    {
-                        "event": "tool-output-delta",
-                        "tool_call_id": tool_call_id,
-                        "delta": delta,
-                    },
-                )
-            )
-
-        token = _tool_call_writer.set(writer)
-        self._run_to_call[run_id] = (ns, tool_call_id, token)
-
-        payload: dict[str, Any] = {
-            "event": "tool-started",
-            "tool_call_id": tool_call_id,
-            "tool_name": tool_name,
-        }
-        if inputs is not None:
-            payload["input"] = inputs
-        self.stream((ns, "tools", payload))
-
-    def _end(self, output: Any, *, run_id: UUID) -> None:
-        info = self._run_to_call.pop(run_id, None)
-        if info is None:
-            return
-        ns, tool_call_id, token = info
-        self._reset_writer(token)
-        self.stream(
-            (
-                ns,
-                "tools",
-                {
-                    "event": "tool-finished",
-                    "tool_call_id": tool_call_id,
-                    "output": output,
-                },
-            )
-        )
-
-    def _error(self, error: BaseException, *, run_id: UUID) -> None:
-        info = self._run_to_call.pop(run_id, None)
-        if info is None:
-            return
-        ns, tool_call_id, token = info
-        self._reset_writer(token)
-        self.stream(
-            (
-                ns,
-                "tools",
-                {
-                    "event": "tool-error",
-                    "tool_call_id": tool_call_id,
-                    "message": str(error),
-                },
-            )
-        )
-
-    def tap_output_aiter(
-        self, run_id: UUID, output: AsyncIterator[T]
-    ) -> AsyncIterator[T]:
-        """Pass-through — required by the `_StreamingCallbackHandler` protocol."""
-        return output
-
-    def tap_output_iter(self, run_id: UUID, output: Iterator[T]) -> Iterator[T]:
-        """Pass-through — sync counterpart to `tap_output_aiter`."""
-        return output
-
-    @staticmethod
-    def _reset_writer(token: Token[ToolCallWriter | None]) -> None:
-        # Token is invalid if `on_tool_end` runs in a different context
-        # than `on_tool_start` (e.g. langchain may hand off to a thread
-        # worker without copying the context). Swallow that case; the
-        # ContextVar lifetime is bounded by the enclosing task anyway.
-        try:
-            _tool_call_writer.reset(token)
-        except ValueError:
-            pass
-
-    # ------------------------------------------------------------------
-    # Sync callbacks
-    # ------------------------------------------------------------------
-
-    def on_tool_start(
-        self,
-        serialized: dict[str, Any],
-        input_str: str,
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        tags: list[str] | None = None,
-        metadata: dict[str, Any] | None = None,
-        inputs: dict[str, Any] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        self._start(
-            serialized,
-            input_str,
-            run_id=run_id,
-            metadata=metadata,
-            inputs=inputs,
-            kwargs=kwargs,
-        )
-
-    def on_tool_end(
-        self,
-        output: Any,
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        self._end(output, run_id=run_id)
-
-    def on_tool_error(
-        self,
-        error: BaseException,
-        *,
-        run_id: UUID,
-        parent_run_id: UUID | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        self._error(error, run_id=run_id)

libs/langgraph/langgraph/pregel/main.py

@@ -17,7 +17,7 @@ from collections.abc import (
     Sequence,
 )
 from dataclasses import is_dataclass, replace
-from functools import partial
+from functools import cached_property, partial
 from inspect import isclass
 from typing import (
     Any,
@@ -44,6 +44,7 @@ from langgraph.cache.base import BaseCache
 from langgraph.checkpoint.base import (
     BaseCheckpointSaver,
     Checkpoint,
+    CheckpointHydrationPlan,
     CheckpointTuple,
 )
 from langgraph.store.base import BaseStore
@@ -73,7 +74,6 @@ from langgraph._internal._constants import (
     CONFIG_KEY_RUNTIME,
     CONFIG_KEY_SEND,
     CONFIG_KEY_STREAM,
-    CONFIG_KEY_STREAM_MESSAGES_V2,
     CONFIG_KEY_TASK_ID,
     CONFIG_KEY_THREAD_ID,
     ERROR,
@@ -124,25 +124,21 @@ from langgraph.pregel._algo import (
 from langgraph.pregel._call import identifier
 from langgraph.pregel._checkpoint import (
     channels_from_checkpoint,
+    checkpoint_hydration_plan,
     copy_checkpoint,
     create_checkpoint,
     empty_checkpoint,
 )
 from langgraph.pregel._draw import draw_graph
 from langgraph.pregel._io import map_input, read_channels
-from langgraph.pregel._lifecycle import StreamLifecycleHandler
 from langgraph.pregel._loop import (
     AsyncPregelLoop,
     SyncPregelLoop,
 )
-from langgraph.pregel._messages import (
-    StreamMessagesHandler,
-    StreamMessagesHandlerV2,
-)
+from langgraph.pregel._messages import StreamMessagesHandler
 from langgraph.pregel._read import DEFAULT_BOUND, PregelNode
 from langgraph.pregel._retry import RetryPolicy
 from langgraph.pregel._runner import PregelRunner
-from langgraph.pregel._tools import StreamToolCallHandler
 from langgraph.pregel._utils import get_new_channel_versions
 from langgraph.pregel._validate import validate_graph, validate_keys
 from langgraph.pregel._write import ChannelWrite, ChannelWriteEntry
@@ -346,65 +342,6 @@ class NodeBuilder:
         )
 
 
-def _collect_stream_modes(mux: Any) -> list[StreamMode]:
-    """Return the union of `required_stream_modes` across registered transformers.
-
-    Transformers declare the stream modes they need to function, and
-    `stream_v2` asks the graph for exactly that union — no hardcoded
-    default set. If zero transformers are registered (or none declares
-    a given mode), the graph does not stream events for that mode.
-    """
-    modes: set[str] = set()
-    for transformer in mux._transformers:
-        modes.update(transformer.required_stream_modes)
-    return cast("list[StreamMode]", list(modes))
-
-
-def _build_stream_factories(
-    compile_time: Sequence[Callable[..., Any]],
-    call_site: Sequence[Any] | None,
-) -> list[Callable[..., Any]]:
-    """Assemble the factory list handed to `StreamMux(factories=...)`.
-
-    Prepends the built-in `ValuesTransformer`, `MessagesTransformer`,
-    and `SubgraphTransformer` factories, then appends the graph's
-    compile-time `stream_transformers` followed by any call-site
-    additions. Factories flow down into subgraph mini-muxes, so
-    per-scope instances propagate automatically.
-    """
-    from langgraph.stream.transformers import (
-        MessagesTransformer,
-        SubgraphTransformer,
-        ToolLifecycleTransformer,
-        ValuesTransformer,
-    )
-
-    builtins: list[Callable[..., Any]] = [
-        ValuesTransformer,
-        ToolLifecycleTransformer,
-        MessagesTransformer,
-        SubgraphTransformer,
-    ]
-    return [*builtins, *compile_time, *(call_site or ())]
-
-
-def _merge_v2_messages_flag(
-    config: RunnableConfig | None,
-) -> RunnableConfig:
-    """Return a config with the v2 messages flag set in `configurable`.
-
-    Signals to pregel that `stream_mode="messages"` should attach
-    `StreamMessagesHandlerV2` for this call so invoke-time model runs
-    route through the v2 event generator and their protocol events
-    reach the messages channel.
-    """
-    merged: RunnableConfig = dict(config or {})  # type: ignore[assignment]
-    configurable = dict(merged.get(CONF) or {})
-    configurable[CONFIG_KEY_STREAM_MESSAGES_V2] = True
-    merged[CONF] = configurable
-    return merged
-
-
 class Pregel(
     PregelProtocol[StateT, ContextT, InputT, OutputT],
     Generic[StateT, ContextT, InputT, OutputT],
@@ -736,7 +673,6 @@ class Pregel(
         config: RunnableConfig | None = None,
         trigger_to_nodes: Mapping[str, Sequence[str]] | None = None,
         name: str = "LangGraph",
-        stream_transformers: Sequence[Callable[..., Any]] | None = None,
         **deprecated_kwargs: Unpack[DeprecatedKwargs],
     ) -> None:
         if (
@@ -783,9 +719,6 @@ class Pregel(
         self.config = config
         self.trigger_to_nodes = trigger_to_nodes or {}
         self.name = name
-        self._stream_transformers: tuple[Callable[..., Any], ...] = tuple(
-            stream_transformers or ()
-        )
         self._serde_allowlist: set[tuple[str, ...]] | None = None
         if auto_validate:
             self.validate()
@@ -797,6 +730,54 @@ class Pregel(
             return checkpointer
         return _serde.apply_checkpointer_allowlist(checkpointer, self._serde_allowlist)
 
+    @cached_property
+    def _checkpoint_hydration_plan(self) -> CheckpointHydrationPlan | None:
+        return checkpoint_hydration_plan(self.channels)
+
+    def _materialize_saved_checkpoint(
+        self,
+        checkpointer: BaseCheckpointSaver | None,
+        saved: CheckpointTuple | None,
+    ) -> CheckpointTuple | None:
+        if saved is None or checkpointer is None:
+            return saved
+        return checkpointer.materialize_checkpoint_tuple(
+            saved, self._checkpoint_hydration_plan
+        )
+
+    async def _amaterialize_saved_checkpoint(
+        self,
+        checkpointer: BaseCheckpointSaver | None,
+        saved: CheckpointTuple | None,
+    ) -> CheckpointTuple | None:
+        if saved is None or checkpointer is None:
+            return saved
+        return await checkpointer.amaterialize_checkpoint_tuple(
+            saved, self._checkpoint_hydration_plan
+        )
+
+    def _materialize_saved_checkpoints(
+        self,
+        checkpointer: BaseCheckpointSaver | None,
+        saved: Sequence[CheckpointTuple],
+    ) -> list[CheckpointTuple]:
+        if checkpointer is None or not saved:
+            return list(saved)
+        return checkpointer.materialize_checkpoint_tuples(
+            saved, self._checkpoint_hydration_plan
+        )
+
+    async def _amaterialize_saved_checkpoints(
+        self,
+        checkpointer: BaseCheckpointSaver | None,
+        saved: Sequence[CheckpointTuple],
+    ) -> list[CheckpointTuple]:
+        if checkpointer is None or not saved:
+            return list(saved)
+        return await checkpointer.amaterialize_checkpoint_tuples(
+            saved, self._checkpoint_hydration_plan
+        )
+
     def get_graph(
         self, config: RunnableConfig | None = None, *, xray: int | bool = False
     ) -> Graph:
@@ -876,15 +857,6 @@ class Pregel(
 
     def copy(self, update: dict[str, Any] | None = None) -> Self:
         attrs = {k: v for k, v in self.__dict__.items() if k != "__orig_class__"}
-        # ``__init__`` accepts ``stream_transformers`` (public parameter) but
-        # the attribute is stored as ``_stream_transformers`` (private). Map
-        # the private key back onto the public kwarg so compile-time
-        # transformers survive ``copy()`` / ``with_config()``. Without this,
-        # ``_stream_transformers`` gets captured by ``**deprecated_kwargs``
-        # and the resulting instance silently has an empty transformer
-        # pipeline.
-        if "_stream_transformers" in attrs:
-            attrs["stream_transformers"] = attrs.pop("_stream_transformers")
         attrs.update(update or {})
         return self.__class__(**attrs)
 
@@ -1127,13 +1099,14 @@ class Pregel(
 
         step = saved.metadata.get("step", -1) + 1
         stop = step + 2
+        checkpoint = saved.checkpoint
         channels, managed = channels_from_checkpoint(
             self.channels,
-            saved.checkpoint,
+            checkpoint,
         )
         # tasks for this checkpoint
         next_tasks = prepare_next_tasks(
-            saved.checkpoint,
+            checkpoint,
             saved.pending_writes or [],
             self.nodes,
             channels,
@@ -1246,13 +1219,14 @@ class Pregel(
 
         step = saved.metadata.get("step", -1) + 1
         stop = step + 2
+        checkpoint = saved.checkpoint
         channels, managed = channels_from_checkpoint(
             self.channels,
-            saved.checkpoint,
+            checkpoint,
         )
         # tasks for this checkpoint
         next_tasks = prepare_next_tasks(
-            saved.checkpoint,
+            checkpoint,
             saved.pending_writes or [],
             self.nodes,
             channels,
@@ -1378,7 +1352,9 @@ class Pregel(
         if not isinstance(thread_id, str):
             config[CONF][CONFIG_KEY_THREAD_ID] = str(thread_id)
 
-        saved = checkpointer.get_tuple(config)
+        saved = self._materialize_saved_checkpoint(
+            checkpointer, checkpointer.get_tuple(config)
+        )
         return self._prepare_state_snapshot(
             config,
             saved,
@@ -1422,7 +1398,9 @@ class Pregel(
         if not isinstance(thread_id, str):
             config[CONF][CONFIG_KEY_THREAD_ID] = str(thread_id)
 
-        saved = await checkpointer.aget_tuple(config)
+        saved = await self._amaterialize_saved_checkpoint(
+            checkpointer, await checkpointer.aget_tuple(config)
+        )
         return await self._aprepare_state_snapshot(
             config,
             saved,
@@ -1476,9 +1454,11 @@ class Pregel(
             },
         )
         # eagerly consume list() to avoid holding up the db cursor
-        for checkpoint_tuple in list(
-            checkpointer.list(config, before=before, limit=limit, filter=filter)
-        ):
+        checkpoint_tuples = self._materialize_saved_checkpoints(
+            checkpointer,
+            list(checkpointer.list(config, before=before, limit=limit, filter=filter)),
+        )
+        for checkpoint_tuple in checkpoint_tuples:
             yield self._prepare_state_snapshot(
                 checkpoint_tuple.config, checkpoint_tuple
             )
@@ -1530,12 +1510,16 @@ class Pregel(
             },
         )
         # eagerly consume list() to avoid holding up the db cursor
-        for checkpoint_tuple in [
-            c
-            async for c in checkpointer.alist(
-                config, before=before, limit=limit, filter=filter
-            )
-        ]:
+        checkpoint_tuples = await self._amaterialize_saved_checkpoints(
+            checkpointer,
+            [
+                c
+                async for c in checkpointer.alist(
+                    config, before=before, limit=limit, filter=filter
+                )
+            ],
+        )
+        for checkpoint_tuple in checkpoint_tuples:
             yield await self._aprepare_state_snapshot(
                 checkpoint_tuple.config, checkpoint_tuple
             )
@@ -1595,12 +1579,13 @@ class Pregel(
         ) -> RunnableConfig:
             # get last checkpoint
             config = ensure_config(self.config, input_config)
-            saved = checkpointer.get_tuple(config)
+            saved = self._materialize_saved_checkpoint(
+                checkpointer, checkpointer.get_tuple(config)
+            )
             if saved is not None:
                 self._migrate_checkpoint(saved.checkpoint)
-            checkpoint = (
-                copy_checkpoint(saved.checkpoint) if saved else empty_checkpoint()
-            )
+            base_checkpoint = saved.checkpoint if saved else empty_checkpoint()
+            checkpoint = copy_checkpoint(base_checkpoint) if saved else base_checkpoint
             checkpoint_previous_versions = (
                 saved.checkpoint["channel_versions"].copy() if saved else {}
             )
@@ -2041,12 +2026,13 @@ class Pregel(
         ) -> RunnableConfig:
             # get last checkpoint
             config = ensure_config(self.config, input_config)
-            saved = await checkpointer.aget_tuple(config)
+            saved = await self._amaterialize_saved_checkpoint(
+                checkpointer, await checkpointer.aget_tuple(config)
+            )
             if saved is not None:
                 self._migrate_checkpoint(saved.checkpoint)
-            checkpoint = (
-                copy_checkpoint(saved.checkpoint) if saved else empty_checkpoint()
-            )
+            base_checkpoint = saved.checkpoint if saved else empty_checkpoint()
+            checkpoint = copy_checkpoint(base_checkpoint) if saved else base_checkpoint
             checkpoint_previous_versions = (
                 saved.checkpoint["channel_versions"].copy() if saved else {}
             )
@@ -2704,34 +2690,14 @@ class Pregel(
             # set up messages stream mode
             if "messages" in stream_modes:
                 ns_ = cast(str | None, config[CONF].get(CONFIG_KEY_CHECKPOINT_NS))
-                messages_handler_cls = (
-                    StreamMessagesHandlerV2
-                    if config[CONF].get(CONFIG_KEY_STREAM_MESSAGES_V2)
-                    else StreamMessagesHandler
-                )
                 run_manager.inheritable_handlers.append(
-                    messages_handler_cls(
+                    StreamMessagesHandler(
                         stream.put,
                         subgraphs,
                         parent_ns=tuple(ns_.split(NS_SEP)) if ns_ else None,
                     )
                 )
 
-            # set up lifecycle stream mode
-            if "lifecycle" in stream_modes:
-                run_manager.inheritable_handlers.append(
-                    StreamLifecycleHandler(
-                        stream.put,
-                        root_graph_name=self.name,
-                    )
-                )
-
-            # set up tools stream mode
-            if "tools" in stream_modes:
-                run_manager.inheritable_handlers.append(
-                    StreamToolCallHandler(stream.put)
-                )
-
             # set up custom stream mode
             if "custom" in stream_modes:
 
@@ -3101,34 +3067,14 @@ class Pregel(
             if "messages" in stream_modes:
                 # namespace can be None in a root level graph?
                 ns_ = cast(str | None, config[CONF].get(CONFIG_KEY_CHECKPOINT_NS))
-                messages_handler_cls = (
-                    StreamMessagesHandlerV2
-                    if config[CONF].get(CONFIG_KEY_STREAM_MESSAGES_V2)
-                    else StreamMessagesHandler
-                )
                 run_manager.inheritable_handlers.append(
-                    messages_handler_cls(
+                    StreamMessagesHandler(
                         stream_put,
                         subgraphs,
                         parent_ns=tuple(ns_.split(NS_SEP)) if ns_ else None,
                     )
                 )
 
-            # set up lifecycle stream mode
-            if "lifecycle" in stream_modes:
-                run_manager.inheritable_handlers.append(
-                    StreamLifecycleHandler(
-                        stream_put,
-                        root_graph_name=self.name,
-                    )
-                )
-
-            # set up tools stream mode
-            if "tools" in stream_modes:
-                run_manager.inheritable_handlers.append(
-                    StreamToolCallHandler(stream_put)
-                )
-
             # set up custom stream mode
             def stream_writer(c: Any) -> None:
                 aioloop.call_soon_threadsafe(
@@ -3355,106 +3301,6 @@ class Pregel(
             await asyncio.shield(run_manager.on_chain_error(e))
             raise
 
-    def stream_v2(
-        self,
-        input: InputT | Command | None,
-        config: RunnableConfig | None = None,
-        *,
-        interrupt_before: All | Sequence[str] | None = None,
-        interrupt_after: All | Sequence[str] | None = None,
-        transformers: Sequence[Any] | None = None,
-        stream_modes: Sequence[StreamMode] | None = None,
-        output_keys: str | Sequence[str] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        """Start a sync v2 streaming run driven by transformer projections.
-
-        Builds a `StreamMux` from the built-in `ValuesTransformer` /
-        `MessagesTransformer`, this graph's compile-time
-        `stream_transformers`, and any additional `transformers=`
-        supplied at the call site. Returns a `GraphRunStream` that the
-        caller drives by iterating any projection — no background
-        thread.
-
-        Args:
-            input: Graph input.
-            config: Optional runnable config forwarded to the graph.
-            interrupt_before: Nodes to interrupt before, if any.
-            interrupt_after: Nodes to interrupt after, if any.
-            transformers: Extra transformer instances appended after
-                compile-time `stream_transformers`.
-
-        Returns:
-            A `GraphRunStream` the caller iterates to drive the run.
-        """
-        from langgraph.stream._mux import StreamMux
-        from langgraph.stream.run_stream import GraphRunStream
-
-        factories = _build_stream_factories(self._stream_transformers, transformers)
-        mux = StreamMux(factories=factories, is_async=False)
-        requested_stream_modes = set(_collect_stream_modes(mux))
-        requested_stream_modes.update(stream_modes or ())
-        graph_iter = iter(
-            self.stream(
-                input,
-                _merge_v2_messages_flag(config),
-                stream_mode=list(requested_stream_modes),
-                subgraphs=True,
-                version="v2",
-                output_keys=output_keys,
-                interrupt_before=interrupt_before,
-                interrupt_after=interrupt_after,
-                **kwargs,
-            )
-        )
-        return GraphRunStream(graph_iter, mux)
-
-    async def astream_v2(
-        self,
-        input: InputT | Command | None,
-        config: RunnableConfig | None = None,
-        *,
-        interrupt_before: All | Sequence[str] | None = None,
-        interrupt_after: All | Sequence[str] | None = None,
-        transformers: Sequence[Any] | None = None,
-        stream_modes: Sequence[StreamMode] | None = None,
-        output_keys: str | Sequence[str] | None = None,
-        **kwargs: Any,
-    ) -> Any:
-        """Async counterpart to `stream_v2`.
-
-        Returns an `AsyncGraphRunStream` whose projections can be awaited
-        concurrently; each subscribed cursor drives the pump when its
-        buffer is empty.
-
-        Args:
-            input: Graph input.
-            config: Optional runnable config forwarded to the graph.
-            interrupt_before: Nodes to interrupt before, if any.
-            interrupt_after: Nodes to interrupt after, if any.
-            transformers: Extra transformer instances appended after
-                compile-time `stream_transformers`.
-        """
-        from langgraph.stream._mux import StreamMux
-        from langgraph.stream.run_stream import AsyncGraphRunStream
-
-        factories = _build_stream_factories(self._stream_transformers, transformers)
-        mux = StreamMux(factories=factories, is_async=True)
-        requested_stream_modes = set(_collect_stream_modes(mux))
-        requested_stream_modes.update(stream_modes or ())
-        graph_aiter = self.astream(
-            input,
-            _merge_v2_messages_flag(config),
-            stream_mode=list(requested_stream_modes),
-            subgraphs=True,
-            version="v2",
-            output_keys=output_keys,
-            interrupt_before=interrupt_before,
-            interrupt_after=interrupt_after,
-            **kwargs,
-        ).__aiter__()
-        return AsyncGraphRunStream(graph_aiter, mux)
-
     @overload
     def invoke(
         self,

libs/langgraph/langgraph/pregel/remote.py

@@ -650,46 +650,22 @@ class RemoteGraph(PregelProtocol):
         """
         updated_stream_modes: list[StreamModeSDK] = []
         req_single = True
-        # `"lifecycle"` is emitted locally by the `StreamLifecycleHandler`
-        # attached inside `Pregel.stream` / `astream`. The remote graph
-        # API has no corresponding mode, so requests for it against a
-        # `RemoteGraph` are silently stripped here and a warning is
-        # logged so the caller isn't left wondering why no lifecycle
-        # events arrive.
-        dropped_lifecycle = False
         # coerce to list, or add default stream mode
         if stream_mode:
             if isinstance(stream_mode, str):
-                if stream_mode != "lifecycle":
-                    updated_stream_modes.append(cast(StreamModeSDK, stream_mode))
-                else:
-                    dropped_lifecycle = True
+                updated_stream_modes.append(stream_mode)
             else:
                 req_single = False
-                for m in stream_mode:
-                    if m == "lifecycle":
-                        dropped_lifecycle = True
-                    else:
-                        updated_stream_modes.append(cast(StreamModeSDK, m))
+                updated_stream_modes.extend(stream_mode)
         else:
-            updated_stream_modes.append(default)  # type: ignore[arg-type]
+            updated_stream_modes.append(default)
         requested_stream_modes = updated_stream_modes.copy()
         # add any from parent graph
         stream: StreamProtocol | None = (
             (config or {}).get(CONF, {}).get(CONFIG_KEY_STREAM)
         )
         if stream:
-            for m in stream.modes:
-                if m == "lifecycle":
-                    dropped_lifecycle = True
-                else:
-                    updated_stream_modes.append(cast(StreamModeSDK, m))
-        if dropped_lifecycle:
-            logger.warning(
-                "Stream mode 'lifecycle' is not supported by RemoteGraph "
-                "and was stripped from the request; no lifecycle events "
-                "will be emitted for this remote run."
-            )
+            updated_stream_modes.extend(stream.modes)
         # map "messages" to "messages-tuple"
         if "messages" in updated_stream_modes:
             updated_stream_modes.remove("messages")

libs/langgraph/langgraph/stream/__init__.py

@@ -1,20 +0,0 @@
-"""Streaming infrastructure for LangGraph.
-
-Compile a graph with `transformers=[...]` and call `graph.stream_v2()` /
-`graph.astream_v2()` to drive a transformer pipeline that projects the
-graph's raw events into ergonomic per-channel streams.
-"""
-
-from langgraph.stream._event_log import EventLog
-from langgraph.stream._types import ProtocolEvent, StreamTransformer
-from langgraph.stream.run_stream import AsyncGraphRunStream, GraphRunStream
-from langgraph.stream.stream_channel import StreamChannel
-
-__all__ = [
-    "AsyncGraphRunStream",
-    "EventLog",
-    "GraphRunStream",
-    "ProtocolEvent",
-    "StreamChannel",
-    "StreamTransformer",
-]

libs/langgraph/langgraph/stream/_convert.py

@@ -1,71 +0,0 @@
-from __future__ import annotations
-
-import time
-from typing import Any, cast
-
-from langgraph.stream._types import ProtocolEvent, _ProtocolEventParams
-from langgraph.types import StreamPart
-
-
-def _is_v2_messages_payload(data: Any) -> bool:
-    return isinstance(data, dict) and isinstance(data.get("event"), str)
-
-
-def _normalize_messages_data(data: dict[str, Any]) -> dict[str, Any]:
-    """Normalize Python Core message fields to the protocol wire shape."""
-    normalized = {**data}
-    if (
-        normalized["event"] == "message-start"
-        and "id" not in normalized
-        and isinstance(normalized.get("message_id"), str)
-    ):
-        normalized["id"] = normalized["message_id"]
-    if (
-        normalized["event"]
-        in ("content-block-start", "content-block-delta", "content-block-finish")
-        and "content" not in normalized
-        and isinstance(normalized.get("content_block"), dict)
-    ):
-        normalized["content"] = normalized["content_block"]
-    normalized.pop("message_id", None)
-    normalized.pop("content_block", None)
-    return normalized
-
-
-def convert_to_protocol_event(part: StreamPart) -> ProtocolEvent:
-    """Convert a v2 StreamPart to a ProtocolEvent.
-
-    Args:
-        part: A stream part with keys `type`, `ns`, `data`, and
-            optionally `interrupts` (present on values events).
-
-    Returns:
-        The equivalent ProtocolEvent.
-    """
-    part_dict = cast(dict[str, Any], part)
-    data = part_dict["data"]
-    params: _ProtocolEventParams = {
-        "namespace": list(part_dict["ns"]),
-        "timestamp": int(time.time() * 1000),
-        "data": data,
-    }
-    if (
-        part_dict["type"] == "messages"
-        and isinstance(data, tuple)
-        and len(data) == 2
-        and _is_v2_messages_payload(data[0])
-        and isinstance(data[1], dict)
-    ):
-        payload, metadata = data
-        params["data"] = _normalize_messages_data(payload)
-        if isinstance(metadata.get("langgraph_node"), str):
-            params["node"] = metadata["langgraph_node"]
-        if isinstance(metadata.get("run_id"), str):
-            params["run_id"] = metadata["run_id"]
-    if "interrupts" in part_dict:
-        params["interrupts"] = part_dict["interrupts"]
-    return {
-        "type": "event",
-        "method": part_dict["type"],
-        "params": params,
-    }

libs/langgraph/langgraph/stream/_event_log.py

@@ -1,306 +0,0 @@
-from __future__ import annotations
-
-import asyncio
-from collections import deque
-from collections.abc import AsyncIterator, Awaitable, Callable, Iterator
-from typing import Generic, TypeVar
-
-T = TypeVar("T")
-
-
-class EventLog(Generic[T]):
-    """Single-consumer drainable queue for streaming events.
-
-    Items are popped off the front as the consumer advances — there is
-    no retention beyond what's currently queued. A log accepts exactly
-    one subscriber; a second `__iter__` / `__aiter__` call raises. Use
-    `tee(n)` / `atee(n)` for fan-out.
-
-    Starts unbound — neither `__iter__` nor `__aiter__` is available
-    until the StreamMux calls `_bind(is_async)`. After binding, only
-    the matching iteration protocol works; the other raises `TypeError`.
-
-    Pump wiring (set by the run stream, not by `_bind`):
-        - `_request_more`: sync pump callable, returns True if a new
-          event was produced.
-        - `_arequest_more`: async pump coroutine factory, same contract.
-
-    Memory is bounded by caller pace: both sync and async use caller-
-    driven pumps, so each cursor advance produces at most one event.
-    The only shape where a log can accumulate meaningfully is
-    concurrent async consumers at unequal rates — a slow consumer's
-    log grows while fast consumers drive the shared pump. That's the
-    documented tradeoff for concurrent consumption; consume at similar
-    rates or use a single consumer if memory matters.
-
-    Lazy-subscribe: `push` is a no-op when no subscriber has registered.
-    Transformers still execute `process()` (so scalar state like
-    `ValuesTransformer._latest` stays current); only the log append is
-    skipped.
-    """
-
-    def __init__(self, maxlen: int | None = None) -> None:
-        """Initialize an empty, unbound log.
-
-        Args:
-            maxlen: Accepted for forward compatibility; currently unused.
-                The caller-driven pump bounds memory naturally for
-                single-consumer use.
-
-        Raises:
-            ValueError: If `maxlen` is not a positive integer or `None`.
-        """
-        if maxlen is not None and maxlen <= 0:
-            raise ValueError("EventLog maxlen must be a positive int or None")
-        self._items: deque[T] = deque()
-        self._maxlen: int | None = maxlen
-        self._closed = False
-        self._error: BaseException | None = None
-
-        # Binding state — None means unbound.
-        self._is_async: bool | None = None
-
-        # Flipped on first __iter__ / __aiter__. Pre-subscription
-        # pushes are silent no-ops.
-        self._subscribed = False
-
-        # Pump wiring set by the run stream after bind.
-        self._request_more: Callable[[], bool] | None = None
-        self._arequest_more: Callable[[], Awaitable[bool]] | None = None
-
-    # ------------------------------------------------------------------
-    # Binding
-    # ------------------------------------------------------------------
-
-    def _bind(self, *, is_async: bool) -> None:
-        """Bind this log to sync or async mode.
-
-        Called by the StreamMux after transformer registration. Must be
-        called exactly once before any iteration.
-
-        Args:
-            is_async: True to enable async iteration, False for sync.
-
-        Raises:
-            RuntimeError: If the log has already been bound.
-        """
-        if self._is_async is not None:
-            raise RuntimeError("EventLog is already bound")
-        self._is_async = is_async
-
-    # ------------------------------------------------------------------
-    # Producer API
-    # ------------------------------------------------------------------
-
-    def push(self, item: T) -> None:
-        """Append an item. No-op when no subscriber is registered.
-
-        Non-blocking in both sync and async — matches v1's
-        `put_nowait` producer shape. Memory is bounded by caller pace
-        via the caller-driven pump.
-
-        Raises:
-            RuntimeError: If the log is closed (and subscribed).
-        """
-        if not self._subscribed:
-            return
-        if self._closed:
-            raise RuntimeError("Cannot push to a closed EventLog")
-        self._items.append(item)
-
-    def close(self) -> None:
-        """Mark the log as complete."""
-        self._closed = True
-
-    def fail(self, err: BaseException) -> None:
-        """Mark the log as errored.
-
-        Args:
-            err: The exception to surface to the subscriber.
-        """
-        self._error = err
-        self._closed = True
-
-    # ------------------------------------------------------------------
-    # Sync iteration (caller-driven pump)
-    # ------------------------------------------------------------------
-
-    def __iter__(self) -> Iterator[T]:
-        """Subscribe and return a sync cursor. Can be called only once.
-
-        Raises:
-            TypeError: If the log is unbound or bound to async mode.
-            RuntimeError: If the log already has a subscriber.
-        """
-        if self._is_async is None:
-            raise TypeError(
-                "EventLog has not been bound yet. "
-                "Register the transformer with a StreamMux first."
-            )
-        if self._is_async:
-            raise TypeError(
-                "This EventLog is bound to async mode — use 'async for' instead."
-            )
-        if self._subscribed:
-            raise RuntimeError(
-                "EventLog already has a subscriber; use .tee(n) for fan-out."
-            )
-        self._subscribed = True
-        return self._sync_cursor()
-
-    def _sync_cursor(self) -> Iterator[T]:
-        while True:
-            if self._items:
-                yield self._items.popleft()
-            elif self._closed:
-                if self._error is not None:
-                    raise self._error
-                return
-            elif self._request_more is not None:
-                if not self._request_more():
-                    if not self._items and not self._closed:
-                        return
-            else:
-                return
-
-    # ------------------------------------------------------------------
-    # Async iteration (caller-driven pump)
-    # ------------------------------------------------------------------
-
-    def __aiter__(self) -> AsyncIterator[T]:
-        """Subscribe and return an async cursor. Can be called only once.
-
-        Raises:
-            TypeError: If the log is unbound or bound to sync mode.
-            RuntimeError: If the log already has a subscriber.
-        """
-        if self._is_async is None:
-            raise TypeError(
-                "EventLog has not been bound yet. "
-                "Register the transformer with a StreamMux first."
-            )
-        if not self._is_async:
-            raise TypeError("This EventLog is bound to sync mode — use 'for' instead.")
-        if self._subscribed:
-            raise RuntimeError(
-                "EventLog already has a subscriber; use .atee(n) for fan-out."
-            )
-        self._subscribed = True
-        return self._async_cursor()
-
-    async def _async_cursor(self) -> AsyncIterator[T]:
-        while True:
-            if self._items:
-                yield self._items.popleft()
-            elif self._closed:
-                if self._error is not None:
-                    raise self._error
-                return
-            elif self._arequest_more is not None:
-                if not await self._arequest_more():
-                    if not self._items and not self._closed:
-                        return
-            else:
-                return
-
-    # ------------------------------------------------------------------
-    # Fan-out via tee
-    # ------------------------------------------------------------------
-
-    def tee(self, n: int = 2) -> tuple[Iterator[T], ...]:
-        """Subscribe and return `n` independent sync iterators.
-
-        Each branch has its own buffer; items pulled from the
-        underlying cursor are copied into every branch. Branches are
-        naturally bounded by caller pace since the sync pump is
-        caller-driven.
-
-        Args:
-            n: Number of branches to create. Must be >= 1.
-
-        Returns:
-            A tuple of `n` iterators over the same underlying stream.
-
-        Raises:
-            TypeError: If the log is unbound or bound to async mode.
-            RuntimeError: If the log already has a subscriber.
-            ValueError: If `n` < 1.
-        """
-        if n < 1:
-            raise ValueError("tee() requires n >= 1")
-        source = self.__iter__()
-        buffers: list[deque[T]] = [deque() for _ in range(n)]
-        exhausted = [False]
-
-        def branch(i: int) -> Iterator[T]:
-            buf = buffers[i]
-            while True:
-                if buf:
-                    yield buf.popleft()
-                elif exhausted[0]:
-                    return
-                else:
-                    try:
-                        item = next(source)
-                    except StopIteration:
-                        exhausted[0] = True
-                        return
-                    for b in buffers:
-                        b.append(item)
-
-        return tuple(branch(i) for i in range(n))
-
-    def atee(self, n: int = 2) -> tuple[AsyncIterator[T], ...]:
-        """Subscribe and return `n` independent async iterators.
-
-        Caller-driven fan-out: each branch's `__anext__` either pops
-        from its own buffer or, under a shared `asyncio.Lock`, pulls
-        one item from the underlying cursor and distributes it to
-        every branch's buffer.
-
-        Args:
-            n: Number of branches to create. Must be >= 1.
-
-        Returns:
-            A tuple of `n` async iterators over the same underlying
-            stream.
-
-        Raises:
-            TypeError: If the log is unbound or bound to sync mode.
-            RuntimeError: If the log already has a subscriber.
-            ValueError: If `n` < 1.
-        """
-        if n < 1:
-            raise ValueError("atee() requires n >= 1")
-        source = self.__aiter__()
-        buffers: list[deque[T]] = [deque() for _ in range(n)]
-        exhausted = [False]
-        error: list[BaseException | None] = [None]
-        lock = asyncio.Lock()
-
-        async def branch(i: int) -> AsyncIterator[T]:
-            buf = buffers[i]
-            while True:
-                if buf:
-                    yield buf.popleft()
-                    continue
-                if exhausted[0]:
-                    if error[0] is not None:
-                        raise error[0]
-                    return
-                async with lock:
-                    if buf or exhausted[0]:
-                        continue
-                    try:
-                        item = await source.__anext__()
-                    except StopAsyncIteration:
-                        exhausted[0] = True
-                        continue
-                    except Exception as e:
-                        error[0] = e
-                        exhausted[0] = True
-                        continue
-                    for b in buffers:
-                        b.append(item)
-
-        return tuple(branch(i) for i in range(n))

libs/langgraph/langgraph/stream/_mux.py

@@ -1,497 +0,0 @@
-from __future__ import annotations
-
-import asyncio
-import time
-from collections.abc import Awaitable, Callable
-from typing import Any
-
-from langgraph.stream._event_log import EventLog
-from langgraph.stream._types import (
-    ProtocolEvent,
-    StreamTransformer,
-    transformer_requires_async,
-)
-from langgraph.stream.stream_channel import StreamChannel
-
-TransformerFactory = Callable[["tuple[str, ...]"], StreamTransformer]
-"""Factory that builds a scoped transformer for a mux.
-
-Called once per `StreamMux` (root or mini-mux) with the mux's scope
-— typically a subgraph's namespace or `()` for the root. Standard
-transformer classes (`ValuesTransformer`, `MessagesTransformer`,
-`SubgraphTransformer`) accept a single positional scope argument, so
-the class itself is a valid factory. User transformers can close over
-their config: `lambda scope: MyTransformer(scope, foo=...)`.
-"""
-
-
-class StreamMux:
-    """Central event dispatcher for the streaming infrastructure.
-
-    Owns the main event log and routes events through a transformer
-    pipeline. StreamChannels discovered in transformer projections are
-    auto-wired so that every `push()` also injects a `ProtocolEvent`
-    into the main log.
-
-    Pass `is_async=True` when the mux will be consumed via async
-    iteration (`handler.astream()`). All EventLog and StreamChannel
-    instances discovered during registration are automatically bound
-    to the matching mode.
-
-    Attributes:
-        extensions: Merged projection dict across all registered
-            transformers. Treat as read-only — mutations won't be
-            reflected back in individual transformers' state.
-        native_keys: Projection keys contributed by transformers with
-            `_native = True`.
-    """
-
-    def __init__(
-        self,
-        transformers: list[StreamTransformer] | None = None,
-        *,
-        is_async: bool = False,
-        factories: list[TransformerFactory] | None = None,
-        scope: tuple[str, ...] = (),
-    ) -> None:
-        """Initialize the mux and register transformers in order.
-
-        Callers pass either `transformers` (pre-built instances) or
-        `factories` (callables producing fresh instances per mux). A
-        factory list is preferred — mini-muxes built by `make_child()`
-        inherit the factory list, so transformers propagate naturally
-        into every subgraph's scope. `transformers` is kept for
-        back-compat tests that exercise the mux directly.
-
-        Each transformer's `init()` is called once during registration,
-        projections are merged into `extensions`, `_native` keys are
-        recorded in `native_keys`, and any EventLog / StreamChannel
-        instances are bound and wired.
-
-        Args:
-            transformers: Already-built transformer instances. Mutually
-                exclusive with `factories`.
-            is_async: True for async dispatch (`apush` / `aclose` /
-                `afail`), False for the sync path.
-            factories: Zero-or-one-argument callables producing
-                transformers. Called with this mux's `scope`.
-            scope: The namespace the mux operates within. The root mux
-                is `()`; mini-muxes for subgraphs use the subgraph's
-                namespace tuple.
-
-        Raises:
-            RuntimeError: If any transformer requires an async run but
-                the mux is in sync mode.
-            TypeError: If a transformer's `init()` doesn't return a dict.
-            ValueError: If transformers' projection keys collide, or if
-                both `transformers` and `factories` are supplied.
-        """
-        if transformers is not None and factories is not None:
-            raise ValueError("Pass either `transformers` or `factories`, not both.")
-
-        self._is_async = is_async
-        self._factories: list[TransformerFactory] = list(factories or ())
-        self.scope: tuple[str, ...] = scope
-        self._pump_fn: Callable[[], bool] | None = None
-        self._apump_fn: Callable[[], Awaitable[bool]] | None = None
-
-        self._events: EventLog[ProtocolEvent] = EventLog()
-        self._events._bind(is_async=is_async)
-        self._transformers: list[StreamTransformer] = []
-        self._channels: list[StreamChannel[Any]] = []
-        self._logs: list[EventLog[Any]] = []
-        self._seq = 0
-
-        self.extensions: dict[str, Any] = {}
-        self.native_keys: set[str] = set()
-        self._projection_owners: dict[str, str] = {}
-        self._transformer_by_key: dict[str, StreamTransformer] = {}
-
-        if factories is not None:
-            for factory in factories:
-                self._register(factory(scope))
-        else:
-            for transformer in transformers or ():
-                self._register(transformer)
-
-    def make_child(self, scope: tuple[str, ...]) -> StreamMux:
-        """Build a mini-mux with the same factories scoped to `scope`.
-
-        Used by `SubgraphTransformer` to attach a fresh transformer
-        pipeline to each discovered subgraph handle. The child mux
-        inherits the current pump binding (so cursors on its projection
-        logs drive the root pump) and carries the same factory list
-        forward to any grandchild subgraphs.
-
-        Raises:
-            RuntimeError: If the mux was not built from a factory list
-                (i.e., constructed with `transformers=`). Mini-muxes
-                require factories so each scope gets its own fresh
-                transformer instances.
-        """
-        if not self._factories:
-            raise RuntimeError(
-                "StreamMux.make_child requires the mux to be constructed "
-                "with factories; pre-built transformers can't be cloned "
-                "to a new scope."
-            )
-        child = StreamMux(
-            factories=self._factories,
-            is_async=self._is_async,
-            scope=scope,
-        )
-        if self._pump_fn is not None:
-            child.bind_pump(self._pump_fn)
-        if self._apump_fn is not None:
-            child.bind_apump(self._apump_fn)
-        return child
-
-    def bind_pump(self, fn: Callable[[], bool]) -> None:
-        """Wire the sync pull callback onto every EventLog in the mux.
-
-        Also propagates to transformers that expose `_bind_pump` so
-        nested handles (e.g., `ChatModelStream` instances produced by
-        `MessagesTransformer`) can drive the graph pump from their
-        projection cursors.
-        """
-        self._pump_fn = fn
-        self._events._request_more = fn
-        for value in self.extensions.values():
-            if isinstance(value, EventLog):
-                value._request_more = fn
-            elif isinstance(value, StreamChannel):
-                value._log._request_more = fn
-        for transformer in self._transformers:
-            bind = getattr(transformer, "_bind_pump", None)
-            if bind is not None:
-                bind(fn)
-
-    def bind_apump(self, fn: Callable[[], Awaitable[bool]]) -> None:
-        """Async counterpart to `bind_pump`."""
-        self._apump_fn = fn
-        self._events._arequest_more = fn
-        for value in self.extensions.values():
-            if isinstance(value, EventLog):
-                value._arequest_more = fn
-            elif isinstance(value, StreamChannel):
-                value._log._arequest_more = fn
-        for transformer in self._transformers:
-            abind = getattr(transformer, "_bind_apump", None)
-            if abind is not None:
-                abind(fn)
-
-    def _register(self, transformer: StreamTransformer) -> None:
-        """Register a single transformer.
-
-        Calls `transformer.init()`, stores the transformer for event
-        processing, binds any EventLog or StreamChannel instances in
-        the projection, and merges the projection into `extensions`.
-        """
-        if transformer_requires_async(transformer) and not self._is_async:
-            raise RuntimeError(
-                f"{type(transformer).__name__} requires an async run — "
-                "it overrides aprocess/afinalize/afail or sets "
-                "requires_async=True. Use astream(), not stream()."
-            )
-        projection = transformer.init()
-        if not isinstance(projection, dict):
-            raise TypeError(
-                f"StreamTransformer.init() must return a dict, "
-                f"got {type(projection).__name__}"
-            )
-        conflicts = set(projection) & set(self.extensions)
-        if conflicts:
-            attributions = ", ".join(
-                f"{key!r} (owned by {self._projection_owners[key]})"
-                for key in sorted(conflicts)
-            )
-            raise ValueError(
-                f"Transformer {type(transformer).__name__} returned "
-                f"projection keys that conflict with already-registered "
-                f"keys: {attributions}"
-            )
-        self._transformers.append(transformer)
-        self._bind_and_wire(projection)
-        self.extensions.update(projection)
-        owner_name = type(transformer).__name__
-        for key in projection:
-            self._projection_owners[key] = owner_name
-            self._transformer_by_key[key] = transformer
-        if getattr(transformer, "_native", False):
-            self.native_keys.update(projection.keys())
-        on_register = getattr(transformer, "_on_register", None)
-        if on_register is not None:
-            on_register(self)
-
-    def transformer_by_key(self, key: str) -> StreamTransformer | None:
-        """Return the transformer that owns the projection at `key`, if any."""
-        return self._transformer_by_key.get(key)
-
-    def emit(self, event: ProtocolEvent) -> None:
-        """Append a protocol event directly to the main log.
-
-        Built-in transformers use this for protocol repair events that
-        must appear before the source event they are processing. Direct
-        emission intentionally bypasses the transformer pipeline, but
-        still lets this mux remain the only local sequencing authority.
-        """
-        self._seq += 1
-        event["seq"] = self._seq
-        self._events.push(event)
-
-    def push(self, event: ProtocolEvent) -> None:
-        """Route an event through all transformers, then append to the main log.
-
-        Each transformer's `process()` is called in registration order
-        — except when the transformer has `scope_exact = True` (the
-        default) and the event's namespace differs from the mux's
-        `scope`, in which case the transformer is skipped. Transformers
-        that need to see cross-scope events opt out by setting
-        `scope_exact = False` (e.g. `SubgraphTransformer`).
-
-        If any transformer returns False, the event is suppressed from
-        the main log, but transformers that already saw it keep their
-        side effects.
-
-        Seq is assigned right before an event enters the main log, not
-        before the transformer pipeline runs. This ensures that events
-        auto-forwarded from StreamChannels during `process()` get
-        earlier seq numbers than the original event, preserving
-        monotonic ordering in the log.
-
-        Args:
-            event: The protocol event to dispatch.
-        """
-        ns = tuple(event["params"]["namespace"])
-        in_scope = ns == self.scope
-        keep = True
-        for transformer in self._transformers:
-            if transformer.scope_exact and not in_scope:
-                continue
-            if not transformer.process(event):
-                keep = False
-        if keep:
-            self._seq += 1
-            event["seq"] = self._seq
-            self._events.push(event)
-
-    def close(self) -> None:
-        """Finalize all transformers, close all projections and the main log.
-
-        EventLogs and StreamChannels discovered in transformer
-        projections are auto-closed after `finalize()` runs —
-        transformers don't need to close them manually. If any
-        transformer's `finalize()` raises, the remaining transformers,
-        projections, and the main log are still closed; the first error
-        is re-raised after cleanup completes.
-
-        Raises:
-            BaseException: The first error raised by a transformer's
-                `finalize()`, re-raised after cleanup finishes.
-        """
-        first_error: BaseException | None = None
-        for transformer in self._transformers:
-            try:
-                transformer.finalize()
-            except BaseException as e:
-                if first_error is None:
-                    first_error = e
-        for log in self._logs:
-            if not log._closed:
-                log.close()
-        for ch in self._channels:
-            if not ch._log._closed:
-                ch._close()
-        self._events.close()
-        if first_error is not None:
-            raise first_error
-
-    def fail(self, err: BaseException) -> None:
-        """Fail all transformers, projections, and the main log.
-
-        EventLogs and StreamChannels discovered in transformer
-        projections are auto-failed — transformers don't need to fail
-        them manually. If any transformer's `fail()` raises, the
-        remaining transformers, projections, and the main log are still
-        failed.
-
-        Args:
-            err: The exception that ended the run.
-        """
-        for transformer in self._transformers:
-            try:
-                transformer.fail(err)
-            except BaseException:
-                pass
-        for log in self._logs:
-            if not log._closed:
-                log.fail(err)
-        for ch in self._channels:
-            if not ch._log._closed:
-                ch._fail(err)
-        self._events.fail(err)
-
-    # ------------------------------------------------------------------
-    # Async dispatch
-    # ------------------------------------------------------------------
-
-    async def apush(self, event: ProtocolEvent) -> None:
-        """Dispatch an event on the async lane.
-
-        Awaits each transformer's `aprocess` in registration order
-        before appending to the main log — except when the transformer
-        has `scope_exact = True` and the event's namespace differs from
-        `self.scope`, in which case it is skipped. A slow `aprocess`
-        serializes the pipeline by design — that's the guarantee that
-        lets a later transformer (or a synchronous consumer) see the
-        result of the async work. For decoupled work, use `schedule()`
-        from inside `process` / `aprocess` instead.
-
-        The main log append is a non-blocking `push` — matching v1's
-        `put_nowait` shape. Memory is bounded by caller pace via the
-        caller-driven pump; see `EventLog` for the full tradeoff story.
-
-        Args:
-            event: The protocol event to dispatch.
-        """
-        ns = tuple(event["params"]["namespace"])
-        in_scope = ns == self.scope
-        keep = True
-        for transformer in self._transformers:
-            if transformer.scope_exact and not in_scope:
-                continue
-            if not await transformer.aprocess(event):
-                keep = False
-        if keep:
-            self._seq += 1
-            event["seq"] = self._seq
-            self._events.push(event)
-
-    async def aclose(self) -> None:
-        """Finalize on the async lane.
-
-        Awaits every task started via `StreamTransformer.schedule()`
-        across all transformers, then calls `afinalize()` on each,
-        then auto-closes logs, channels, and the main event log.
-
-        If any scheduled task raised under `on_error="raise"`, or any
-        transformer's `afinalize` raises, the exception propagates.
-        The caller (the pump) handles it by routing into `afail`.
-
-        Raises:
-            BaseException: The first scheduled-task or `afinalize`
-                error, re-raised after cleanup.
-        """
-        pending = self._collect_scheduled_tasks()
-        if pending:
-            results = await asyncio.gather(*pending, return_exceptions=True)
-            first_err = next(
-                (
-                    r
-                    for r in results
-                    if isinstance(r, BaseException)
-                    and not isinstance(r, asyncio.CancelledError)
-                ),
-                None,
-            )
-            if first_err is not None:
-                raise first_err
-
-        first_error: BaseException | None = None
-        for transformer in self._transformers:
-            try:
-                await transformer.afinalize()
-            except BaseException as e:
-                if first_error is None:
-                    first_error = e
-        for log in self._logs:
-            if not log._closed:
-                log.close()
-        for ch in self._channels:
-            if not ch._log._closed:
-                ch._close()
-        self._events.close()
-        if first_error is not None:
-            raise first_error
-
-    async def afail(self, err: BaseException) -> None:
-        """Fail on the async lane.
-
-        Cancels every scheduled task across all transformers, awaits
-        them to completion, then runs each transformer's `afail` hook
-        and auto-fails logs, channels, and the main event log.
-
-        Args:
-            err: The exception that ended the run.
-        """
-        pending = self._collect_scheduled_tasks()
-        for task in pending:
-            task.cancel()
-        if pending:
-            await asyncio.gather(*pending, return_exceptions=True)
-
-        for transformer in self._transformers:
-            try:
-                await transformer.afail(err)
-            except BaseException:
-                pass
-        for log in self._logs:
-            if not log._closed:
-                log.fail(err)
-        for ch in self._channels:
-            if not ch._log._closed:
-                ch._fail(err)
-        if not self._events._closed:
-            self._events.fail(err)
-
-    def _collect_scheduled_tasks(self) -> list[asyncio.Task[Any]]:
-        """Return a snapshot of in-flight tasks scheduled via transformers."""
-        return [
-            task
-            for transformer in self._transformers
-            for task in getattr(transformer, "_stream_scheduled_tasks", ())
-            if not task.done()
-        ]
-
-    # ------------------------------------------------------------------
-    # Binding and StreamChannel auto-wiring
-    # ------------------------------------------------------------------
-
-    def _bind_and_wire(self, projection: dict[str, Any]) -> None:
-        """Bind and wire EventLog / StreamChannel instances in a projection."""
-        for value in projection.values():
-            if isinstance(value, StreamChannel):
-                value._bind(is_async=self._is_async)
-                self._channels.append(value)
-                channel_name = value.name
-
-                def _make_forward(name: str) -> Callable[[Any], None]:
-                    def _forward(item: Any) -> None:
-                        self._forward(name, item)
-
-                    return _forward
-
-                value._wire(_make_forward(channel_name))
-            elif isinstance(value, EventLog):
-                value._bind(is_async=self._is_async)
-                self._logs.append(value)
-
-    def _forward(self, channel_name: str, item: Any) -> None:
-        """Inject a ProtocolEvent for a StreamChannel push.
-
-        Forwarded events bypass the transformer pipeline to avoid
-        infinite recursion (a transformer that pushes to a channel
-        during `process()` would re-trigger itself). These events are
-        visible in the main event log but are not passed through
-        transformers' `process()` methods.
-        """
-        event: ProtocolEvent = {
-            "type": "event",
-            "method": f"custom:{channel_name}",
-            "params": {
-                "namespace": [],
-                "timestamp": int(time.time() * 1000),
-                "data": item,
-            },
-        }
-        self.emit(event)

libs/langgraph/langgraph/stream/_types.py

@@ -1,312 +0,0 @@
-from __future__ import annotations
-
-import asyncio
-import logging
-from abc import ABC, abstractmethod
-from collections.abc import Coroutine
-from typing import Any, ClassVar, Literal
-
-from typing_extensions import NotRequired, TypedDict
-
-_logger = logging.getLogger(__name__)
-
-
-class _ProtocolEventParams(TypedDict):
-    """Parameters for a protocol event.
-
-    `timestamp` is wall-clock milliseconds since the epoch and can go
-    backwards across NTP adjustments — use `ProtocolEvent.seq` for
-    ordering.
-    """
-
-    namespace: list[str]
-    timestamp: int
-    data: Any
-    node: NotRequired[str]
-    run_id: NotRequired[str]
-    interrupts: NotRequired[tuple[Any, ...]]
-
-
-class ProtocolEvent(TypedDict):
-    """A protocol event emitted by the streaming infrastructure.
-
-    Wraps a raw stream part (values, messages, custom, etc.) in a uniform
-    envelope with a monotonic sequence number assigned by the StreamMux.
-    Consumers that need a total order across events should use `seq`, not
-    `params.timestamp` (which is wall-clock and not monotonic).
-    """
-
-    type: Literal["event"]
-    event_id: NotRequired[str]
-    seq: NotRequired[int]
-    method: str  # StreamMode value: "values", "messages", "custom", etc.
-    params: _ProtocolEventParams
-
-
-class StreamTransformer(ABC):
-    """Extension point for custom stream projections.
-
-    Transformers observe protocol events flowing through the StreamMux and
-    build typed derived projections (EventLogs, StreamChannels, promises,
-    etc.).
-
-    Set `_native = True` on a transformer to have its projection keys
-    exposed as direct attributes on the run stream (in addition to
-    appearing in `run.extensions`).
-
-    Subclasses must implement `init` and override at least one of
-    `process` / `aprocess`. The `finalize` / `afinalize` and `fail` /
-    `afail` hooks are optional — the default implementations are no-ops.
-    EventLog and StreamChannel instances in the projection dict are
-    auto-closed / auto-failed by the mux, so most transformers don't
-    need `finalize` or `fail` at all.
-
-    Transformers that need async work pick the async lane by:
-
-    1. Overriding `aprocess` (and optionally `afinalize` / `afail`), or
-    2. Calling `self.schedule(coro)` from inside a sync `process`, or
-    3. Setting `requires_async = True` explicitly.
-
-    The mux detects these cases at registration and raises if they're
-    used under sync `stream()` — they only work under `astream()`.
-
-    Use `aprocess` when the pump must wait for async work before the
-    next transformer sees the event (e.g. PII redaction that mutates
-    `event` in place). Use `schedule()` for decoupled async work whose
-    result lands on an independent projection (e.g. async moderation
-    scoring, cost lookup, external tracing).
-
-    Attributes:
-        scope: Namespace the transformer operates within — `()` for the
-            root mux, a subgraph's namespace tuple inside a mini-mux.
-            Set at construction from the mux's scope (each factory is
-            called as `factory(scope)`). Transformers that only care
-            about events at their own namespace compare against
-            `self.scope`; subgraph-aware transformers can treat it as
-            a parent path.
-        scope_exact: If True (the default), the mux only calls
-            `process` / `aprocess` for events whose namespace equals
-            `self.scope` — user transformers get scope-scoped events
-            for free with no boilerplate. Set False for transformers
-            that need to see events across scopes (e.g.
-            `SubgraphTransformer` forwards deeper events into child
-            mini-muxes).
-        requires_async: Explicit opt-in for transformers that need a
-            running event loop but don't override any async method (for
-            example, transformers that call `schedule()` from a sync
-            `process`). The mux also auto-detects the async lane when
-            `aprocess`, `afinalize`, or `afail` is overridden.
-        required_stream_modes: Stream modes the graph must emit for
-            this transformer to have anything to process. Computed as
-            the union across all registered transformers to determine
-            which modes a `GraphStreamer` run requests from the
-            graph. Empty tuple means the transformer consumes only
-            synthetic events (or is purely passive).
-    """
-
-    requires_async: ClassVar[bool] = False
-    scope_exact: ClassVar[bool] = True
-    required_stream_modes: ClassVar[tuple[str, ...]] = ()
-
-    def __init__(self, scope: tuple[str, ...] = ()) -> None:
-        """Initialize the transformer with its mux's scope.
-
-        Args:
-            scope: The namespace tuple the owning mux is scoped to.
-                `()` for the root, the subgraph's namespace inside a
-                mini-mux. Factories receive this at construction time
-                (`factory(scope)` in `StreamMux`).
-        """
-        self.scope: tuple[str, ...] = scope
-
-    @abstractmethod
-    def init(self) -> dict[str, Any]:
-        """Return the projection dict.
-
-        Keys become entries in `run.extensions`. If the transformer has
-        `_native = True`, keys are also set as direct attributes on the
-        run stream.
-
-        StreamChannel instances in the return value are automatically
-        wired by the StreamMux for protocol event auto-forwarding.
-        """
-        ...
-
-    def process(self, event: ProtocolEvent) -> bool:
-        """Handle an event on the sync lane.
-
-        Called for every event before it is appended to the main event
-        log. Subclasses must override either `process` or `aprocess`.
-        The default raises so a missing override fails loudly rather
-        than silently passing every event through.
-
-        Args:
-            event: The protocol event to observe.
-
-        Returns:
-            True to keep the event in the main log, False to suppress it.
-        """
-        raise NotImplementedError(
-            f"{type(self).__name__} must override process() or aprocess()"
-        )
-
-    async def aprocess(self, event: ProtocolEvent) -> bool:
-        """Handle an event on the async lane.
-
-        The mux awaits this before dispatching to the next transformer,
-        so a slow `aprocess` serializes the pipeline. Use it only when
-        a later transformer — or a consumer reading the event
-        synchronously — must see the result of the async work (e.g.
-        PII redaction that mutates `event` in place).
-
-        The default delegates to `process`, so purely-sync transformers
-        run unchanged under `astream()`.
-
-        Args:
-            event: The protocol event to observe.
-
-        Returns:
-            True to keep the event in the main log, False to suppress it.
-        """
-        return self.process(event)
-
-    def finalize(self) -> None:
-        """Called when the run ends normally (sync lane).
-
-        Override to close EventLogs, resolve promises, or perform other
-        teardown. StreamChannel instances are auto-closed by the mux.
-        """
-
-    async def afinalize(self) -> None:
-        """Called when the run ends normally (async lane).
-
-        By the time this runs, the mux has already awaited every task
-        started via `schedule()`, so EventLogs can be closed here
-        without a last-task-wins race.
-
-        The default delegates to `finalize`.
-        """
-        self.finalize()
-
-    def fail(self, err: BaseException) -> None:
-        """Called when the run ends with an error (sync lane).
-
-        Override to fail EventLogs, reject promises, or perform other
-        teardown. StreamChannel instances are auto-failed by the mux.
-
-        Args:
-            err: The exception that ended the run.
-        """
-
-    async def afail(self, err: BaseException) -> None:
-        """Called when the run ends with an error (async lane).
-
-        The mux cancels and awaits every task started via `schedule()`
-        before calling this, so cleanup doesn't race with in-flight work.
-
-        The default delegates to `fail`.
-
-        Args:
-            err: The exception that ended the run.
-        """
-        self.fail(err)
-
-    # ------------------------------------------------------------------
-    # Scheduled async work
-    # ------------------------------------------------------------------
-
-    def schedule(
-        self,
-        coro: Coroutine[Any, Any, Any],
-        *,
-        on_error: Literal["log", "raise"] = "log",
-    ) -> asyncio.Task[Any]:
-        """Schedule a coroutine tied to this transformer's lifecycle.
-
-        The mux holds the task reference, awaits all scheduled tasks
-        during `aclose()` before calling `afinalize()`, and cancels
-        them on `afail()`. Authors don't need to track tasks or
-        implement the last-task-closes-the-log dance.
-
-        Requires a running event loop — call only under `astream()`.
-        Set `requires_async = True` on the class so registration under
-        sync `stream()` fails fast with a clear message.
-
-        Args:
-            coro: The coroutine to run. Its lifecycle is owned by the
-                mux from this point on.
-            on_error: `"log"` (default) catches and logs any exception
-                the coroutine raises, so a single failure doesn't tear
-                down the run. `"raise"` lets the exception propagate
-                when the mux joins pendings, converting the close path
-                into the fail path.
-
-        Returns:
-            The asyncio Task. Authors rarely need to await it directly
-            — consumers read results from whatever projection the
-            coroutine pushes into.
-
-        Raises:
-            RuntimeError: If called without a running event loop (i.e.
-                under sync `stream()` rather than `astream()`).
-        """
-        try:
-            asyncio.get_running_loop()
-        except RuntimeError:
-            raise RuntimeError(
-                f"{type(self).__name__}.schedule() requires a running "
-                "event loop; this transformer must run under astream(), "
-                "not stream(). Set requires_async=True on the class so "
-                "this fails at registration rather than at first event."
-            ) from None
-
-        wrapped = self._wrap_scheduled(coro) if on_error == "log" else coro
-        task = asyncio.create_task(wrapped)
-        tasks = self._scheduled_task_set()
-        tasks.add(task)
-        task.add_done_callback(tasks.discard)
-        return task
-
-    @staticmethod
-    async def _wrap_scheduled(coro: Coroutine[Any, Any, Any]) -> Any:
-        try:
-            return await coro
-        except asyncio.CancelledError:
-            raise
-        except BaseException:
-            _logger.exception("Scheduled StreamTransformer task failed")
-
-    def _scheduled_task_set(self) -> set[asyncio.Task[Any]]:
-        """Return the lazily-allocated task set.
-
-        Avoids requiring subclasses to call `super().__init__()`.
-        """
-        tasks: set[asyncio.Task[Any]] | None = getattr(
-            self, "_stream_scheduled_tasks", None
-        )
-        if tasks is None:
-            tasks = set()
-            self._stream_scheduled_tasks = tasks
-        return tasks
-
-
-def transformer_requires_async(transformer: StreamTransformer) -> bool:
-    """Return True if the transformer needs a running event loop.
-
-    A transformer requires async if it explicitly opts in
-    (`requires_async = True`) or overrides any of the async-lane methods
-    (`aprocess`, `afinalize`, `afail`).
-
-    Args:
-        transformer: The transformer to inspect.
-
-    Returns:
-        True if the transformer cannot run under sync `stream()`.
-    """
-    if transformer.requires_async:
-        return True
-    cls = type(transformer)
-    for name in ("aprocess", "afinalize", "afail"):
-        if getattr(cls, name) is not getattr(StreamTransformer, name):
-            return True
-    return False

libs/langgraph/langgraph/stream/run_stream.py

@@ -1,412 +0,0 @@
-from __future__ import annotations
-
-import asyncio
-from collections.abc import AsyncIterator, Awaitable, Callable, Iterator, Mapping
-from types import MappingProxyType, TracebackType
-from typing import TYPE_CHECKING, Any
-
-from langgraph.stream._convert import convert_to_protocol_event
-from langgraph.stream._mux import StreamMux
-from langgraph.stream._types import ProtocolEvent
-
-if TYPE_CHECKING:
-    from langgraph.stream.transformers import ValuesTransformer
-
-
-def _drive_until_done(pump: Callable[[], bool]) -> None:
-    """Call the sync pump until it returns False."""
-    while pump():
-        pass
-
-
-async def _adrive_until_done(pump: Callable[[], Awaitable[bool]]) -> None:
-    """Call the async pump until it returns False."""
-    while await pump():
-        pass
-
-
-class BaseRunStream:
-    """Shared shape for any object that wraps a `StreamMux`.
-
-    Root (`GraphRunStream` / `AsyncGraphRunStream`) and scoped
-    (`SubgraphRunStream`) streams both compose a `StreamMux`. The mux
-    owns the projections — `values`, `messages`, `subgraphs`, and any
-    user-registered keys — all exposed via `extensions`. Native
-    projections (`_native = True`) are also bound as direct attributes
-    (`run.values`, `run.messages`, …) for ergonomics.
-
-    Raw iteration (`for event in run` / `async for event in run`) and
-    the `interleave(...)` helper both live here so every subclass
-    behaves consistently. Subclasses only add pump ownership, scope
-    metadata, or sync/async flavor.
-    """
-
-    def __init__(self, mux: StreamMux) -> None:
-        self._mux = mux
-        self.extensions: Mapping[str, Any] = MappingProxyType(mux.extensions)
-        for key in mux.native_keys:
-            setattr(self, key, mux.extensions[key])
-
-    @property
-    def _values_transformer(self) -> ValuesTransformer:
-        """Look up the `ValuesTransformer` backing `output` / `interrupted`.
-
-        Resolved lazily off the mux so subclasses don't have to thread
-        it through their constructors. Raises if no `ValuesTransformer`
-        is registered — `output` / `interrupted` / `interrupts` have
-        nothing to return in that case, so failing loudly is better
-        than returning `None` silently.
-        """
-        from langgraph.stream.transformers import ValuesTransformer
-
-        vt = self._mux.transformer_by_key("values")
-        if not isinstance(vt, ValuesTransformer):
-            raise RuntimeError(
-                "No ValuesTransformer is registered on this mux — "
-                "`output`, `interrupted`, and `interrupts` require one. "
-                "Add it to your GraphStreamer subclass's "
-                "`builtin_factories` or pass it via `transformers=`."
-            )
-        return vt
-
-    def __iter__(self) -> Iterator[ProtocolEvent]:
-        """Sync iteration of protocol events on this mux's main log.
-
-        Raises at the EventLog level if the mux is async-bound.
-        """
-        return iter(self._mux._events)
-
-    def __aiter__(self) -> AsyncIterator[ProtocolEvent]:
-        """Async iteration of protocol events on this mux's main log.
-
-        Raises at the EventLog level if the mux is sync-bound.
-        """
-        return self._mux._events.__aiter__()
-
-    def interleave(self, *names: str) -> Iterator[tuple[str, Any]]:
-        """Iterate multiple projections round-robin, yielding ``(name, item)``.
-
-        Each turn advances one projection's cursor; when a cursor's
-        buffer is empty, pulling from it drives the pump once, which
-        fans out to every subscribed projection log. Projections whose
-        items aren't consumed on this turn sit in their own buffers
-        only until the next turn reaches them, bounding memory by the
-        skew between projection rates rather than letting any single
-        log grow to the full run length.
-
-        Projections are exhausted independently; a projection that
-        finishes early drops out of the rotation while others
-        continue. The overall iterator ends once all named projections
-        are done.
-
-        Args:
-            *names: Projection keys to interleave. Must match keys in
-                `extensions`.
-
-        Yields:
-            `(name, item)` tuples in round-robin order across the named
-            projections.
-
-        Raises:
-            KeyError: If a name doesn't match a registered projection.
-
-        Example:
-            ```python
-            for name, item in run.interleave("messages", "values"):
-                if name == "messages":
-                    print("msg:", item)
-                else:
-                    print("val:", item)
-            ```
-        """
-        cursors: dict[str, Iterator[Any]] = {
-            name: iter(self.extensions[name]) for name in names
-        }
-        done: set[str] = set()
-        while len(done) < len(cursors):
-            for name, cursor in cursors.items():
-                if name in done:
-                    continue
-                try:
-                    item = next(cursor)
-                except StopIteration:
-                    done.add(name)
-                    continue
-                yield (name, item)
-
-
-class GraphRunStream(BaseRunStream):
-    """Sync run stream with caller-driven pumping.
-
-    The caller's iteration on any projection (`values`, `messages`,
-    raw events, or `output`) drives the graph forward. No background
-    thread is used — the caller's `for` loop is the pump.
-
-    Projections are single-consumer — iterating `run.values` twice
-    raises. Use `projection.tee(n)` if you genuinely need fan-out.
-    """
-
-    def __init__(
-        self,
-        graph_iter: Iterator[Any],
-        mux: StreamMux,
-    ) -> None:
-        """Initialize the run stream.
-
-        Args:
-            graph_iter: Pull-based iterator over the graph's stream.
-            mux: The StreamMux owning projections and the main log.
-                Must have a `ValuesTransformer` registered under the
-                `"values"` key — `output` / `interrupted` / `interrupts`
-                read from it lazily.
-        """
-        super().__init__(mux)
-        self._graph_iter = graph_iter
-        self._exhausted = False
-        mux.bind_pump(self._pump_next)
-
-    def _pump_next(self) -> bool:
-        """Pull one event from the graph and push it through the mux.
-
-        Returns:
-            True if an event was pulled, False if the graph is
-            exhausted or has raised.
-        """
-        if self._exhausted:
-            return False
-        try:
-            part = next(self._graph_iter)
-        except StopIteration:
-            self._mux.close()
-            self._exhausted = True
-            return False
-        except Exception as e:
-            self._mux.fail(e)
-            self._exhausted = True
-            return False
-        self._mux.push(convert_to_protocol_event(part))
-        return True
-
-    def abort(self) -> None:
-        """Stop the run early.
-
-        Closes the mux and marks the stream exhausted. The graph
-        iterator is dropped; any in-flight nodes see the closure on
-        their next yield point. Idempotent.
-        """
-        if self._exhausted:
-            return
-        self._exhausted = True
-        try:
-            self._mux.close()
-        except Exception:
-            pass
-
-    def __enter__(self) -> GraphRunStream:
-        return self
-
-    def __exit__(
-        self,
-        exc_type: type[BaseException] | None,
-        exc: BaseException | None,
-        tb: TracebackType | None,
-    ) -> None:
-        self.abort()
-
-    @property
-    def output(self) -> dict[str, Any] | None:
-        """Drive the run to completion and return the final state."""
-        _drive_until_done(self._pump_next)
-        vt = self._values_transformer
-        if vt.error is not None:
-            raise vt.error
-        return vt._latest
-
-    @property
-    def interrupted(self) -> bool:
-        """Drive the run to completion, then return whether it was interrupted.
-
-        Raises:
-            BaseException: If the run ended with an error.
-        """
-        _drive_until_done(self._pump_next)
-        vt = self._values_transformer
-        if vt.error is not None:
-            raise vt.error
-        return vt._interrupted
-
-    @property
-    def interrupts(self) -> list[Any]:
-        """Drive the run to completion, then return interrupt payloads.
-
-        Raises:
-            BaseException: If the run ended with an error.
-        """
-        _drive_until_done(self._pump_next)
-        vt = self._values_transformer
-        if vt.error is not None:
-            raise vt.error
-        return vt._interrupts
-
-
-class AsyncGraphRunStream(BaseRunStream):
-    """Async run stream with caller-driven pumping.
-
-    Async iteration on any projection drives the graph forward — there
-    is no background task. Concurrent consumers share a single-flight
-    pump via an `asyncio.Lock`, so each awaiting cursor contributes
-    one event per acquisition. Backpressure comes from the logs: when
-    a subscribed log's buffer reaches `maxlen`, `apush` awaits the
-    subscriber to drain, which holds back the pump and paces the
-    graph.
-
-    Projections are single-consumer — a second `aiter(run.values)`
-    raises. Use `projection.tee(n)` for fan-out.
-
-    Use as an async context manager to guarantee clean shutdown on
-    early exit:
-
-    ```python
-    async with await handler.astream(input) as run:
-        async for msg in run.messages:
-            ...
-    ```
-    """
-
-    def __init__(
-        self,
-        graph_aiter: AsyncIterator[Any],
-        mux: StreamMux,
-    ) -> None:
-        """Initialize the async run stream.
-
-        Args:
-            graph_aiter: Async iterator over the graph's stream.
-            mux: The StreamMux owning projections and the main log.
-                Must have a `ValuesTransformer` registered under the
-                `"values"` key — `output` / `interrupted` / `interrupts`
-                read from it lazily.
-        """
-        super().__init__(mux)
-        self._graph_aiter = graph_aiter
-        self._exhausted = False
-        self._pump_cond = asyncio.Condition()
-        self._pumping = False
-        mux.bind_apump(self._apump_next)
-
-    async def _apump_next(self) -> bool:
-        """Drive one pump step, or wait for the active pumper to drive one.
-
-        "Take-a-number" semantics: at most one task at a time calls
-        `graph_aiter.__anext__()` (asyncio iterators can't be advanced
-        concurrently). Other callers wait on a Condition that the
-        active pumper notifies after each step. This lets a "passive"
-        consumer — one whose projection's buffer is being filled by the
-        active pumper's push — wake up as soon as its data lands,
-        instead of queueing on the pump and only observing its data one
-        graph event late.
-
-        `except Exception` is intentional — `CancelledError` and other
-        `BaseException` subclasses propagate, matching asyncio's
-        cancellation contract.
-
-        Returns:
-            True if a pump step completed (by this task or another),
-            False if the graph is exhausted.
-        """
-        async with self._pump_cond:
-            if self._exhausted:
-                return False
-            if self._pumping:
-                # Another task is pumping; wait for its progress signal.
-                await self._pump_cond.wait()
-                return not self._exhausted
-            self._pumping = True
-
-        try:
-            try:
-                part = await self._graph_aiter.__anext__()
-            except StopAsyncIteration:
-                self._exhausted = True
-                await self._mux.aclose()
-                return False
-            except Exception as e:
-                self._exhausted = True
-                await self._mux.afail(e)
-                return False
-            await self._mux.apush(convert_to_protocol_event(part))
-            return True
-        finally:
-            async with self._pump_cond:
-                self._pumping = False
-                self._pump_cond.notify_all()
-
-    async def abort(self) -> None:
-        """Stop the run early.
-
-        Marks the stream exhausted, wakes any pump-waiters, and closes
-        the mux. Any `apush` blocked on backpressure wakes and returns
-        without appending. Idempotent.
-        """
-        async with self._pump_cond:
-            if self._exhausted:
-                return
-            self._exhausted = True
-            self._pump_cond.notify_all()
-        try:
-            await self._mux.aclose()
-        except Exception:
-            pass
-
-    async def __aenter__(self) -> AsyncGraphRunStream:
-        return self
-
-    async def __aexit__(
-        self,
-        exc_type: type[BaseException] | None,
-        exc: BaseException | None,
-        tb: TracebackType | None,
-    ) -> None:
-        await self.abort()
-
-    async def output(self) -> dict[str, Any] | None:
-        """Drive the run to completion and return the final state.
-
-        Methods (not properties) on the async lane so `run.output`
-        without `await` raises at type-check time instead of silently
-        yielding a coroutine object.
-
-        Example:
-            ```python
-            output = await run.output()
-            ```
-
-        Raises:
-            BaseException: If the run ended with an error.
-        """
-        await _adrive_until_done(self._apump_next)
-        if (err := self._values_transformer.error) is not None:
-            raise err
-        return self._values_transformer._latest
-
-    async def interrupted(self) -> bool:
-        """Drive the run to completion and return whether it was interrupted.
-
-        Raises:
-            BaseException: If the run ended with an error.
-        """
-        await _adrive_until_done(self._apump_next)
-        if (err := self._values_transformer.error) is not None:
-            raise err
-        return self._values_transformer._interrupted
-
-    async def interrupts(self) -> list[Any]:
-        """Drive the run to completion and return interrupt payloads.
-
-        Raises:
-            BaseException: If the run ended with an error.
-        """
-        await _adrive_until_done(self._apump_next)
-        if (err := self._values_transformer.error) is not None:
-            raise err
-        return self._values_transformer._interrupts

libs/langgraph/langgraph/stream/stream_channel.py

@@ -1,109 +0,0 @@
-from __future__ import annotations
-
-from collections.abc import AsyncIterator, Callable, Iterator
-from typing import Generic, TypeVar
-
-from langgraph.stream._event_log import EventLog
-
-T = TypeVar("T")
-
-
-class StreamChannel(Generic[T]):
-    """A named projection channel with optional protocol auto-forwarding.
-
-    Wraps an event log and declares a protocol channel name. When the
-    StreamMux detects a StreamChannel in a transformer's `init()`
-    return value, it automatically wires every `push()` to inject a
-    `ProtocolEvent` into the main event stream using the channel's
-    name as the method.
-
-    Auto-forwarded events bypass the transformer pipeline — other
-    transformers' `process()` / `aprocess()` methods do not see
-    `custom:<name>` events produced by a channel push. This prevents a
-    transformer that pushes to its own channel during `process()` from
-    re-triggering itself, but it also means filter- or tap-style
-    transformers cannot observe channel output from peer transformers.
-    Consumers that need that should iterate the main event stream.
-
-    In-process consumers iterate the channel directly (`for item in ch`
-    or `async for item in ch`). Remote SDK clients subscribe via
-    `session.subscribe("custom:<channelName>")`.
-
-    Like EventLog, a StreamChannel starts unbound. The mux calls
-    `_bind(is_async)` during registration so the correct iteration
-    protocol is available by the time user code sees it.
-
-    Lifecycle (`_close` / `_fail`) is managed by the mux — transformers
-    using only StreamChannels don't need `finalize` or `fail` hooks.
-    """
-
-    def __init__(self, name: str, *, maxlen: int | None = None) -> None:
-        """Initialize the channel with an empty inner log.
-
-        Args:
-            name: The protocol channel name used for auto-forwarded
-                events (`custom:<name>` on the wire).
-            maxlen: Optional retention cap on the inner EventLog. See
-                `EventLog.__init__` for semantics.
-        """
-        self.name = name
-        self._log: EventLog[T] = EventLog(maxlen=maxlen)
-        self._wire_fn: Callable[[T], None] | None = None
-
-    def _bind(self, *, is_async: bool) -> None:
-        """Bind the underlying event log to sync or async mode.
-
-        Args:
-            is_async: True for async iteration, False for sync.
-        """
-        self._log._bind(is_async=is_async)
-
-    def push(self, item: T) -> None:
-        """Append an item to the log and auto-forward if wired.
-
-        Args:
-            item: The item to push.
-        """
-        self._log.push(item)
-        if self._wire_fn is not None:
-            self._wire_fn(item)
-
-    # ------------------------------------------------------------------
-    # Mux lifecycle hooks (not called by transformers directly)
-    # ------------------------------------------------------------------
-
-    def _wire(self, fn: Callable[[T], None]) -> None:
-        """Install the auto-forward callback (called by StreamMux)."""
-        self._wire_fn = fn
-
-    def _close(self) -> None:
-        """Close the underlying log (called by StreamMux on run end)."""
-        self._log.close()
-
-    def _fail(self, err: BaseException) -> None:
-        """Fail the underlying log (called by StreamMux on run error)."""
-        self._log.fail(err)
-
-    # ------------------------------------------------------------------
-    # Iteration — delegates to the inner event log (multi-cursor)
-    # ------------------------------------------------------------------
-
-    def __iter__(self) -> Iterator[T]:
-        return iter(self._log)
-
-    def __aiter__(self) -> AsyncIterator[T]:
-        return self._log.__aiter__()
-
-    def tee(self, n: int = 2) -> tuple[Iterator[T], ...]:
-        """Fan out the channel into `n` independent sync iterators.
-
-        Delegates to the underlying EventLog's `tee()`.
-        """
-        return self._log.tee(n)
-
-    def atee(self, n: int = 2) -> tuple[AsyncIterator[T], ...]:
-        """Fan out the channel into `n` independent async iterators.
-
-        Delegates to the underlying EventLog's `atee()`.
-        """
-        return self._log.atee(n)

libs/langgraph/langgraph/stream/transformers.py

@@ -1,750 +0,0 @@
-from __future__ import annotations
-
-import logging
-from typing import TYPE_CHECKING, Any, Literal, cast
-
-from langchain_core.language_models._compat_bridge import message_to_events
-from langchain_core.language_models.chat_model_stream import (
-    AsyncChatModelStream,
-    ChatModelStream,
-)
-from langchain_core.messages import AIMessageChunk, BaseMessage
-from langchain_protocol.protocol import (
-    CheckpointRef,
-    LifecycleCause,
-    LifecycleData,
-    MessagesData,
-)
-
-from langgraph.errors import GraphInterrupt
-from langgraph.stream._event_log import EventLog
-from langgraph.stream._types import ProtocolEvent, StreamTransformer
-from langgraph.stream.run_stream import BaseRunStream
-
-if TYPE_CHECKING:
-    from collections.abc import Awaitable, Callable
-
-    from langgraph.stream._mux import StreamMux
-
-
-logger = logging.getLogger(__name__)
-
-
-SubgraphStatus = Literal["started", "running", "completed", "failed", "interrupted"]
-_TERMINAL_STATUSES: frozenset[SubgraphStatus] = frozenset(
-    {"completed", "failed", "interrupted"}
-)
-
-
-def _is_record(value: Any) -> bool:
-    return isinstance(value, dict)
-
-
-def _to_chat_model_stream_event(event: MessagesData) -> MessagesData:
-    """Convert wire-shaped message fields to ChatModelStream's internal shape."""
-    event_type = event.get("event")
-    converted: dict[str, Any] = dict(event)
-    if (
-        event_type == "message-start"
-        and "message_id" not in converted
-        and isinstance(converted.get("id"), str)
-    ):
-        converted["message_id"] = converted["id"]
-    if (
-        event_type in ("content-block-start", "content-block-delta", "content-block-finish")
-        and "content_block" not in converted
-        and isinstance(converted.get("content"), dict)
-    ):
-        converted["content_block"] = converted["content"]
-    return cast("MessagesData", converted)
-
-
-def _message_event_id(event: MessagesData) -> str | None:
-    raw_id = event.get("id") or event.get("message_id")
-    return str(raw_id) if raw_id is not None else None
-
-
-def _content_block_start_skeleton(content: Any) -> dict[str, Any] | None:
-    """Return a minimal content-block-start payload for a delta/finish block."""
-    if not _is_record(content) or not isinstance(content.get("type"), str):
-        return None
-
-    block_type = content["type"]
-    skeleton: dict[str, Any] = {"type": block_type}
-    if block_type == "text":
-        skeleton["text"] = ""
-    elif block_type == "reasoning":
-        skeleton["reasoning"] = ""
-    elif block_type in ("tool_call", "tool_call_chunk"):
-        skeleton["type"] = "tool_call_chunk"
-        if isinstance(content.get("id"), str):
-            skeleton["id"] = content["id"]
-        if isinstance(content.get("name"), str):
-            skeleton["name"] = content["name"]
-        skeleton["args"] = ""
-    elif block_type in ("server_tool_call", "server_tool_call_chunk"):
-        skeleton["type"] = "server_tool_call_chunk"
-        if isinstance(content.get("id"), str):
-            skeleton["id"] = content["id"]
-        if isinstance(content.get("name"), str):
-            skeleton["name"] = content["name"]
-        skeleton["args"] = ""
-    return skeleton
-
-
-def _copy_event(
-    source: ProtocolEvent,
-    *,
-    method: str,
-    namespace: list[str],
-    data: Any,
-) -> ProtocolEvent:
-    params = {**source["params"], "namespace": namespace, "data": data}
-    return {"type": "event", "method": method, "params": params}
-
-
-def _message_repair_key(event: ProtocolEvent, run_id: str) -> str:
-    namespace_key = "\x1f".join(event["params"]["namespace"])
-    return f"{namespace_key}\x1e{run_id}"
-
-
-def _extract_tool_calls_from_values(data: Any) -> dict[str, dict[str, Any]]:
-    if not _is_record(data):
-        return {}
-    messages = data.get("messages")
-    if not isinstance(messages, list):
-        return {}
-    known: dict[str, dict[str, Any]] = {}
-    for message in messages:
-        if not _is_record(message):
-            continue
-        tool_calls = message.get("tool_calls")
-        if not isinstance(tool_calls, list):
-            continue
-        for tool_call in tool_calls:
-            if not _is_record(tool_call):
-                continue
-            tool_call_id = tool_call.get("id")
-            if not isinstance(tool_call_id, str):
-                continue
-            name = tool_call.get("name")
-            args = tool_call.get("args")
-            known[tool_call_id] = {
-                "tool_name": name if isinstance(name, str) else "",
-                "input": args if _is_record(args) else {},
-            }
-    return known
-
-
-class ValuesTransformer(StreamTransformer):
-    """Capture values events as a drainable stream of state snapshots.
-
-    Keeps `_latest` / `_interrupted` / `_interrupts` as scalar state
-    regardless of whether the log has a subscriber — so `run.output()`
-    and `run.interrupted` work without forcing the caller to iterate
-    `run.values`. Log pushes are silent no-ops when unsubscribed.
-
-    Native transformer — projection keys are exposed as direct
-    attributes on the run stream (e.g. `run.values`).
-
-    `scope` (inherited from `StreamTransformer`) is the namespace the
-    transformer captures values for. `()` matches the root graph;
-    subgraph mini-muxes pass their subgraph's namespace, so each
-    instance sees only its own level.
-    """
-
-    _native = True
-    required_stream_modes = ("values",)
-
-    def __init__(self, scope: tuple[str, ...] = ()) -> None:
-        super().__init__(scope)
-        self._log: EventLog[dict[str, Any]] = EventLog()
-        self._latest: dict[str, Any] | None = None
-        self._interrupted = False
-        self._interrupts: list[Any] = []
-
-    def init(self) -> dict[str, Any]:
-        return {"values": self._log}
-
-    @property
-    def error(self) -> BaseException | None:
-        """The error that ended the run, or `None` if it succeeded.
-
-        Set by the mux when it auto-fails the projection log.
-        """
-        return self._log._error
-
-    def process(self, event: ProtocolEvent) -> bool:
-        # Namespace filtering is handled by the mux via `scope_exact`.
-        if event["method"] != "values":
-            return True
-        params = event["params"]
-        self._latest = params["data"]
-        interrupts = params.get("interrupts", ())
-        if interrupts:
-            self._interrupted = True
-            self._interrupts.extend(interrupts)
-        self._log.push(params["data"])
-        return True
-
-
-class ToolLifecycleTransformer(StreamTransformer):
-    """Repair tool-start events needed for deterministic subagent discovery.
-
-    Some subagent frameworks expose a tool-caused subgraph lifecycle before
-    a LangChain tool callback has emitted the matching `tool-started`
-    frame. Core can infer the missing start from the latest values snapshot
-    (`messages[*].tool_calls`) and emit it before the lifecycle event leaves
-    the mux, keeping remote clients from guessing from values snapshots.
-    """
-
-    scope_exact = False
-    required_stream_modes = ("values", "tools", "lifecycle")
-
-    def __init__(self, scope: tuple[str, ...] = ()) -> None:
-        super().__init__(scope)
-        self._known_tool_calls: dict[str, dict[str, Any]] = {}
-        self._emitted_tool_starts: set[str] = set()
-        self._mux: StreamMux | None = None
-
-    def init(self) -> dict[str, Any]:
-        return {}
-
-    def _on_register(self, mux: StreamMux) -> None:
-        self._mux = mux
-
-    def process(self, event: ProtocolEvent) -> bool:
-        method = event["method"]
-        data = event["params"]["data"]
-        if method == "values":
-            self._known_tool_calls.update(_extract_tool_calls_from_values(data))
-            return True
-        if method == "tools" and _is_record(data):
-            if (
-                data.get("event") == "tool-started"
-                and isinstance(data.get("tool_call_id"), str)
-            ):
-                tool_call_id = cast("str", data["tool_call_id"])
-                if tool_call_id in self._emitted_tool_starts:
-                    return False
-                self._emitted_tool_starts.add(tool_call_id)
-            return True
-        if method == "lifecycle":
-            self._emit_missing_tool_started(event)
-        return True
-
-    def _emit_missing_tool_started(self, event: ProtocolEvent) -> None:
-        if self._mux is None:
-            return
-        data = event["params"]["data"]
-        if not _is_record(data) or data.get("event") != "started":
-            return
-        cause = data.get("cause")
-        if not _is_record(cause) or cause.get("type") != "toolCall":
-            return
-        tool_call_id = cause.get("tool_call_id")
-        if not isinstance(tool_call_id, str):
-            return
-        if tool_call_id in self._emitted_tool_starts:
-            return
-        known = self._known_tool_calls.get(tool_call_id)
-        if known is None:
-            return
-
-        self._emitted_tool_starts.add(tool_call_id)
-        namespace = event["params"]["namespace"]
-        self._mux.emit(
-            _copy_event(
-                event,
-                method="tools",
-                namespace=namespace[:-1],
-                data={
-                    "event": "tool-started",
-                    "tool_call_id": tool_call_id,
-                    "tool_name": known["tool_name"],
-                    "input": known["input"],
-                },
-            )
-        )
-
-
-class MessagesTransformer(StreamTransformer):
-    """Capture messages events as ChatModelStream objects.
-
-    The messages projection yields one `ChatModelStream` (or
-    `AsyncChatModelStream`) per LLM call. Consumers iterate
-    `run.messages` to get stream handles, then use each handle's typed
-    projections (`.text`, `.reasoning`, `.tool_calls`, `.usage`,
-    `.output`) for per-message content.
-
-    Two input shapes are handled (via `params["data"] = (payload,
-    metadata)` from `StreamMessagesHandler`):
-
-    1. Protocol event (dict with `"event"` key) — emitted by
-       `stream_v2()` / `astream_v2()` via the `on_stream_event`
-       callback. Routed to an existing `ChatModelStream` by
-       `metadata["run_id"]`. A `message-start` event creates a new
-       stream; `message-finish` closes it.
-    2. Whole `AIMessage` — emitted from `on_chain_end` when a node
-       returns a finalized message. Replayed as a synthetic protocol
-       event lifecycle via `message_to_events`, then the
-       already-complete stream is pushed to the log.
-
-    V1 `AIMessageChunk` tuples (from `on_llm_new_token`) are not
-    streamed into this projection: chat models that want to populate
-    `run.messages` with content-block streaming must use
-    `stream_v2()` / `astream_v2()`. Models called via the legacy
-    `stream()` method still surface their final `AIMessage` via
-    `on_chain_end` when a node returns it as state.
-
-    `scope` (inherited from `StreamTransformer`) is the namespace the
-    transformer captures messages for. `()` matches the root graph;
-    subgraph mini-muxes pass their subgraph's namespace, so each
-    instance sees only its own level.
-
-    Native transformer — the `messages` projection is exposed as a
-    direct attribute on the run stream.
-
-    `scope_exact = False`: matches events at the transformer's own
-    namespace **or** exactly one segment deeper (the chat-model /
-    node's own task ns). Mirrors JS's root-feed filter
-    (`namespaces=[[]], depth=1`) — root accepts depth-0 events plus
-    its own nodes' depth-1 tokens; subgraph mini-muxes accept their
-    own scope plus their internal nodes' tokens. Events deeper than
-    scope + 1 are dropped (the enclosing `SubgraphTransformer` has
-    already forwarded them to the matching child mini-mux).
-    """
-
-    _native = True
-    scope_exact = False
-    required_stream_modes = ("messages",)
-
-    def __init__(self, scope: tuple[str, ...] = ()) -> None:
-        super().__init__(scope)
-        self._log: EventLog[ChatModelStream] = EventLog()
-        # Correlate protocol events back to a ChatModelStream by run_id
-        # (attached to the event's metadata by StreamMessagesHandler).
-        self._by_run: dict[str, ChatModelStream] = {}
-        self._started_blocks: dict[str, set[int]] = {}
-        self._mux: StreamMux | None = None
-        self._pump_fn: Callable[[], bool] | None = None
-        self._apump_fn: Callable[[], Awaitable[bool]] | None = None
-
-    def init(self) -> dict[str, Any]:
-        return {"messages": self._log}
-
-    def _on_register(self, mux: StreamMux) -> None:
-        self._mux = mux
-
-    def _bind_pump(self, fn: Callable[[], bool]) -> None:
-        """Wire the sync pull callback. Called by GraphRunStream._wire_request_more."""
-        self._pump_fn = fn
-
-    def _bind_apump(self, fn: Callable[[], Awaitable[bool]]) -> None:
-        """Wire the async pull callback.
-
-        Called by `AsyncGraphRunStream._wire_arequest_more` so each
-        `AsyncChatModelStream` this transformer creates can drive the
-        shared graph pump from its projection cursors.
-        """
-        self._apump_fn = fn
-
-    def _make_stream(
-        self,
-        *,
-        namespace: list[str],
-        node: str | None,
-        message_id: str | None,
-    ) -> ChatModelStream:
-        """Create a ChatModelStream (sync) or AsyncChatModelStream (async).
-
-        Wires whichever pump is bound. Prefers the async pump so nested
-        iteration under `AsyncGraphRunStream` drives the graph forward
-        without a background task. The unwired fallback (no pump bound)
-        is used by unit tests that dispatch events manually.
-        """
-        if self._apump_fn is not None:
-            astream = AsyncChatModelStream(
-                namespace=namespace,
-                node=node,
-                message_id=message_id,
-            )
-            astream.set_arequest_more(self._apump_fn)
-            return astream
-        if self._pump_fn is not None:
-            stream: ChatModelStream = ChatModelStream(
-                namespace=namespace,
-                node=node,
-                message_id=message_id,
-            )
-            stream.set_request_more(self._pump_fn)
-            return stream
-        return AsyncChatModelStream(
-            namespace=namespace,
-            node=node,
-            message_id=message_id,
-        )
-
-    def process(self, event: ProtocolEvent) -> bool:
-        if event["method"] != "messages":
-            return True
-        params = event["params"]
-        # Accept events at our scope or exactly one segment deeper
-        # (the chat-model / node's own task ns). Deeper events belong
-        # to a subgraph and are routed by `SubgraphTransformer`.
-        ns = tuple(params["namespace"])
-        depth = len(self.scope)
-        if ns[:depth] != self.scope:
-            return True
-
-        raw_data = params["data"]
-        metadata: dict[str, Any] = {}
-        if isinstance(raw_data, tuple) and len(raw_data) == 2:
-            payload, raw_metadata = raw_data
-            metadata = raw_metadata if isinstance(raw_metadata, dict) else {}
-        else:
-            payload = raw_data
-        node = params.get("node")
-        if not isinstance(node, str):
-            node = metadata.get("langgraph_node")
-            if not isinstance(node, str):
-                node = None
-        raw_run_id = params.get("run_id", metadata.get("run_id"))
-        run_id = str(raw_run_id) if raw_run_id is not None else ""
-
-        if isinstance(payload, dict) and "event" in payload:
-            self._repair_content_block_lifecycle(
-                event, cast("MessagesData", payload), run_id=run_id
-            )
-            if len(ns) > depth + 1:
-                return True
-            self._route_protocol_event(
-                cast("MessagesData", payload), run_id=run_id, node=node
-            )
-        elif isinstance(payload, BaseMessage) and not isinstance(
-            payload, AIMessageChunk
-        ):
-            self._route_whole_message(payload, node=node)
-        # Legacy AIMessageChunk tuples (from on_llm_new_token) are ignored;
-        # v1 streaming callers must switch to stream_v2() to populate this
-        # projection.
-
-        return True
-
-    def _route_protocol_event(
-        self,
-        event: MessagesData,
-        *,
-        run_id: str,
-        node: str | None,
-    ) -> None:
-        stream_event = _to_chat_model_stream_event(event)
-        event_type = event.get("event")
-        if event_type == "message-start":
-            message_id = _message_event_id(event)
-            stream = self._make_stream(
-                namespace=list(self.scope),
-                node=node,
-                message_id=message_id,
-            )
-            self._by_run[run_id or message_id or ""] = stream
-            self._log.push(stream)
-            stream.dispatch(stream_event)
-        elif run_id in self._by_run:
-            stream = self._by_run[run_id]
-            stream.dispatch(stream_event)
-            if event_type == "message-finish":
-                del self._by_run[run_id]
-
-    def _repair_content_block_lifecycle(
-        self,
-        source: ProtocolEvent,
-        event: MessagesData,
-        *,
-        run_id: str,
-    ) -> None:
-        if self._mux is None:
-            return
-        event_type = event.get("event")
-        key = _message_repair_key(source, run_id)
-        if event_type == "message-start":
-            self._started_blocks[key] = set()
-            return
-        if event_type == "content-block-start":
-            index = event.get("index")
-            if isinstance(index, int):
-                self._started_blocks.setdefault(key, set()).add(index)
-            return
-        if event_type in ("content-block-delta", "content-block-finish"):
-            index = event.get("index")
-            if not isinstance(index, int):
-                return
-            started = self._started_blocks.setdefault(key, set())
-            if index in started:
-                return
-            skeleton = _content_block_start_skeleton(event.get("content"))
-            if skeleton is None:
-                return
-            started.add(index)
-            self._mux.emit(
-                _copy_event(
-                    source,
-                    method="messages",
-                    namespace=list(source["params"]["namespace"]),
-                    data={
-                        "event": "content-block-start",
-                        "index": index,
-                        "content": skeleton,
-                    },
-                )
-            )
-        elif event_type == "message-finish":
-            self._started_blocks.pop(key, None)
-
-    def _route_whole_message(self, message: BaseMessage, *, node: str | None) -> None:
-        stream = self._make_stream(
-            namespace=list(self.scope),
-            node=node,
-            message_id=message.id,
-        )
-        for evt in message_to_events(message, message_id=message.id):
-            stream.dispatch(evt)
-        self._log.push(stream)
-
-    def finalize(self) -> None:
-        """Clear any routing state — streams close themselves via `message-finish`."""
-        self._by_run.clear()
-        self._started_blocks.clear()
-
-    def fail(self, err: BaseException) -> None:
-        """Propagate run error to any streams still open when the graph fails."""
-        for stream in list(self._by_run.values()):
-            stream.fail(err)
-        self._by_run.clear()
-        self._started_blocks.clear()
-
-
-class SubgraphRunStream(BaseRunStream):
-    """Scoped view of a single nested subgraph execution.
-
-    Yielded on `run.subgraphs` (or `parent.subgraphs` for grandchildren)
-    when a nested `Pregel` spawns. Wraps a mini-`StreamMux` built with
-    the same transformer factories as the root mux, so `.values`,
-    `.messages`, `.subgraphs` are populated by the standard
-    transformers scoped to this handle's namespace — no duplicated
-    routing logic. The mini-mux borrows the root's pump via
-    `make_child`'s pump inheritance, so any cursor on a subagent
-    projection drives the whole run forward.
-
-    Lifecycle fields update in place as events arrive:
-
-    - `path`: the namespace tuple — stable for the life of the handle.
-    - `graph_name` / `cause`: set once from the `started` payload.
-      `cause` is populated by product-specific stream transformers
-      (see `LifecycleCause` in the protocol definition); pregel itself
-      emits no `cause`, so it may be `None` for subgraphs not covered
-      by a product transformer.
-    - `status`: advances `started` → `running` → `completed` /
-      `failed` / `interrupted`.
-    - `error` / `checkpoint`: set on the terminal event when present.
-
-    `.output` is a snapshot of the latest values seen at this
-    namespace — it doesn't drive the pump (unlike root's
-    `GraphRunStream.output`), because advancing a subgraph to
-    completion is only meaningful as part of advancing the whole run.
-    """
-
-    def __init__(
-        self,
-        path: tuple[str, ...],
-        mux: StreamMux,
-        *,
-        graph_name: str | None = None,
-        cause: LifecycleCause | None = None,
-    ) -> None:
-        super().__init__(mux)
-        self.path: tuple[str, ...] = path
-        self.graph_name: str | None = graph_name
-        self.cause: LifecycleCause | None = cause
-        self.status: SubgraphStatus = "started"
-        self.error: str | None = None
-        self.checkpoint: CheckpointRef | None = None
-
-    @property
-    def output(self) -> dict[str, Any] | None:
-        """Latest values snapshot at this namespace, or `None`.
-
-        Snapshot-only — iterating other projections or the root's
-        `.output` is what drives the pump.
-        """
-        values_t = self._mux.transformer_by_key("values")
-        if isinstance(values_t, ValuesTransformer):
-            return values_t._latest
-        return None
-
-
-class SubgraphTransformer(StreamTransformer):
-    """Discover subgraphs and route events into per-subgraph mini-muxes.
-
-    Thin state-machine + dispatcher. At its own `scope` (inherited
-    from `StreamTransformer`, determined by the enclosing mux), it
-    watches for `lifecycle` events at exactly one level deeper to
-    discover direct children. Each discovered child gets its own
-    `SubgraphRunStream` backed by a mini-`StreamMux` — built via
-    `parent_mux.make_child(path)`, so the same factory list produces
-    fresh transformer instances at the child's scope.
-
-    Every incoming event that falls under one of the direct children
-    (ns starts with a child's `path`) is forwarded into that child's
-    mini-mux via `push`. The standard transformers in that mini-mux
-    (`ValuesTransformer`, `MessagesTransformer`, and another
-    `SubgraphTransformer` for grandchildren) handle the rest. No
-    duplicated routing or assembly logic.
-
-    Lifecycle state for each handle (running / completed / failed /
-    interrupted) is updated in place as events fire. On terminal
-    events, the handle's mini-mux is closed so any subscribed cursors
-    unblock. `finalize` / `fail` handle dangling handles left mid-run.
-
-    Native transformer — `subgraphs` exposes the direct-children log.
-
-    `scope_exact = False`: this transformer sees events at any
-    namespace, because it forwards out-of-scope events to the matching
-    direct-child mini-mux.
-    """
-
-    _native = True
-    scope_exact = False
-    required_stream_modes = ("lifecycle",)
-
-    def __init__(self, scope: tuple[str, ...] = ()) -> None:
-        super().__init__(scope)
-        self._root_log: EventLog[SubgraphRunStream] = EventLog()
-        # Direct children only (namespace = scope + one segment).
-        self._by_ns: dict[tuple[str, ...], SubgraphRunStream] = {}
-        self._mux: StreamMux | None = None
-
-    def init(self) -> dict[str, Any]:
-        return {"subgraphs": self._root_log}
-
-    def _on_register(self, mux: StreamMux) -> None:
-        """Capture the enclosing mux so we can build child mini-muxes."""
-        self._mux = mux
-
-    def process(self, event: ProtocolEvent) -> bool:
-        ns = tuple(event["params"]["namespace"])
-        method = event["method"]
-        depth = len(self.scope)
-
-        # 1. On `started` for a direct child (ns depth = mine + 1 and
-        #    ns prefix matches mine), register the handle.
-        if method == "lifecycle" and len(ns) == depth + 1 and ns[:-1] == self.scope:
-            data = cast(LifecycleData, event["params"]["data"])
-            if data.get("event") == "started":
-                self._on_started(ns, data)
-
-        # 2. Forward the event to the matching direct-child mini-mux
-        #    before the status-change step below so that terminal events
-        #    reach the child's log and grandchild transformers *before*
-        #    the child's mini-mux is closed. Prefix-match: ns must start
-        #    with some child's path.
-        direct_child_ns = ns[: depth + 1] if len(ns) > depth else None
-        if direct_child_ns is not None and direct_child_ns in self._by_ns:
-            self._by_ns[direct_child_ns]._mux.push(event)
-
-        # 3. Status change for a direct child (ns = child's path, method
-        #    = lifecycle). Update handle fields, close mini-mux on
-        #    terminal.
-        if (
-            method == "lifecycle"
-            and ns in self._by_ns
-            and len(ns) == depth + 1
-            and ns[:-1] == self.scope
-        ):
-            data = cast(LifecycleData, event["params"]["data"])
-            event_type = data.get("event")
-            if event_type in ("running", "completed", "failed", "interrupted"):
-                self._on_status_change(ns, event_type, data)
-
-        return True
-
-    def _on_started(self, ns: tuple[str, ...], data: LifecycleData) -> None:
-        if ns in self._by_ns:
-            # Duplicate started — ignore.
-            return
-        # `_on_register` is called by the mux during registration, which
-        # happens before any event can be dispatched — so this should
-        # always be set by the time we process an event.
-        assert self._mux is not None, (
-            "SubgraphTransformer processed an event before _on_register; "
-            "transformer registration ordering is broken."
-        )
-        child_mux = self._mux.make_child(ns)
-        handle = SubgraphRunStream(
-            path=ns,
-            mux=child_mux,
-            graph_name=data.get("graph_name"),
-            cause=data.get("cause"),
-        )
-        self._by_ns[ns] = handle
-        self._root_log.push(handle)
-
-    def _on_status_change(
-        self,
-        ns: tuple[str, ...],
-        event_type: SubgraphStatus,
-        data: LifecycleData,
-    ) -> None:
-        handle = self._by_ns[ns]
-        handle.status = event_type
-        err = data.get("error")
-        if err is not None:
-            handle.error = err
-        checkpoint = data.get("checkpoint")
-        if checkpoint is not None:
-            handle.checkpoint = checkpoint
-        if event_type in _TERMINAL_STATUSES:
-            self._close_handle_mux(handle)
-
-    @staticmethod
-    def _close_handle_mux(handle: SubgraphRunStream) -> None:
-        # Idempotent close — mux.close() runs finalize on its transformers
-        # (which cascades through grandchildren) and closes projection logs.
-        if not handle._mux._events._closed:
-            try:
-                handle._mux.close()
-            except Exception:
-                logger.warning(
-                    "Error closing subgraph mini-mux at %s; subscribers "
-                    "may not see a clean close.",
-                    handle.path,
-                    exc_info=True,
-                )
-
-    def finalize(self) -> None:
-        """Transition any still-open direct children to `completed`."""
-        for handle in self._by_ns.values():
-            if handle.status not in _TERMINAL_STATUSES:
-                handle.status = "completed"
-            self._close_handle_mux(handle)
-
-    def fail(self, err: BaseException) -> None:
-        """Transition any still-open direct children to `failed` / `interrupted`."""
-        is_interrupt = isinstance(err, GraphInterrupt)
-        terminal: SubgraphStatus = "interrupted" if is_interrupt else "failed"
-        error_str = None if is_interrupt else str(err)
-        for handle in self._by_ns.values():
-            if handle.status not in _TERMINAL_STATUSES:
-                handle.status = terminal
-                if error_str is not None and handle.error is None:
-                    handle.error = error_str
-            if not handle._mux._events._closed:
-                try:
-                    handle._mux.fail(err)
-                except Exception:
-                    logger.warning(
-                        "Error failing subgraph mini-mux at %s; subscribers "
-                        "may not see the terminal error.",
-                        handle.path,
-                        exc_info=True,
-                    )

libs/langgraph/langgraph/types.py

@@ -116,15 +116,7 @@ def ensure_valid_checkpointer(checkpointer: Checkpointer) -> Checkpointer:
 
 
 StreamMode = Literal[
-    "values",
-    "updates",
-    "checkpoints",
-    "tasks",
-    "debug",
-    "messages",
-    "custom",
-    "lifecycle",
-    "tools",
+    "values", "updates", "checkpoints", "tasks", "debug", "messages", "custom"
 ]
 """How the stream method should emit outputs.
 
@@ -137,8 +129,6 @@ StreamMode = Literal[
 - `"checkpoints"`: Emit an event when a checkpoint is created, in the same format as returned by `get_state()`.
 - `"tasks"`: Emit events when tasks start and finish, including their results and errors.
 - `"debug"`: Emit `"checkpoints"` and `"tasks"` events for debugging purposes.
-- `"lifecycle"`: Emit subgraph lifecycle events (`started`, `running`, `completed`, `failed`, `interrupted`) with payloads matching `LifecycleData`.
-- `"tools"`: Emit tool-call lifecycle events (`tool-started`, `tool-output-delta`, `tool-finished`, `tool-error`) keyed by `tool_call_id`.
 """
 
 StreamWriter = Callable[[Any], None]

libs/langgraph/pyproject.toml

@@ -24,7 +24,7 @@ classifiers = [
     'Programming Language :: Python :: 3.13',
 ]
 dependencies = [
-    "langchain-core>=1.3.2",
+    "langchain-core==1.3.0a2",
     "langgraph-checkpoint>=2.1.0,<5.0.0",
     "langgraph-sdk>=0.3.0,<0.4.0",
     "langgraph-prebuilt>=1.0.9,<1.1.0",

libs/langgraph/tests/test_channels.py

@@ -117,3 +117,609 @@ def test_untracked_value() -> None:
     new_channel = UntrackedValue(dict).from_checkpoint(checkpoint)
     with pytest.raises(EmptyChannelError):
         new_channel.get()
+
+
+def test_delta_channel_basic_two_steps() -> None:
+    from langchain_core.messages import AIMessage, HumanMessage
+    from langgraph.checkpoint.base import DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    ch = DeltaChannel(add_messages).from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+
+    # Step 1: one message added
+    ch.update([HumanMessage(content="hi", id="h1")])
+    d1 = ch.checkpoint()
+    assert isinstance(d1, DeltaValue)
+    assert len(d1.delta) == 1
+    assert d1.prev_checkpoint_id is None  # first ever step
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+
+    # Step 2: another message
+    ch.update([AIMessage(content="hello", id="a1")])
+    d2 = ch.checkpoint()
+    assert d2.prev_checkpoint_id == "cid1"
+    assert len(d2.delta) == 1
+    ch.after_checkpoint("v2")
+
+    # Full accumulated value is preserved in memory
+    assert len(ch.get()) == 2
+    assert ch.get()[0].content == "hi"
+    assert ch.get()[1].content == "hello"
+
+
+def test_delta_channel_after_checkpoint_no_op_when_unchanged() -> None:
+    from langchain_core.messages import HumanMessage
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    ch = DeltaChannel(add_messages).from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+    ch.update([HumanMessage(content="hi", id="h1")])
+    ch.after_checkpoint("v1")
+
+    # Same version: no-op
+    ch.after_checkpoint("v1")
+    assert ch._base_version == "v1"
+    assert ch._pending == []
+
+
+def test_delta_channel_from_checkpoint_chain() -> None:
+    from langchain_core.messages import AIMessage, HumanMessage
+    from langgraph.checkpoint.base import DeltaChainValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    spec = DeltaChannel(add_messages)
+    chain = DeltaChainValue(
+        base=None,
+        deltas=[
+            [HumanMessage(content="hi", id="h1")],
+            [AIMessage(content="hello", id="a1")],
+            [HumanMessage(content="bye", id="h2")],
+        ],
+    )
+    ch = spec.from_checkpoint(chain)
+    msgs = ch.get()
+    assert len(msgs) == 3
+    assert msgs[0].content == "hi"
+    assert msgs[1].content == "hello"
+    assert msgs[2].content == "bye"
+
+
+def test_delta_channel_from_checkpoint_backwards_compat() -> None:
+    from langchain_core.messages import HumanMessage
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    # Old BinaryOperatorAggregate checkpoint: plain list
+    spec = DeltaChannel(add_messages)
+    old_value = [HumanMessage(content="old", id="h1")]
+    ch = spec.from_checkpoint(old_value)
+    assert ch.get() == old_value
+
+
+def test_delta_channel_overwrite_resets_chain() -> None:
+    from langchain_core.messages import HumanMessage
+    from langgraph.checkpoint.base import DeltaChainValue, DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+    from langgraph.types import Overwrite
+
+    ch = DeltaChannel(add_messages).from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+    ch.update([HumanMessage(content="old", id="h1")])
+    ch.after_checkpoint("v1")
+
+    # Overwrite should create a root blob (prev_checkpoint_id=None)
+    ch.update([Overwrite([HumanMessage(content="new", id="h2")])])
+    d = ch.checkpoint()
+    assert isinstance(d, DeltaValue)
+    assert d.prev_checkpoint_id is None  # chain root
+    assert len(d.delta) == 1
+    assert len(d.delta[0]) == 1
+    assert d.delta[0][0].content == "new"
+
+    spec = DeltaChannel(add_messages)
+    replayed = spec.from_checkpoint(DeltaChainValue(base=None, deltas=[d.delta]))
+    assert replayed.get()[0].content == "new"
+
+
+def test_delta_channel_assembly_fallback_via_get_tuple() -> None:
+    """Materialization falls back to get_tuple for savers without get_channel_blob."""
+    from langgraph.checkpoint.base import (
+        BaseCheckpointSaver,
+        CheckpointHydrationPlan,
+        CheckpointTuple,
+        DeltaChainValue,
+        DeltaValue,
+        IncrementalChannelSpec,
+        empty_checkpoint,
+    )
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    msg1 = {"type": "human", "content": "hello"}
+    msg2 = {"type": "ai", "content": "world"}
+
+    cp1 = empty_checkpoint()
+    cp1["id"] = "cp1"
+    cp1["channel_values"]["messages"] = [msg1]
+
+    cp2 = empty_checkpoint()
+    cp2["id"] = "cp2"
+    cp2["channel_values"]["messages"] = DeltaValue(
+        delta=[msg2], prev_checkpoint_id="cp1"
+    )
+
+    class TestSaver(BaseCheckpointSaver[str]):
+        def get_tuple(self, config):
+            return CheckpointTuple(
+                config={
+                    "configurable": {
+                        "thread_id": "t1",
+                        "checkpoint_ns": "",
+                        "checkpoint_id": "cp1",
+                    }
+                },
+                checkpoint=cp1,
+                metadata={},
+                parent_config=None,
+                pending_writes=[],
+            )
+
+        def list(self, config, *, filter=None, before=None, limit=None):
+            raise NotImplementedError
+
+        def put(self, config, checkpoint, metadata, new_versions):
+            raise NotImplementedError
+
+    saver = TestSaver()
+
+    config = {"configurable": {"thread_id": "t1", "checkpoint_ns": ""}}
+    materialized = saver.materialize_checkpoint(
+        config,
+        cp2,
+        CheckpointHydrationPlan(
+            channels=(IncrementalChannelSpec(name="messages", kind="delta"),)
+        ),
+    )
+
+    chain = materialized["channel_values"]["messages"]
+    assert isinstance(chain, DeltaChainValue)
+    assert chain.base == [msg1]
+    assert chain.deltas == [[msg2]]
+
+    from langchain_core.messages import AIMessage, HumanMessage
+
+    spec = DeltaChannel(add_messages)
+    ch = spec.from_checkpoint(chain)
+    result = ch.get()
+    assert len(result) == 2
+    assert isinstance(result[0], HumanMessage) and result[0].content == "hello"
+    assert isinstance(result[1], AIMessage) and result[1].content == "world"
+
+
+def test_delta_channel_remove_message_delta_and_replay() -> None:
+    """RemoveMessage stored in a delta must round-trip correctly through the chain."""
+    from langchain_core.messages import AIMessage, HumanMessage, RemoveMessage
+    from langgraph.checkpoint.base import DeltaChainValue, DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    spec = DeltaChannel(add_messages)
+    ch = spec.from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+
+    # Step 1: add two messages
+    ch.update([HumanMessage(content="hi", id="h1")])
+    ch.update([AIMessage(content="hello", id="a1")])
+    d1 = ch.checkpoint()
+    assert isinstance(d1, DeltaValue)
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+    assert ch.get() == [
+        HumanMessage(content="hi", id="h1"),
+        AIMessage(content="hello", id="a1"),
+    ]
+
+    # Step 2: remove the AI message
+    ch.update([RemoveMessage(id="a1")])
+    d2 = ch.checkpoint()
+    assert isinstance(d2, DeltaValue)
+    assert d2.prev_checkpoint_id == "cid1"
+    assert any(isinstance(w, RemoveMessage) for w in d2.delta)
+    ch.after_checkpoint("v2", checkpoint_id="cid2")
+    assert ch.get() == [HumanMessage(content="hi", id="h1")]
+
+    # Replay the full chain from scratch — must reproduce the post-remove state
+    chain = DeltaChainValue(base=None, deltas=[d1.delta, d2.delta])
+    ch2 = spec.from_checkpoint(chain)
+    assert ch2.get() == [HumanMessage(content="hi", id="h1")]
+
+
+def test_delta_channel_update_by_id_delta_and_replay() -> None:
+    """Updating a message by ID stored in a delta must round-trip correctly."""
+    from langchain_core.messages import HumanMessage
+    from langgraph.checkpoint.base import DeltaChainValue, DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    spec = DeltaChannel(add_messages)
+    ch = spec.from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+
+    # Step 1: add a message
+    ch.update([HumanMessage(content="original", id="h1")])
+    d1 = ch.checkpoint()
+    assert isinstance(d1, DeltaValue)
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+
+    # Step 2: update the same message by ID
+    ch.update([HumanMessage(content="updated", id="h1")])
+    d2 = ch.checkpoint()
+    assert isinstance(d2, DeltaValue)
+    assert d2.prev_checkpoint_id == "cid1"
+    ch.after_checkpoint("v2", checkpoint_id="cid2")
+    assert ch.get() == [HumanMessage(content="updated", id="h1")]
+
+    # Replay the full chain — must produce the updated message, not the original
+    chain = DeltaChainValue(base=None, deltas=[d1.delta, d2.delta])
+    ch2 = spec.from_checkpoint(chain)
+    assert len(ch2.get()) == 1
+    assert ch2.get()[0].content == "updated"
+
+
+def test_delta_channel_snapshot_every_emits_plain_list() -> None:
+    """snapshot_every=N causes a plain-list snapshot after N steps; next deltas chain to it."""
+    from langchain_core.messages import HumanMessage
+    from langgraph.checkpoint.base import DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    SNAP = 3
+    spec = DeltaChannel(add_messages, snapshot_every=SNAP)
+    ch = spec.from_checkpoint(MISSING)
+    # First after_checkpoint anchors _base_version without counting a step.
+    ch.after_checkpoint("v0", checkpoint_id="cid0")
+
+    # Steps 1..SNAP: each should stay as DeltaValue; counter increments each step.
+    for i in range(1, SNAP + 1):
+        ch.update([HumanMessage(content=f"m{i}", id=f"h{i}")])
+        ckpt = ch.checkpoint()
+        assert isinstance(ckpt, DeltaValue), f"expected DeltaValue at step {i}"
+        ch.after_checkpoint(f"v{i}", checkpoint_id=f"cid{i}")
+
+    # Step SNAP+1: _steps_since_snapshot == SNAP → snapshot fires
+    ch.update([HumanMessage(content="snap", id="hsnap")])
+    snap = ch.checkpoint()
+    assert isinstance(snap, list), "expected plain-list snapshot at snapshot_every step"
+    assert len(snap) == SNAP + 1
+
+    # After snapshot, counter resets — next step is DeltaValue again
+    ch.after_checkpoint("vsnap", checkpoint_id="cidsnap")
+    ch.update([HumanMessage(content="post", id="hpost")])
+    post = ch.checkpoint()
+    assert isinstance(post, DeltaValue)
+    assert post.prev_checkpoint_id == "cidsnap"
+
+
+def test_delta_channel_snapshot_every_end_to_end() -> None:
+    """Graph with snapshot_every: get_state returns correct accumulated value after snapshot."""
+    from typing import Annotated
+
+    from langchain_core.messages import AIMessage, HumanMessage
+    from langgraph.checkpoint.memory import InMemorySaver
+    from typing_extensions import TypedDict
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph import START, StateGraph
+    from langgraph.graph.message import add_messages
+
+    class State(TypedDict):
+        messages: Annotated[list, DeltaChannel(add_messages, snapshot_every=2)]
+
+    counter = {"n": 0}
+
+    def respond(state: State) -> dict:
+        counter["n"] += 1
+        return {
+            "messages": [
+                AIMessage(content=f"ai-{counter['n']}", id=f"ai-{counter['n']}")
+            ]
+        }
+
+    builder = StateGraph(State)
+    builder.add_node("respond", respond)
+    builder.add_edge(START, "respond")
+    graph = builder.compile(checkpointer=InMemorySaver())
+    config = {"configurable": {"thread_id": "snap-test"}}
+
+    # Run 5 turns — snapshot fires after 2 steps, then again after 2 more
+    for i in range(5):
+        graph.invoke({"messages": [HumanMessage(content=f"h{i}", id=f"h{i}")]}, config)
+
+    state = graph.get_state(config)
+    msgs = state.values["messages"]
+    # 5 human + 5 AI = 10 total
+    assert len(msgs) == 10, f"expected 10 messages, got {len(msgs)}: {msgs}"
+
+
+def test_delta_channel_dict_reducer_overwrite_preserves_mapping() -> None:
+    """Overwrite should preserve dict values instead of coercing them to keys."""
+    from langgraph.checkpoint.base import DeltaChainValue, DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.types import Overwrite
+
+    def merge_dicts(left: dict, right: dict) -> dict:
+        return {**left, **right}
+
+    ch = DeltaChannel(merge_dicts, dict).from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+    ch.update([{"a": 1}])
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+
+    ch.update([Overwrite({"b": 2})])
+    d = ch.checkpoint()
+    assert isinstance(d, DeltaValue)
+    assert d.prev_checkpoint_id is None
+    assert d.delta == [{"b": 2}]
+    assert ch.get() == {"b": 2}
+
+    spec = DeltaChannel(merge_dicts, dict)
+    replayed = spec.from_checkpoint(DeltaChainValue(base=None, deltas=[d.delta]))
+    assert replayed.get() == {"b": 2}
+
+
+def test_delta_channel_dict_snapshot_every_round_trip() -> None:
+    """Full snapshots should preserve non-list reducers across reload."""
+    from langgraph.checkpoint.base import DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+
+    def merge_dicts(left: dict, right: dict) -> dict:
+        return {**left, **right}
+
+    ch = DeltaChannel(merge_dicts, dict, snapshot_every=1).from_checkpoint(MISSING)
+    ch.after_checkpoint("v0", checkpoint_id="cid0")
+
+    ch.update([{"a": 1}])
+    first = ch.checkpoint()
+    assert isinstance(first, DeltaValue)
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+
+    ch.update([{"b": 2}])
+    snap = ch.checkpoint()
+    assert isinstance(snap, dict)
+    assert snap == {"a": 1, "b": 2}
+
+    rehydrated = DeltaChannel(merge_dicts, dict, snapshot_every=1).from_checkpoint(snap)
+    assert rehydrated.get() == {"a": 1, "b": 2}
+
+
+def test_delta_channel_assembly_fast_path_returns_delta_value() -> None:
+    """get_channel_blob returning a DeltaValue continues chain traversal (fast-path)."""
+    from langgraph.checkpoint.base import (
+        BaseCheckpointSaver,
+        CheckpointHydrationPlan,
+        DeltaChainValue,
+        DeltaValue,
+        IncrementalChannelSpec,
+        empty_checkpoint,
+    )
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph.message import add_messages
+
+    msg1 = {"type": "human", "content": "one"}
+    msg2 = {"type": "ai", "content": "two"}
+    msg3 = {"type": "human", "content": "three"}
+
+    # cp3 → cp2 (DeltaValue) → cp1 (base list)
+    dv_cp2 = DeltaValue(delta=[msg2], prev_checkpoint_id="cp1")
+    cp3 = empty_checkpoint()
+    cp3["id"] = "cp3"
+    cp3["channel_values"]["messages"] = DeltaValue(
+        delta=[msg3], prev_checkpoint_id="cp2"
+    )
+
+    class TestSaver(BaseCheckpointSaver[str]):
+        def get_tuple(self, config):
+            raise NotImplementedError
+
+        def list(self, config, *, filter=None, before=None, limit=None):
+            raise NotImplementedError
+
+        def put(self, config, checkpoint, metadata, new_versions):
+            raise NotImplementedError
+
+        def get_channel_blob(self, thread_id, checkpoint_ns, checkpoint_id, channel):
+            if checkpoint_id == "cp2":
+                return dv_cp2
+            if checkpoint_id == "cp1":
+                return [msg1]
+            return NotImplemented
+
+    saver = TestSaver()
+
+    config = {"configurable": {"thread_id": "t1", "checkpoint_ns": ""}}
+    materialized = saver.materialize_checkpoint(
+        config,
+        cp3,
+        CheckpointHydrationPlan(
+            channels=(IncrementalChannelSpec(name="messages", kind="delta"),)
+        ),
+    )
+
+    chain = materialized["channel_values"]["messages"]
+    assert isinstance(chain, DeltaChainValue)
+    assert chain.base == [msg1]
+    assert chain.deltas == [[msg2], [msg3]]
+
+    spec = DeltaChannel(add_messages)
+    ch = spec.from_checkpoint(chain)
+    # add_messages converts dicts to message objects; check by type and content
+
+    result = ch.get()
+    assert len(result) == 3
+    assert result[0].content == "one"
+    assert result[1].content == "two"
+    assert result[2].content == "three"
+
+
+def test_delta_channel_dict_reducer_fresh_channel() -> None:
+    """DeltaChannel with a dict reducer starts as empty dict on MISSING checkpoint."""
+    from langgraph.channels.delta import DeltaChannel
+
+    def merge_dicts(left: dict, right: dict) -> dict:
+        return {**left, **right}
+
+    ch = DeltaChannel(merge_dicts, dict).from_checkpoint(MISSING)
+    # Should be available (not raise EmptyChannelError) and start empty
+    assert ch.is_available()
+    assert ch.get() == {}
+
+
+def test_delta_channel_dict_reducer_basic_updates() -> None:
+    """DeltaChannel with a dict reducer accumulates key/value pairs across steps."""
+    from langgraph.checkpoint.base import DeltaValue
+
+    from langgraph.channels.delta import DeltaChannel
+
+    def merge_dicts(left: dict, right: dict) -> dict:
+        return {**left, **right}
+
+    ch = DeltaChannel(merge_dicts, dict).from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+
+    ch.update([{"a": 1}])
+    d1 = ch.checkpoint()
+    assert isinstance(d1, DeltaValue)
+    assert d1.delta == [{"a": 1}]
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+
+    ch.update([{"b": 2}])
+    d2 = ch.checkpoint()
+    assert d2.delta == [{"b": 2}]
+    assert d2.prev_checkpoint_id == "cid1"
+    ch.after_checkpoint("v2")
+
+    assert ch.get() == {"a": 1, "b": 2}
+
+
+def test_delta_channel_dict_reducer_chain_reconstruction() -> None:
+    """DeltaChainValue replays correctly through a dict merge reducer."""
+    from langgraph.checkpoint.base import DeltaChainValue
+
+    from langgraph.channels.delta import DeltaChannel
+
+    def merge_dicts(left: dict, right: dict) -> dict:
+        return {**left, **right}
+
+    spec = DeltaChannel(merge_dicts, dict)
+    chain = DeltaChainValue(
+        base={"a": 1},
+        deltas=[[{"b": 2}], [{"c": 3}]],
+    )
+    ch = spec.from_checkpoint(chain)
+    assert ch.get() == {"a": 1, "b": 2, "c": 3}
+    assert ch._steps_since_snapshot == 2
+
+
+def test_delta_channel_dict_reducer_with_deletions() -> None:
+    """Dict reducer that treats None values as deletions works end-to-end (deepagents pattern)."""
+    from langgraph.checkpoint.base import DeltaChainValue
+
+    from langgraph.channels.delta import DeltaChannel
+
+    def merge_files(left: dict | None, right: dict) -> dict:
+        if left is None:
+            return {k: v for k, v in right.items() if v is not None}
+        result = {**left}
+        for k, v in right.items():
+            if v is None:
+                result.pop(k, None)
+            else:
+                result[k] = v
+        return result
+
+    ch = DeltaChannel(merge_files, dict).from_checkpoint(MISSING)
+    ch.after_checkpoint(None)
+
+    ch.update([{"file1.py": "content1", "file2.py": "content2"}])
+    ch.after_checkpoint("v1", checkpoint_id="cid1")
+
+    # Delete file1, add file3
+    ch.update([{"file1.py": None, "file3.py": "content3"}])
+    ch.after_checkpoint("v2", checkpoint_id="cid2")
+
+    assert ch.get() == {"file2.py": "content2", "file3.py": "content3"}
+
+    # Confirm chain reconstruction produces the same result
+    chain = DeltaChainValue(
+        base={},
+        deltas=[
+            [{"file1.py": "content1", "file2.py": "content2"}],
+            [{"file1.py": None, "file3.py": "content3"}],
+        ],
+    )
+    spec = DeltaChannel(merge_files, dict)
+    ch2 = spec.from_checkpoint(chain)
+    assert ch2.get() == {"file2.py": "content2", "file3.py": "content3"}
+
+
+def test_delta_channel_assembly_broken_chain_logs_warning() -> None:
+    """If a prev_checkpoint_id points to a missing checkpoint, log a warning and use partial chain."""
+    from langgraph.checkpoint.base import (
+        BaseCheckpointSaver,
+        CheckpointHydrationPlan,
+        DeltaValue,
+        IncrementalChannelSpec,
+        empty_checkpoint,
+    )
+
+    cp = empty_checkpoint()
+    cp["id"] = "cp2"
+    cp["channel_values"]["messages"] = DeltaValue(
+        delta=["msg2"], prev_checkpoint_id="cp-missing"
+    )
+
+    class TestSaver(BaseCheckpointSaver[str]):
+        def get_tuple(self, config):
+            return None
+
+        def list(self, config, *, filter=None, before=None, limit=None):
+            raise NotImplementedError
+
+        def put(self, config, checkpoint, metadata, new_versions):
+            raise NotImplementedError
+
+    saver = TestSaver()
+
+    config = {"configurable": {"thread_id": "t1", "checkpoint_ns": ""}}
+
+    materialized = saver.materialize_checkpoint(
+        config,
+        cp,
+        CheckpointHydrationPlan(
+            channels=(IncrementalChannelSpec(name="messages", kind="delta"),)
+        ),
+    )
+
+    # Should still assemble — with partial chain (just the current delta, base=None)
+    from langgraph.checkpoint.base import DeltaChainValue
+
+    chain = materialized["channel_values"]["messages"]
+    assert isinstance(chain, DeltaChainValue)
+    assert chain.base is None
+    assert chain.deltas == [["msg2"]]

libs/langgraph/tests/test_delta_channel_benchmark.py

@@ -0,0 +1,356 @@
+"""Benchmark: DeltaChannel vs BinaryOperatorAggregate storage and time.
+
+Run directly:  python tests/test_delta_channel_benchmark.py
+Run via pytest: pytest tests/test_delta_channel_benchmark.py -s
+
+Simulates realistic multi-turn conversations with paragraph-length messages
+(~100 tokens each) scaling up to 1M-token-equivalent histories.
+
+Token estimates: 1 token ≈ 4 chars; each turn ≈ 200 tokens (human + AI).
+A 1M-token conversation ≈ 5,000 turns of realistic messages.
+"""
+
+from __future__ import annotations
+
+import sys
+import time
+from typing import Annotated, Any
+
+from langchain_core.messages import AIMessage, HumanMessage
+from langgraph.checkpoint.memory import MemorySaver
+from typing_extensions import TypedDict
+
+from langgraph.channels.delta import DeltaChannel
+from langgraph.graph import END, StateGraph
+from langgraph.graph.message import add_messages
+
+try:
+    from langgraph.checkpoint.sqlite import SqliteSaver
+
+    _SQLITE_AVAILABLE = True
+except ImportError:
+    _SQLITE_AVAILABLE = False
+
+SNAPSHOT_EVERY = 50
+
+# ---------------------------------------------------------------------------
+# Realistic message payload (~100 tokens / ~400 chars each)
+# ---------------------------------------------------------------------------
+
+_HUMAN_TEMPLATE = (
+    "I need help understanding the implications of {topic} on our system architecture. "
+    "Specifically, I'm concerned about how this interacts with our existing {concern} "
+    "and whether we need to refactor the {component} layer before proceeding."
+)
+
+_AI_TEMPLATE = (
+    "Great question about {topic}. The key insight here is that {concern} introduces "
+    "a subtle ordering dependency that most teams overlook until they hit it in production. "
+    "For your {component} layer specifically, I'd recommend starting with a careful audit "
+    "of the interface boundaries before making any structural changes. This will give you "
+    "a clear picture of the blast radius and let you sequence the migration safely."
+)
+
+_TOPICS = [
+    "distributed tracing",
+    "eventual consistency",
+    "schema migration",
+    "backpressure handling",
+    "idempotency guarantees",
+    "cache invalidation",
+    "connection pooling",
+    "rate limiting",
+    "circuit breaking",
+    "observability pipelines",
+]
+
+_CONCERNS = [
+    "concurrency model",
+    "retry semantics",
+    "state management",
+    "error propagation",
+    "latency budget",
+]
+
+_COMPONENTS = [
+    "persistence",
+    "routing",
+    "ingestion",
+    "aggregation",
+    "serialization",
+]
+
+
+def _human_content(i: int) -> str:
+    return _HUMAN_TEMPLATE.format(
+        topic=_TOPICS[i % len(_TOPICS)],
+        concern=_CONCERNS[i % len(_CONCERNS)],
+        component=_COMPONENTS[i % len(_COMPONENTS)],
+    )
+
+
+def _ai_content(i: int) -> str:
+    return _AI_TEMPLATE.format(
+        topic=_TOPICS[i % len(_TOPICS)],
+        concern=_CONCERNS[i % len(_CONCERNS)],
+        component=_COMPONENTS[i % len(_COMPONENTS)],
+    )
+
+
+# ---------------------------------------------------------------------------
+# State definitions
+# ---------------------------------------------------------------------------
+
+
+class BinaryState(TypedDict):
+    messages: Annotated[list, add_messages]
+
+
+class DeltaState(TypedDict):
+    messages: Annotated[list, DeltaChannel(add_messages)]
+
+
+class DeltaSnapshotState(TypedDict):
+    messages: Annotated[list, DeltaChannel(add_messages, snapshot_every=SNAPSHOT_EVERY)]
+
+
+# ---------------------------------------------------------------------------
+# Graph factory
+# ---------------------------------------------------------------------------
+
+
+def _make_graph(state_cls: type, checkpointer: Any = None) -> Any:
+    def human_node(state: Any) -> dict:
+        return {}
+
+    def ai_node(state: Any) -> dict:
+        i = len(state["messages"]) // 2
+        return {"messages": [AIMessage(content=_ai_content(i), id=f"a{i}")]}
+
+    g = StateGraph(state_cls)
+    g.add_node("human", human_node)
+    g.add_node("ai", ai_node)
+    g.add_edge("human", "ai")
+    g.add_edge("ai", END)
+    g.set_entry_point("human")
+    return g.compile(checkpointer=checkpointer or MemorySaver())
+
+
+# ---------------------------------------------------------------------------
+# Measurement helpers
+# ---------------------------------------------------------------------------
+
+
+def _total_blob_bytes(saver: MemorySaver) -> int:
+    total = 0
+    for (_, _, _, _), (type_tag, blob) in saver.blobs.items():
+        if blob is not None:
+            total += len(blob)
+    return total
+
+
+def _run_turns(
+    n_turns: int,
+    state_cls: type,
+    checkpointer: Any = None,
+) -> tuple[float, float, int]:
+    """Run n_turns conversation turns.
+
+    Returns (write_elapsed_s, read_elapsed_s, total_blob_bytes).
+    blob_bytes is -1 for savers without in-memory blob stores (e.g. SQLite).
+    Read latency is measured as the time to invoke the graph with no new
+    messages after the full history is built — this forces state rehydration.
+    """
+    graph = _make_graph(state_cls, checkpointer)
+    config = {"configurable": {"thread_id": "bench"}}
+
+    t0 = time.perf_counter()
+    for i in range(n_turns):
+        graph.invoke(
+            {"messages": [HumanMessage(content=_human_content(i), id=f"h{i}")]},
+            config,
+        )
+    write_elapsed = time.perf_counter() - t0
+
+    # Measure read/rehydration: get_state forces the channel to rebuild
+    t1 = time.perf_counter()
+    for _ in range(5):
+        graph.get_state(config)
+    read_elapsed = (time.perf_counter() - t1) / 5
+
+    if isinstance(graph.checkpointer, MemorySaver):
+        blob_bytes = _total_blob_bytes(graph.checkpointer)
+    else:
+        blob_bytes = -1
+    return write_elapsed, read_elapsed, blob_bytes
+
+
+def _fmt_bytes(n: int) -> str:
+    if n >= 1_000_000:
+        return f"{n / 1_000_000:.1f} MB"
+    if n >= 1_000:
+        return f"{n / 1_000:.1f} KB"
+    return f"{n} B"
+
+
+def _approx_tokens(n_turns: int) -> str:
+    # ~100 tokens human + ~100 tokens AI per turn
+    tokens = n_turns * 200
+    if tokens >= 1_000_000:
+        return f"~{tokens / 1_000_000:.1f}M tok"
+    if tokens >= 1_000:
+        return f"~{tokens / 1_000:.0f}K tok"
+    return f"~{tokens} tok"
+
+
+# ---------------------------------------------------------------------------
+# Benchmark matrix
+# ---------------------------------------------------------------------------
+
+# Turn counts chosen to span from a short session to a long-running agent conversation.
+# Storage and time complexity differences are clearly visible by 500 turns.
+# Extrapolation: 5,000 turns × ~200 tokens/turn ≈ 1M tokens (Claude's full context window).
+TURN_COUNTS = [50, 100, 200, 500]
+
+
+def _checkpointer_factories() -> list[tuple[str, Any]]:
+    """Return (label, context_manager_or_none) pairs for available checkpointers."""
+    factories: list[tuple[str, Any]] = [("InMemory", None)]
+    if _SQLITE_AVAILABLE:
+        import tempfile
+
+        factories.append(("SQLite", tempfile.NamedTemporaryFile(suffix=".db")))
+    return factories
+
+
+def run_benchmark() -> None:
+    print()
+    print(
+        "DeltaChannel vs add_messages (BinaryOperatorAggregate) — checkpoint storage & latency"
+    )
+    print("Simulating realistic multi-turn conversations up to ~1M-token histories")
+    print("(5,000 turns × ~200 tokens/turn ≈ 1M tokens — Claude's full context window)")
+    print()
+
+    checkpointers: list[tuple[str, Any]] = [("InMemory (fast-path)", None)]
+    if _SQLITE_AVAILABLE:
+        checkpointers.append(("SQLite (get_tuple fallback)", "sqlite"))
+
+    for cp_label, cp_hint in checkpointers:
+        print(f"--- Checkpointer: {cp_label} ---")
+        _run_benchmark_for_checkpointer(cp_hint)
+
+
+def _run_benchmark_for_checkpointer(cp_hint: Any) -> None:
+    import contextlib
+    import tempfile
+
+    @contextlib.contextmanager
+    def _make_saver():
+        if cp_hint is None:
+            yield None
+        else:
+            with tempfile.NamedTemporaryFile(suffix=".db") as f:
+                with SqliteSaver.from_conn_string(f.name) as saver:
+                    yield saver
+
+    W = 120
+    print("=" * W)
+    header = (
+        f"{'turns':>6}  {'ctx size':>10}  "
+        f"{'add_msgs (bytes)':>18}  {'delta (bytes)':>15}  {'delta+snap (bytes)':>18}  "
+        f"{'storage saved':>14}  "
+        f"{'read: add_msgs':>14}  {'read: delta+snap':>16}"
+    )
+    print(header)
+    print("-" * W)
+
+    results = []
+    for turns in TURN_COUNTS:
+        with _make_saver() as saver:
+            b_wt, b_rt, b_bytes = _run_turns(turns, BinaryState, saver)
+        with _make_saver() as saver:
+            d_wt, d_rt, d_bytes = _run_turns(turns, DeltaState, saver)
+        with _make_saver() as saver:
+            s_wt, s_rt, s_bytes = _run_turns(turns, DeltaSnapshotState, saver)
+
+        # For non-InMemory savers, blob_bytes are unavailable (-1); use read times only
+        if b_bytes < 0 or s_bytes < 0:
+            b_bytes_str = "n/a"
+            d_bytes_str = "n/a"
+            s_bytes_str = "n/a"
+            storage_ratio_str = "n/a"
+        else:
+            storage_ratio = b_bytes / s_bytes if s_bytes else float("inf")
+            b_bytes_str = _fmt_bytes(b_bytes)
+            d_bytes_str = _fmt_bytes(d_bytes)
+            s_bytes_str = _fmt_bytes(s_bytes)
+            storage_ratio_str = f"{storage_ratio:.1f}x"
+            results.append((turns, b_bytes, s_bytes, b_rt, s_rt, storage_ratio))
+
+        print(
+            f"{turns:>6}  {_approx_tokens(turns):>10}  "
+            f"{b_bytes_str:>18}  {d_bytes_str:>15}  {s_bytes_str:>18}  "
+            f"{storage_ratio_str:>14}  "
+            f"{b_rt * 1000:>12.1f}ms  {s_rt * 1000:>14.1f}ms"
+        )
+
+    print("=" * W)
+    print()
+
+    if results:
+        best = results[-1]
+        turns, b_bytes, s_bytes, b_rt, s_rt, ratio = best
+        print(f"Key findings at max scale ({turns} turns):")
+        print(
+            f"  Storage:  {_fmt_bytes(b_bytes)} (add_messages)  →  {_fmt_bytes(s_bytes)} (DeltaChannel+snapshot) — {ratio:.0f}x reduction"
+        )
+        print(
+            f"  Read latency: {b_rt * 1000:.1f}ms (add_messages)  vs  {s_rt * 1000:.1f}ms (DeltaChannel+snapshot)"
+        )
+        print()
+    print("Legend:")
+    print(
+        "  add_msgs      = Annotated[list, add_messages]  — current default, O(N²) storage"
+    )
+    print(
+        "  delta         = DeltaChannel(add_messages)     — O(N) storage, unbounded chain at read"
+    )
+    print(
+        f"  delta+snap    = DeltaChannel(add_messages, snapshot_every={SNAPSHOT_EVERY})  — O(N) storage, O(1) read depth"
+    )
+    print()
+
+
+# ---------------------------------------------------------------------------
+# Pytest entry point
+# ---------------------------------------------------------------------------
+
+
+def test_delta_channel_benchmark(capsys: Any) -> None:
+    """Storage grows O(N²) for add_messages, O(N) for DeltaChannel."""
+    with capsys.disabled():
+        run_benchmark()
+
+    # Correctness assertion: DeltaChannel must use less storage at scale.
+    for turns in [100, 200]:
+        _, _, b_bytes = _run_turns(turns, BinaryState)
+        _, _, d_bytes = _run_turns(turns, DeltaState)
+        _, _, s_bytes = _run_turns(turns, DeltaSnapshotState)
+        assert d_bytes < b_bytes, (
+            f"DeltaChannel should use less storage at {turns} turns, "
+            f"got delta={d_bytes} binary={b_bytes}"
+        )
+        assert s_bytes < b_bytes, (
+            f"DeltaChannel+snapshot should use less storage at {turns} turns, "
+            f"got snapshot={s_bytes} binary={b_bytes}"
+        )
+
+
+# ---------------------------------------------------------------------------
+# Script entry point
+# ---------------------------------------------------------------------------
+
+if __name__ == "__main__":
+    run_benchmark()
+    sys.exit(0)

libs/langgraph/tests/test_pregel.py

@@ -615,8 +615,11 @@ def test_run_from_checkpoint_id_retains_previous_writes(
         )
     ]
 
-    assert len(new_history) == len(history) + 1
-    for original, new in zip(history, new_history[1:]):
+    # +2: one fork checkpoint from time travel, one from the new execution
+    assert len(new_history) == len(history) + 2
+    # new_history[0] is the new execution result, new_history[1] is the fork
+    assert new_history[1].metadata["source"] == "fork"
+    for original, new in zip(history, new_history[2:]):
         assert original.values == new.values
         assert original.next == new.next
         assert original.metadata["step"] == new.metadata["step"]
@@ -624,7 +627,7 @@ def test_run_from_checkpoint_id_retains_previous_writes(
     def _get_tasks(hist: list, start: int):
         return [h.tasks for h in hist[start:]]
 
-    assert _get_tasks(new_history, 1) == _get_tasks(history, 0)
+    assert _get_tasks(new_history, 2) == _get_tasks(history, 0)
 
 
 def test_batch_two_processes_in_out() -> None:
@@ -9397,3 +9400,190 @@ def test_fork_does_not_apply_pending_writes(
 
     # Should be: 1 (input) + 20 (forked node_a) + 100 (node_b) = 121
     assert result == {"value": 121}
+
+
+async def test_delta_channel_end_to_end_inmemory() -> None:
+    """Full graph run: DeltaChannel accumulates correctly across multiple turns."""
+    from langchain_core.messages import AIMessage, HumanMessage
+    from langgraph.checkpoint.memory import InMemorySaver
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph import START, StateGraph
+    from langgraph.graph.message import add_messages
+
+    class State(TypedDict):
+        messages: Annotated[list, DeltaChannel(add_messages)]
+
+    def respond(state: State) -> dict:
+        n = len(state["messages"])
+        return {"messages": [AIMessage(content=f"reply-{n}", id=f"ai-{n}")]}
+
+    builder = StateGraph(State)
+    builder.add_node("respond", respond)
+    builder.add_edge(START, "respond")
+    graph = builder.compile(checkpointer=InMemorySaver())
+
+    config = {"configurable": {"thread_id": "diff-test-1"}}
+
+    # Turn 1
+    graph.invoke({"messages": [HumanMessage(content="hello", id="h1")]}, config)
+    # Turn 2
+    graph.invoke({"messages": [HumanMessage(content="world", id="h2")]}, config)
+    # Turn 3
+    graph.invoke({"messages": [HumanMessage(content="bye", id="h3")]}, config)
+
+    state = graph.get_state(config)
+    msgs = state.values["messages"]
+    # 3 human + 3 AI = 6 total
+    assert len(msgs) == 6, f"expected 6 messages, got {len(msgs)}: {msgs}"
+    assert msgs[0].content == "hello"
+    assert msgs[2].content == "world"
+    assert msgs[4].content == "bye"
+    assert msgs[1].content == "reply-1"
+    assert msgs[3].content == "reply-3"
+    assert msgs[5].content == "reply-5"
+
+
+async def test_delta_channel_time_travel() -> None:
+    """Time-travel back to turn-1 checkpoint and resume; continuation must not include turn-2 deltas."""
+    from langchain_core.messages import AIMessage, HumanMessage
+    from langgraph.checkpoint.memory import InMemorySaver
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph import START, StateGraph
+    from langgraph.graph.message import add_messages
+
+    class State(TypedDict):
+        messages: Annotated[list, DeltaChannel(add_messages)]
+
+    counter = {"n": 0}
+
+    def respond(state: State) -> dict:
+        counter["n"] += 1
+        return {
+            "messages": [
+                AIMessage(content=f"ai-{counter['n']}", id=f"ai-{counter['n']}")
+            ]
+        }
+
+    builder = StateGraph(State)
+    builder.add_node("respond", respond)
+    builder.add_edge(START, "respond")
+    saver = InMemorySaver()
+    graph = builder.compile(checkpointer=saver)
+
+    config = {"configurable": {"thread_id": "diff-time-travel"}}
+
+    # Run 2 turns: h1→ai-1, h2→ai-2
+    graph.invoke({"messages": [HumanMessage(content="h1", id="h1")]}, config)
+    graph.invoke({"messages": [HumanMessage(content="h2", id="h2")]}, config)
+
+    # Find the checkpoint after turn 1 (2 messages: h1 + ai-1)
+    history = list(graph.get_state_history(config))
+    after_turn1 = next(h for h in history if len(h.values.get("messages", [])) == 2)
+
+    assert len(after_turn1.values["messages"]) == 2
+    assert after_turn1.values["messages"][0].content == "h1"
+    assert after_turn1.values["messages"][1].content == "ai-1"
+
+    # Resume from turn-1 checkpoint: inject h3, expect 3 messages total (h1, ai-1, ai-N)
+    # NOT 5 messages (turn-2 deltas must not bleed into the resumed run)
+    result = graph.invoke(
+        {"messages": [HumanMessage(content="h3", id="h3")]},
+        after_turn1.config,
+    )
+    msgs = result["messages"]
+    # Should be: h1, ai-1, h3, ai-N — 4 messages total
+    assert len(msgs) == 4, (
+        f"expected 4 messages after time-travel resume, got {len(msgs)}: {msgs}"
+    )
+    assert msgs[0].content == "h1"
+    assert msgs[1].content == "ai-1"
+    assert msgs[2].content == "h3"
+
+
+async def test_delta_channel_remove_message_end_to_end() -> None:
+    """RemoveMessage inside a DeltaChannel graph must persist and reload correctly."""
+    from langchain_core.messages import AIMessage, HumanMessage, RemoveMessage
+    from langgraph.checkpoint.memory import InMemorySaver
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph import START, StateGraph
+    from langgraph.graph.message import add_messages
+
+    class State(TypedDict):
+        messages: Annotated[list, DeltaChannel(add_messages)]
+
+    def respond(state: State) -> dict:
+        return {"messages": [AIMessage(content="reply", id="ai-1")]}
+
+    def delete_first(state: State) -> dict:
+        # removes the first message
+        return {"messages": [RemoveMessage(id=state["messages"][0].id)]}
+
+    builder = StateGraph(State)
+    builder.add_node("respond", respond)
+    builder.add_node("delete_first", delete_first)
+    builder.add_edge(START, "respond")
+    builder.add_edge("respond", "delete_first")
+    graph = builder.compile(checkpointer=InMemorySaver())
+
+    config = {"configurable": {"thread_id": "diff-remove-test"}}
+    graph.invoke({"messages": [HumanMessage(content="hello", id="h1")]}, config)
+
+    state = graph.get_state(config)
+    msgs = state.values["messages"]
+    # h1 was removed, only ai-1 should remain
+    assert len(msgs) == 1, f"expected 1 message, got {len(msgs)}: {msgs}"
+    assert msgs[0].id == "ai-1"
+
+    # A subsequent turn must reconstruct from the checkpoint correctly
+    graph.invoke({"messages": [HumanMessage(content="again", id="h2")]}, config)
+    state = graph.get_state(config)
+    msgs = state.values["messages"]
+    # ai-1 + h2 + ai-1(second reply, same id overwrites) + h2 removed
+    # more simply: after second run we expect ai-1 updated + h2 remaining minus deleted h2
+    # just assert h1 is still gone
+    assert all(m.id != "h1" for m in msgs), (
+        "h1 should still be absent after second turn"
+    )
+
+
+async def test_delta_channel_update_by_id_end_to_end() -> None:
+    """Updating a message by ID via DeltaChannel must persist and reload correctly."""
+    from langchain_core.messages import HumanMessage
+    from langgraph.checkpoint.memory import InMemorySaver
+
+    from langgraph.channels.delta import DeltaChannel
+    from langgraph.graph import START, StateGraph
+    from langgraph.graph.message import add_messages
+
+    class State(TypedDict):
+        messages: Annotated[list, DeltaChannel(add_messages)]
+
+    def update_msg(state: State) -> dict:
+        # re-send h1 with updated content
+        return {"messages": [HumanMessage(content="updated", id="h1")]}
+
+    builder = StateGraph(State)
+    builder.add_node("update_msg", update_msg)
+    builder.add_edge(START, "update_msg")
+    graph = builder.compile(checkpointer=InMemorySaver())
+
+    config = {"configurable": {"thread_id": "diff-update-id-test"}}
+    graph.invoke({"messages": [HumanMessage(content="original", id="h1")]}, config)
+
+    state = graph.get_state(config)
+    msgs = state.values["messages"]
+    assert len(msgs) == 1, f"expected 1 message, got {len(msgs)}: {msgs}"
+    assert msgs[0].content == "updated"
+    assert msgs[0].id == "h1"
+
+    # Second turn: verify the updated state is the base for further accumulation
+    graph.invoke({"messages": [HumanMessage(content="new", id="h2")]}, config)
+    state = graph.get_state(config)
+    msgs = state.values["messages"]
+    ids = [m.id for m in msgs]
+    assert "h1" in ids  # h1 persists (updated, not duplicated)
+    assert "h2" in ids
+    assert ids.count("h1") == 1, "h1 must not be duplicated"

libs/langgraph/tests/test_pregel_async.py

@@ -2086,8 +2086,11 @@ async def test_run_from_checkpoint_id_retains_previous_writes(
         )
     ]
 
-    assert len(new_history) == len(history) + 1
-    for original, new in zip(history, new_history[1:]):
+    # +2: one fork checkpoint from time travel, one from the new execution
+    assert len(new_history) == len(history) + 2
+    # new_history[0] is the new execution result, new_history[1] is the fork
+    assert new_history[1].metadata["source"] == "fork"
+    for original, new in zip(history, new_history[2:]):
         assert original.values == new.values
         assert original.next == new.next
         assert original.metadata["step"] == new.metadata["step"]
@@ -2095,7 +2098,7 @@ async def test_run_from_checkpoint_id_retains_previous_writes(
     def _get_tasks(hist: list, start: int):
         return [h.tasks for h in hist[start:]]
 
-    assert _get_tasks(new_history, 1) == _get_tasks(history, 0)
+    assert _get_tasks(new_history, 2) == _get_tasks(history, 0)
 
 
 async def test_cond_edge_after_send() -> None:

libs/langgraph/tests/test_rehydrate_sweep.py

@@ -0,0 +1,185 @@
+"""Sweep snapshot_every values to find the storage vs. time-travel tradeoff.
+
+Run directly:  python tests/test_rehydrate_sweep.py
+Run via pytest: pytest tests/test_rehydrate_sweep.py -s
+"""
+
+from __future__ import annotations
+
+import sys
+import time
+from typing import Annotated, Any
+
+from langchain_core.messages import AIMessage, HumanMessage
+from langgraph.checkpoint.memory import MemorySaver
+from typing_extensions import TypedDict
+
+from langgraph.channels.delta import DeltaChannel
+from langgraph.graph import END, StateGraph
+from langgraph.graph.message import add_messages
+
+# ---------------------------------------------------------------------------
+# Config
+# ---------------------------------------------------------------------------
+
+REHYDRATE_SWEEP = [5, 10, 25, 50, 100, None]  # None = no rehydration (pure diff)
+TURN_COUNTS = [50, 100, 250, 500]
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_state(snapshot_every: int | None) -> type:
+    channel = DeltaChannel(add_messages, snapshot_every=snapshot_every)
+    return TypedDict("S", {"messages": Annotated[list, channel]})
+
+
+def _make_graph(state_cls: type) -> Any:
+    def human_node(state: Any) -> dict:
+        return {}
+
+    def ai_node(state: Any) -> dict:
+        last = state["messages"][-1]
+        return {"messages": [AIMessage(content=f"reply-to-{last.id}")]}
+
+    g = StateGraph(state_cls)
+    g.add_node("human", human_node)
+    g.add_node("ai", ai_node)
+    g.add_edge("human", "ai")
+    g.add_edge("ai", END)
+    g.set_entry_point("human")
+    return g.compile(checkpointer=MemorySaver())
+
+
+def _total_blob_bytes(saver: MemorySaver) -> int:
+    total = 0
+    for (_, _, _, _), (type_tag, blob) in saver.blobs.items():
+        if blob is not None:
+            total += len(blob)
+    return total
+
+
+def _measure_time_travel_ms(graph: Any, config: dict) -> float:
+    """Time how long it takes to get state at the very first checkpoint (worst case)."""
+    history = list(graph.get_state_history(config))
+    if not history:
+        return 0.0
+    oldest = history[-1]
+    t0 = time.perf_counter()
+    graph.get_state(oldest.config)
+    return (time.perf_counter() - t0) * 1000
+
+
+def _run(n_turns: int, snapshot_every: int | None) -> tuple[float, int, float]:
+    """Returns (write_ms, blob_bytes, time_travel_ms)."""
+    state_cls = _make_state(snapshot_every)
+    graph = _make_graph(state_cls)
+    saver: MemorySaver = graph.checkpointer  # type: ignore[assignment]
+    config = {"configurable": {"thread_id": "sweep"}}
+
+    t0 = time.perf_counter()
+    for i in range(n_turns):
+        graph.invoke(
+            {"messages": [HumanMessage(content=f"msg-{i}", id=f"h{i}")]}, config
+        )
+    write_ms = (time.perf_counter() - t0) * 1000
+
+    blob_bytes = _total_blob_bytes(saver)
+    tt_ms = _measure_time_travel_ms(graph, config)
+    return write_ms, blob_bytes, tt_ms
+
+
+# ---------------------------------------------------------------------------
+# ASCII sparkline
+# ---------------------------------------------------------------------------
+
+
+def _sparkline(values: list[float], width: int = 20) -> str:
+    bars = " ▁▂▃▄▅▆▇█"
+    lo, hi = min(values), max(values)
+    span = hi - lo or 1
+    chars = [bars[round((v - lo) / span * (len(bars) - 1))] for v in values]
+    return "".join(chars).ljust(width)
+
+
+# ---------------------------------------------------------------------------
+# Main
+# ---------------------------------------------------------------------------
+
+
+def run_sweep() -> None:
+    label = {v: (str(v) if v is not None else "None(∞)") for v in REHYDRATE_SWEEP}
+
+    print()
+    print("snapshot_every sweep — storage vs time-travel cost")
+    print("=" * 90)
+
+    for turns in TURN_COUNTS:
+        print(f"\n--- {turns} turns ---")
+        col_w = 12
+        header = (
+            f"{'snapshot_every':>18}  "
+            f"{'blob_bytes':>{col_w}}  "
+            f"{'write_ms':>{col_w}}  "
+            f"{'time_travel_ms':>{col_w}}"
+        )
+        print(header)
+        print("-" * 60)
+
+        tt_vals: list[float] = []
+        byte_vals: list[int] = []
+        write_vals: list[float] = []
+        rows: list[tuple] = []
+
+        for rv in REHYDRATE_SWEEP:
+            write_ms, blob_bytes, tt_ms = _run(turns, rv)
+            rows.append((rv, blob_bytes, write_ms, tt_ms))
+            byte_vals.append(blob_bytes)
+            write_vals.append(write_ms)
+            tt_vals.append(tt_ms)
+
+        for rv, blob_bytes, write_ms, tt_ms in rows:
+            print(
+                f"{label[rv]:>18}  "
+                f"{blob_bytes:>{col_w},}  "
+                f"{write_ms:>{col_w}.1f}  "
+                f"{tt_ms:>{col_w}.2f}"
+            )
+
+        print()
+        print(
+            f"  bytes       spark: [{_sparkline(byte_vals)}]  "
+            f"lo={min(byte_vals):,}  hi={max(byte_vals):,}"
+        )
+        print(
+            f"  time-travel spark: [{_sparkline(tt_vals)}]  "
+            f"lo={min(tt_vals):.2f}ms  hi={max(tt_vals):.2f}ms"
+        )
+        print(
+            f"  write       spark: [{_sparkline(write_vals)}]  "
+            f"lo={min(write_vals):.1f}ms  hi={max(write_vals):.1f}ms"
+        )
+
+    print()
+    print("=" * 90)
+    print(
+        "snapshot_every=None means pure diff (no snapshots) — "
+        "lowest storage, highest time-travel cost."
+    )
+    print(
+        "Lower snapshot_every = more frequent full snapshots = "
+        "faster time-travel, more storage."
+    )
+    print()
+
+
+def test_rehydrate_sweep(capsys: Any) -> None:
+    with capsys.disabled():
+        run_sweep()
+
+
+if __name__ == "__main__":
+    run_sweep()
+    sys.exit(0)

libs/langgraph/tests/test_stream_subgraph_transformer.py

@@ -1,628 +0,0 @@
-"""Tests for subgraph lifecycle events and the SubgraphTransformer."""
-
-from __future__ import annotations
-
-import operator
-import time
-from typing import Annotated, Any
-
-import pytest
-from langgraph.checkpoint.memory import InMemorySaver
-from typing_extensions import TypedDict
-
-from langgraph.constants import END, START
-from langgraph.errors import GraphInterrupt
-from langgraph.graph import StateGraph
-from langgraph.stream._event_log import EventLog
-from langgraph.stream._mux import StreamMux
-from langgraph.stream._types import ProtocolEvent
-from langgraph.stream.transformers import (
-    MessagesTransformer,
-    SubgraphRunStream,
-    SubgraphTransformer,
-    ToolLifecycleTransformer,
-    ValuesTransformer,
-)
-from langgraph.types import interrupt
-
-TS = int(time.time() * 1000)
-
-
-def _lifecycle(
-    event: str,
-    *,
-    namespace: list[str] | None = None,
-    graph_name: str | None = None,
-    cause: dict[str, Any] | None = None,
-    error: str | None = None,
-) -> ProtocolEvent:
-    data: dict[str, Any] = {"event": event}
-    if graph_name is not None:
-        data["graph_name"] = graph_name
-    if cause is not None:
-        data["cause"] = cause
-    if error is not None:
-        data["error"] = error
-    return {
-        "type": "event",
-        "method": "lifecycle",
-        "params": {
-            "namespace": namespace or [],
-            "timestamp": TS,
-            "data": data,
-        },
-    }
-
-
-def _values(payload: dict[str, Any], *, namespace: list[str]) -> ProtocolEvent:
-    return {
-        "type": "event",
-        "method": "values",
-        "params": {
-            "namespace": namespace,
-            "timestamp": TS,
-            "data": payload,
-        },
-    }
-
-
-def _subscribe(log: EventLog) -> None:
-    """Flip `_subscribed = True` so pushes retain items for test inspection."""
-    log._subscribed = True
-
-
-# ---------------------------------------------------------------------------
-# Unit tests: feed events directly into the transformer
-# ---------------------------------------------------------------------------
-
-
-_FACTORIES = [
-    ValuesTransformer,
-    ToolLifecycleTransformer,
-    MessagesTransformer,
-    SubgraphTransformer,
-]
-
-
-def _handle_values_items(handle: SubgraphRunStream) -> list:
-    return list(handle._mux.extensions["values"]._items)  # type: ignore[attr-defined]
-
-
-def _handle_subgraphs_items(handle: SubgraphRunStream) -> list:
-    return list(handle._mux.extensions["subgraphs"]._items)  # type: ignore[attr-defined]
-
-
-def _pre_subscribe_handle(handle: SubgraphRunStream) -> None:
-    """Flip `_subscribed` on every EventLog inside the handle's mini-mux.
-
-    The mini-mux is built via `make_child` with the full factory list,
-    so values / messages / subgraphs logs all exist as projections.
-    Tests that feed events directly need them subscribed so pushes
-    retain items in the deque for `_items` inspection.
-    """
-    for value in handle._mux.extensions.values():
-        if isinstance(value, EventLog):
-            _subscribe(value)
-
-
-class TestSubgraphTransformerUnit:
-    def _mux(self) -> tuple[StreamMux, SubgraphTransformer]:
-        mux = StreamMux(factories=_FACTORIES, is_async=False)
-        transformer = mux.transformer_by_key("subgraphs")
-        assert isinstance(transformer, SubgraphTransformer)
-        _subscribe(transformer._root_log)
-        return mux, transformer
-
-    def _handle(self, transformer: SubgraphTransformer) -> SubgraphRunStream:
-        """Return the single root handle after pushing one lifecycle started."""
-        (handle,) = list(transformer._root_log._items)
-        return handle
-
-    def test_root_started_is_ignored(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", graph_name="root"))
-        assert list(transformer._root_log._items) == []
-        assert transformer._by_ns == {}
-
-    def test_child_started_yields_handle(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(
-            _lifecycle(
-                "started",
-                namespace=["task_a:child"],
-                graph_name="child",
-                cause={"type": "toolCall", "tool_call_id": "call_abc"},
-            )
-        )
-
-        handle = self._handle(transformer)
-        assert handle.path == ("task_a:child",)
-        assert handle.graph_name == "child"
-        assert handle.cause == {"type": "toolCall", "tool_call_id": "call_abc"}
-        assert handle.status == "started"
-
-    def test_tool_started_is_synthesized_before_tool_caused_lifecycle(self) -> None:
-        mux, transformer = self._mux()
-        events = iter(mux._events)
-        mux.push(
-            _values(
-                {
-                    "messages": [
-                        {
-                            "tool_calls": [
-                                {
-                                    "id": "call_abc",
-                                    "name": "task",
-                                    "args": {"subagent_type": "researcher"},
-                                }
-                            ]
-                        }
-                    ]
-                },
-                namespace=[],
-            )
-        )
-        mux.push(
-            _lifecycle(
-                "started",
-                namespace=["task:child"],
-                graph_name="child",
-                cause={"type": "toolCall", "tool_call_id": "call_abc"},
-            )
-        )
-
-        mux.close()
-        tool_started, lifecycle_started = list(events)[1:3]
-        assert tool_started["method"] == "tools"
-        assert tool_started["params"]["namespace"] == []
-        assert tool_started["params"]["data"] == {
-            "event": "tool-started",
-            "tool_call_id": "call_abc",
-            "tool_name": "task",
-            "input": {"subagent_type": "researcher"},
-        }
-        assert lifecycle_started["method"] == "lifecycle"
-        assert tool_started["seq"] < lifecycle_started["seq"]
-        assert self._handle(transformer).path == ("task:child",)
-
-    def test_core_golden_trace_uses_js_wire_shape_and_ordering(self) -> None:
-        mux, _transformer = self._mux()
-        events = iter(mux._events)
-        mux.push(
-            _values(
-                {
-                    "messages": [
-                        {
-                            "tool_calls": [
-                                {
-                                    "id": "call_abc",
-                                    "name": "task",
-                                    "args": {"subagent_type": "researcher"},
-                                }
-                            ]
-                        }
-                    ]
-                },
-                namespace=[],
-            )
-        )
-        for data in (
-            {"event": "message-start", "id": "msg-1", "role": "ai"},
-            {
-                "event": "content-block-delta",
-                "index": 0,
-                "content": {"type": "text", "text": "hi"},
-            },
-        ):
-            mux.push(
-                {
-                    "type": "event",
-                    "method": "messages",
-                    "params": {
-                        "namespace": ["call_model:task-1"],
-                        "timestamp": TS,
-                        "data": data,
-                        "run_id": "run-1",
-                    },
-                }
-            )
-        mux.push(
-            _lifecycle(
-                "started",
-                namespace=["task:child"],
-                graph_name="child",
-                cause={"type": "toolCall", "tool_call_id": "call_abc"},
-            )
-        )
-        mux.close()
-
-        trace = [
-            (event["method"], event["params"]["namespace"], event["params"]["data"])
-            for event in events
-        ]
-        assert trace == [
-            (
-                "values",
-                [],
-                {
-                    "messages": [
-                        {
-                            "tool_calls": [
-                                {
-                                    "id": "call_abc",
-                                    "name": "task",
-                                    "args": {"subagent_type": "researcher"},
-                                }
-                            ]
-                        }
-                    ]
-                },
-            ),
-            (
-                "messages",
-                ["call_model:task-1"],
-                {"event": "message-start", "id": "msg-1", "role": "ai"},
-            ),
-            (
-                "messages",
-                ["call_model:task-1"],
-                {
-                    "event": "content-block-start",
-                    "index": 0,
-                    "content": {"type": "text", "text": ""},
-                },
-            ),
-            (
-                "messages",
-                ["call_model:task-1"],
-                {
-                    "event": "content-block-delta",
-                    "index": 0,
-                    "content": {"type": "text", "text": "hi"},
-                },
-            ),
-            (
-                "tools",
-                [],
-                {
-                    "event": "tool-started",
-                    "tool_call_id": "call_abc",
-                    "tool_name": "task",
-                    "input": {"subagent_type": "researcher"},
-                },
-            ),
-            (
-                "lifecycle",
-                ["task:child"],
-                {
-                    "event": "started",
-                    "graph_name": "child",
-                    "cause": {"type": "toolCall", "tool_call_id": "call_abc"},
-                },
-            ),
-        ]
-
-    def test_status_transitions(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        mux.push(_lifecycle("running", namespace=["t:c"]))
-        mux.push(_lifecycle("completed", namespace=["t:c"]))
-
-        handle = self._handle(transformer)
-        assert handle.status == "completed"
-
-    def test_grandchild_surfaces_under_child(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:child"], graph_name="child"))
-
-        child = self._handle(transformer)
-        _pre_subscribe_handle(child)
-
-        mux.push(
-            _lifecycle(
-                "started",
-                namespace=["t:child", "u:grand"],
-                graph_name="grand",
-            )
-        )
-
-        (grand,) = _handle_subgraphs_items(child)
-        assert grand.path == ("t:child", "u:grand")
-        assert grand.graph_name == "grand"
-
-    def test_failed_stores_error(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        mux.push(_lifecycle("failed", namespace=["t:c"], error="boom"))
-
-        handle = self._handle(transformer)
-        assert handle.status == "failed"
-        assert handle.error == "boom"
-
-    def test_values_routed_into_handle(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-
-        handle = self._handle(transformer)
-        _pre_subscribe_handle(handle)
-
-        mux.push(_values({"value": 1}, namespace=["t:c"]))
-        mux.push(_values({"value": 2}, namespace=["t:c"]))
-
-        assert _handle_values_items(handle) == [{"value": 1}, {"value": 2}]
-        assert handle.output == {"value": 2}
-
-    def test_root_values_not_routed(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        handle = self._handle(transformer)
-        _pre_subscribe_handle(handle)
-
-        # Values event at root namespace — must not leak into child handle.
-        mux.push(_values({"value": "root"}, namespace=[]))
-        assert _handle_values_items(handle) == []
-
-    def test_finalize_closes_dangling(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        handle = self._handle(transformer)
-
-        mux.close()
-        assert handle.status == "completed"
-        assert handle._mux.extensions["values"]._closed
-        assert handle._mux.extensions["subgraphs"]._closed
-
-    def test_fail_with_graph_interrupt_marks_interrupted(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        handle = self._handle(transformer)
-
-        mux.fail(GraphInterrupt())
-        assert handle.status == "interrupted"
-
-    def test_fail_with_generic_error_marks_failed(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        handle = self._handle(transformer)
-
-        mux.fail(RuntimeError("explode"))
-        assert handle.status == "failed"
-        assert handle.error == "explode"
-
-    def test_duplicate_started_ignored(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="c"))
-        mux.push(_lifecycle("started", namespace=["t:c"], graph_name="other"))
-
-        handles = list(transformer._root_log._items)
-        assert len(handles) == 1
-        assert handles[0].graph_name == "c"
-
-    def test_non_lifecycle_non_values_passthrough(self) -> None:
-        mux, transformer = self._mux()
-        mux.push(
-            {
-                "type": "event",
-                "method": "messages",
-                "params": {
-                    "namespace": ["t:c"],
-                    "timestamp": TS,
-                    "data": (
-                        {"event": "message-start", "message_id": "m1"},
-                        {"run_id": "m1"},
-                    ),
-                },
-            }
-        )
-        assert list(transformer._root_log._items) == []
-
-
-# ---------------------------------------------------------------------------
-# End-to-end tests via stream_v2 on real graphs
-# ---------------------------------------------------------------------------
-
-
-class SimpleState(TypedDict):
-    value: str
-    items: Annotated[list[str], operator.add]
-
-
-def _build_nested_graph():
-    """Parent graph with a compiled subgraph node."""
-
-    def inner_node(state: SimpleState) -> dict:
-        return {"value": state["value"] + "X", "items": ["x"]}
-
-    inner_builder = StateGraph(SimpleState)
-    inner_builder.add_node("inner_node", inner_node)
-    inner_builder.add_edge(START, "inner_node")
-    inner_builder.add_edge("inner_node", END)
-    inner = inner_builder.compile()
-
-    def outer_node(state: SimpleState) -> dict:
-        return {"value": state["value"] + "Y", "items": ["y"]}
-
-    outer_builder = StateGraph(SimpleState)
-    outer_builder.add_node("outer_node", outer_node)
-    outer_builder.add_node("sub", inner)
-    outer_builder.add_edge(START, "outer_node")
-    outer_builder.add_edge("outer_node", "sub")
-    outer_builder.add_edge("sub", END)
-    return outer_builder.compile()
-
-
-class TestSubgraphTransformerEndToEnd:
-    def test_flat_graph_yields_no_subgraphs(self) -> None:
-        builder = StateGraph(SimpleState)
-        builder.add_node("n", lambda s: {"value": s["value"] + "!", "items": ["!"]})
-        builder.add_edge(START, "n")
-        builder.add_edge("n", END)
-        graph = builder.compile()
-
-        run = graph.stream_v2({"value": "", "items": []})
-
-        collected: list[SubgraphRunStream] = []
-        for sub in run.subgraphs:
-            collected.append(sub)
-        assert collected == []
-        # Output still resolves.
-        assert run.output is not None
-
-    def test_nested_graph_yields_one_child(self) -> None:
-        graph = _build_nested_graph()
-        run = graph.stream_v2({"value": "", "items": []})
-
-        collected: list[SubgraphRunStream] = []
-        for sub in run.subgraphs:
-            collected.append(sub)
-
-        assert len(collected) == 1
-        child = collected[0]
-        assert len(child.path) == 1
-        assert child.path[0].startswith("sub:")
-        assert child.status == "completed"
-
-    def test_error_in_subgraph_fails_child(self) -> None:
-        def boom(state: SimpleState) -> dict:
-            raise RuntimeError("subgraph_failed")
-
-        inner_builder = StateGraph(SimpleState)
-        inner_builder.add_node("inner", boom)
-        inner_builder.add_edge(START, "inner")
-        inner_builder.add_edge("inner", END)
-        inner = inner_builder.compile()
-
-        outer_builder = StateGraph(SimpleState)
-        outer_builder.add_node("sub", inner)
-        outer_builder.add_edge(START, "sub")
-        outer_builder.add_edge("sub", END)
-        graph = outer_builder.compile()
-
-        run = graph.stream_v2({"value": "", "items": []})
-
-        collected: list[SubgraphRunStream] = []
-        with pytest.raises(RuntimeError):
-            for sub in run.subgraphs:
-                collected.append(sub)
-
-        assert len(collected) == 1
-        assert collected[0].status == "failed"
-
-
-class TestSubgraphTransformerAsyncEndToEnd:
-    @pytest.mark.anyio
-    async def test_nested_graph_yields_one_child(self) -> None:
-        async def inner(state: SimpleState) -> dict:
-            return {"value": state["value"] + "X", "items": ["x"]}
-
-        inner_builder = StateGraph(SimpleState)
-        inner_builder.add_node("inner", inner)
-        inner_builder.add_edge(START, "inner")
-        inner_builder.add_edge("inner", END)
-        inner_graph = inner_builder.compile()
-
-        outer_builder = StateGraph(SimpleState)
-        outer_builder.add_node("sub", inner_graph)
-        outer_builder.add_edge(START, "sub")
-        outer_builder.add_edge("sub", END)
-        graph = outer_builder.compile()
-
-        run = await graph.astream_v2({"value": "", "items": []})
-
-        collected: list[SubgraphRunStream] = []
-        async for sub in run.subgraphs:
-            collected.append(sub)
-
-        assert len(collected) == 1
-        child = collected[0]
-        assert child.status == "completed"
-
-
-class TestSubgraphCause:
-    """Pregel core emits no `cause`; product transformers populate it."""
-
-    def test_cause_not_populated_by_pregel(self) -> None:
-        graph = _build_nested_graph()
-        run = graph.stream_v2({"value": "", "items": []})
-
-        collected: list[SubgraphRunStream] = list(run.subgraphs)
-        assert len(collected) == 1
-        child = collected[0]
-
-        # The child's single-segment path still encodes `node_name:task_id`
-        # (that's pregel's internal namespace format), but `cause` is now
-        # product-agnostic and must be populated by a stream transformer,
-        # not by pregel itself.
-        assert ":" in child.path[0]
-        node_name, _, task_id = child.path[0].partition(":")
-        assert node_name == "sub"
-        assert task_id  # non-empty
-        assert child.cause is None
-
-
-class TestSubgraphInterrupt:
-    """Interrupts raised inside a subgraph surface as status=interrupted."""
-
-    def _build_interrupt_subgraph(self):
-        def inner_node(state: SimpleState) -> dict:
-            interrupt("need approval")
-            return {"value": state["value"] + "X", "items": ["x"]}
-
-        inner_builder = StateGraph(SimpleState)
-        inner_builder.add_node("inner_node", inner_node)
-        inner_builder.add_edge(START, "inner_node")
-        inner_builder.add_edge("inner_node", END)
-        inner = inner_builder.compile()
-
-        outer_builder = StateGraph(SimpleState)
-        outer_builder.add_node("sub", inner)
-        outer_builder.add_edge(START, "sub")
-        outer_builder.add_edge("sub", END)
-        return outer_builder.compile(checkpointer=InMemorySaver())
-
-    def test_interrupt_in_subgraph_marks_handle_interrupted(self) -> None:
-        graph = self._build_interrupt_subgraph()
-        run = graph.stream_v2(
-            {"value": "", "items": []},
-            config={"configurable": {"thread_id": "t1"}},
-        )
-
-        collected: list[SubgraphRunStream] = list(run.subgraphs)
-
-        assert run.interrupted is True
-        assert len(collected) == 1
-        assert collected[0].status == "interrupted"
-
-
-class TestSubgraphNameCollision:
-    """The subgraph's compiled `name` equaling its node name is detected.
-
-    Primary detector `name != langgraph_node` fails here; the
-    parent_run_id fallback in `_is_nested_pregel_start` is what keeps
-    the subgraph visible.
-    """
-
-    def test_name_equals_node_name_still_detected(self) -> None:
-        def inner_node(state: SimpleState) -> dict:
-            return {"value": state["value"] + "X", "items": ["x"]}
-
-        inner_builder = StateGraph(SimpleState)
-        inner_builder.add_node("inner_node", inner_node)
-        inner_builder.add_edge(START, "inner_node")
-        inner_builder.add_edge("inner_node", END)
-        # Compile with the same name as the node it will be registered as.
-        inner = inner_builder.compile(name="sub")
-
-        outer_builder = StateGraph(SimpleState)
-        outer_builder.add_node("sub", inner)
-        outer_builder.add_edge(START, "sub")
-        outer_builder.add_edge("sub", END)
-        graph = outer_builder.compile()
-
-        run = graph.stream_v2({"value": "", "items": []})
-
-        collected: list[SubgraphRunStream] = list(run.subgraphs)
-        assert len(collected) == 1
-        child = collected[0]
-        assert child.graph_name == "sub"
-        assert child.status == "completed"

libs/langgraph/tests/test_time_travel.py

@@ -37,6 +37,7 @@ def _checkpoint_summary(history: list) -> list[dict]:
     Returns a list of dicts (newest-first, matching get_state_history order) with:
       - id: short checkpoint id suffix (last 6 chars)
       - parent_id: short parent checkpoint id suffix or None
+      - source: checkpoint metadata source (input, loop, fork, update)
       - next: tuple of next node names
       - values: channel values snapshot
     """
@@ -52,6 +53,7 @@ def _checkpoint_summary(history: list) -> list[dict]:
             {
                 "id": cid[-6:],
                 "parent_id": pid[-6:] if pid else None,
+                "source": s.metadata.get("source"),
                 "next": s.next,
                 "values": s.values,
             }
@@ -280,6 +282,116 @@ def test_replay_from_before_interrupt_refires(
     assert call_count["node_b"] == 1  # NOT re-executed (after interrupt)
 
 
+def test_replay_from_before_interrupt_then_resume(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Replay from checkpoint before interrupt node, then resume with a new
+    answer and verify the graph completes with the new value.
+
+    Graph: START --> node_a --> ask_human (interrupt) --> node_b --> END
+
+    Original run:
+      source=input  next=(__start__,)  values=[]
+      source=loop   next=(node_a,)     values=[]
+      source=loop   next=(ask_human,)  values=[a]            <-- replay from here
+      source=loop   next=(node_b,)     values=[a, human:old_answer]
+      source=loop   next=()            values=[a, human:old_answer, b]
+
+    After replay (fork created) + resume with "new_answer":
+      source=input  next=(__start__,)  values=[]
+      source=loop   next=(node_a,)     values=[]
+      source=loop   next=(ask_human,)  values=[a]            <-- branch point
+      source=loop   next=(node_b,)     values=[a, human:old_answer]
+      source=loop   next=()            values=[a, human:old_answer, b]  (old branch)
+      source=fork   next=(ask_human,)  values=[a]            <-- fork from branch point
+      source=loop   next=(node_b,)     values=[a, human:new_answer]
+      source=loop   next=()            values=[a, human:new_answer, b]  (new branch)
+    """
+
+    called: list[str] = []
+
+    def node_a(state: State) -> State:
+        called.append("node_a")
+        return {"value": ["a"]}
+
+    def ask_human(state: State) -> State:
+        called.append("ask_human")
+        answer = interrupt("What is your input?")
+        return {"value": [f"human:{answer}"]}
+
+    def node_b(state: State) -> State:
+        called.append("node_b")
+        return {"value": ["b"]}
+
+    graph = (
+        StateGraph(State)
+        .add_node("node_a", node_a)
+        .add_node("ask_human", ask_human)
+        .add_node("node_b", node_b)
+        .add_edge(START, "node_a")
+        .add_edge("node_a", "ask_human")
+        .add_edge("ask_human", "node_b")
+        .compile(checkpointer=sync_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # --- Original run: invoke until interrupt, then resume to complete ---
+    graph.invoke({"value": []}, config)
+    graph.invoke(Command(resume="old_answer"), config)
+
+    original_history = list(graph.get_state_history(config))
+    original = _checkpoint_summary(original_history)
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["a", "human:old_answer", "b"]}),
+        ("loop", ("node_b",), {"value": ["a", "human:old_answer"]}),
+        ("loop", ("ask_human",), {"value": ["a"]}),
+        ("loop", ("node_a",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # --- Replay from checkpoint before ask_human ---
+    before_ask = next(s for s in original_history if s.next == ("ask_human",))
+
+    called.clear()
+    replay_result = graph.invoke(None, before_ask.config)
+    assert replay_result["__interrupt__"][0].value == "What is your input?"
+    assert "ask_human" in called
+    assert "node_a" not in called  # before the replay point, not re-executed
+
+    # A fork checkpoint is now the latest — it branches from the replay point
+    post_replay = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"]) for s in post_replay] == [
+        ("fork", ("ask_human",)),  # <-- new fork (latest)
+        ("loop", ()),  # original done
+        ("loop", ("node_b",)),
+        ("loop", ("ask_human",)),  # branch point
+        ("loop", ("node_a",)),
+        ("input", ("__start__",)),
+    ]
+
+    # --- Resume with a new answer ---
+    called.clear()
+    final_result = graph.invoke(Command(resume="new_answer"), config)
+    assert final_result["value"] == ["a", "human:new_answer", "b"]
+    assert "ask_human" in called
+    assert "node_b" in called
+
+    final = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch (from fork)
+        ("loop", (), {"value": ["a", "human:new_answer", "b"]}),
+        ("loop", ("node_b",), {"value": ["a", "human:new_answer"]}),
+        ("fork", ("ask_human",), {"value": ["a"]}),
+        # Original branch (preserved)
+        ("loop", (), {"value": ["a", "human:old_answer", "b"]}),
+        ("loop", ("node_b",), {"value": ["a", "human:old_answer"]}),
+        ("loop", ("ask_human",), {"value": ["a"]}),
+        ("loop", ("node_a",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
 def test_replay_interrupt_stable_across_replays(
     sync_checkpointer: BaseCheckpointSaver,
 ) -> None:
@@ -320,8 +432,14 @@ def test_replay_interrupt_stable_across_replays(
         r = graph.invoke(None, before_ask.config)
         results.append(r)
 
-    assert all(r == results[0] for r in results)
-    assert "__interrupt__" in results[0]
+    # Each replay creates a fork with a unique interrupt ID, so we compare
+    # interrupt values and state values rather than full equality.
+    assert all("__interrupt__" in r for r in results)
+    assert all(
+        r["__interrupt__"][0].value == results[0]["__interrupt__"][0].value
+        for r in results
+    )
+    assert all(r["value"] == results[0]["value"] for r in results)
 
 
 def test_fork_from_before_interrupt_refires(
@@ -854,6 +972,290 @@ def test_subgraph_interrupt_replay_from_interrupt_checkpoint(
     assert "step_b" not in called
 
 
+def test_subgraph_interrupt_replay_from_parent_then_resume(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Replay from the parent checkpoint where a subgraph interrupt fired,
+    then resume with a new answer. Verifies that a fork is created and the
+    full graph completes. Checks full checkpoint history at each stage."""
+
+    called: list[str] = []
+
+    def router(state: State) -> State:
+        called.append("router")
+        return {"value": ["routed"]}
+
+    def step_a(state: State) -> State:
+        called.append("step_a")
+        return {"value": ["sub_a"]}
+
+    def ask_human(state: State) -> State:
+        called.append("ask_human")
+        answer = interrupt("Provide input:")
+        return {"value": [f"human:{answer}"]}
+
+    def step_b(state: State) -> State:
+        called.append("step_b")
+        return {"value": ["sub_b"]}
+
+    subgraph = (
+        StateGraph(State)
+        .add_node("step_a", step_a)
+        .add_node("ask_human", ask_human)
+        .add_node("step_b", step_b)
+        .add_edge(START, "step_a")
+        .add_edge("step_a", "ask_human")
+        .add_edge("ask_human", "step_b")
+        .compile(checkpointer=True)
+    )
+
+    def post_process(state: State) -> State:
+        called.append("post_process")
+        return {"value": ["post"]}
+
+    graph = (
+        StateGraph(State)
+        .add_node("router", router)
+        .add_node("subgraph_node", subgraph)
+        .add_node("post_process", post_process)
+        .add_edge(START, "router")
+        .add_edge("router", "subgraph_node")
+        .add_edge("subgraph_node", "post_process")
+        .compile(checkpointer=sync_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # Run until interrupt, then resume to complete
+    graph.invoke({"value": []}, config)
+    graph.invoke(Command(resume="old_answer"), config)
+
+    # Original parent history (newest first)
+    original_history = list(graph.get_state_history(config))
+    assert [s.next for s in original_history] == [
+        (),  # done
+        ("post_process",),
+        ("subgraph_node",),  # subgraph ran, interrupt fired here
+        ("router",),
+        ("__start__",),
+    ]
+
+    # Find the parent checkpoint where the interrupt fired
+    interrupt_checkpoint = next(
+        s for s in original_history if s.next == ("subgraph_node",)
+    )
+
+    # Replay from parent checkpoint — subgraph re-executes, interrupt re-fires
+    called.clear()
+    replay_result = graph.invoke(None, interrupt_checkpoint.config)
+    assert "__interrupt__" in replay_result
+    assert replay_result["__interrupt__"][0].value == "Provide input:"
+    assert "step_a" in called
+    assert "ask_human" in called
+    assert "step_b" not in called
+
+    # Verify fork checkpoint was created
+    post_replay_history = list(graph.get_state_history(config))
+    assert [s.next for s in post_replay_history] == [
+        ("subgraph_node",),  # fork (interrupt pending)
+        (),  # original done
+        ("post_process",),
+        ("subgraph_node",),
+        ("router",),
+        ("__start__",),
+    ]
+    assert [s.metadata["source"] for s in post_replay_history] == [
+        "fork",
+        "loop",
+        "loop",
+        "loop",
+        "loop",
+        "input",
+    ]
+    fork = post_replay_history[0]
+    assert (
+        fork.parent_config["configurable"]["checkpoint_id"]
+        == interrupt_checkpoint.config["configurable"]["checkpoint_id"]
+    )
+
+    # Resume with a new answer — full graph should complete
+    called.clear()
+    final_result = graph.invoke(Command(resume="new_answer"), config)
+    assert "__interrupt__" not in final_result
+    assert "human:new_answer" in final_result["value"]
+    assert "sub_b" in final_result["value"]
+    assert "post" in final_result["value"]
+    assert "ask_human" in called
+    assert "step_b" in called
+    assert "post_process" in called
+
+    # Final checkpoint history
+    final_history = list(graph.get_state_history(config))
+    assert [s.next for s in final_history] == [
+        (),  # new branch done
+        ("post_process",),  # new branch post_process
+        ("subgraph_node",),  # fork
+        (),  # original done
+        ("post_process",),
+        ("subgraph_node",),
+        ("router",),
+        ("__start__",),
+    ]
+    assert [s.metadata["source"] for s in final_history] == [
+        "loop",
+        "loop",
+        "fork",
+        "loop",
+        "loop",
+        "loop",
+        "loop",
+        "input",
+    ]
+
+
+def test_subgraph_replay_loads_accumulated_state_then_resume(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Two parent invocations, then replay from before the subgraph in the
+    2nd invocation. The subgraph (checkpointer=True) should load its
+    accumulated state from the 1st invocation via ReplayState, re-fire
+    the interrupt, and then resume + complete.
+
+    This tests the ReplayState path: the parent is replaying and the
+    subgraph uses list(before=parent_checkpoint_id) to find its
+    corresponding checkpoint from the original execution.
+    """
+
+    class SubState(TypedDict):
+        value: Annotated[list[str], operator.add]
+
+    class ParentState(TypedDict):
+        results: Annotated[list[str], operator.add]
+
+    started_state: list[dict] = []
+
+    def step_a(state: SubState) -> SubState:
+        started_state.append(dict(state))
+        answer = interrupt("question_a")
+        return {"value": [f"a:{answer}"]}
+
+    subgraph = (
+        StateGraph(SubState)
+        .add_node("step_a", step_a)
+        .add_edge(START, "step_a")
+        .compile(checkpointer=True)
+    )
+
+    def parent_node(state: ParentState) -> ParentState:
+        return {"results": ["p"]}
+
+    graph = (
+        StateGraph(ParentState)
+        .add_node("parent_node", parent_node)
+        .add_node("sub_node", subgraph)
+        .add_edge(START, "parent_node")
+        .add_edge("parent_node", "sub_node")
+        .compile(checkpointer=sync_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # === 1st invocation: complete with answer "a1" ===
+    graph.invoke({"results": []}, config)
+    graph.invoke(Command(resume="a1"), config)
+
+    # step_a saw empty state (fresh subgraph)
+    assert started_state[0] == {"value": []}
+
+    # === 2nd invocation: complete with answer "a2" ===
+    started_state.clear()
+    graph.invoke({"results": []}, config)
+    graph.invoke(Command(resume="a2"), config)
+
+    # Stateful subgraph retained state from 1st invocation
+    assert started_state[0] == {"value": ["a:a1"]}
+
+    # Original history (newest first)
+    original_history = list(graph.get_state_history(config))
+    assert [s.next for s in original_history] == [
+        (),  # 2nd done
+        ("sub_node",),  # 2nd sub_node
+        ("parent_node",),  # 2nd parent_node
+        ("__start__",),  # 2nd input
+        (),  # 1st done
+        ("sub_node",),  # 1st sub_node
+        ("parent_node",),  # 1st parent_node
+        ("__start__",),  # 1st input
+    ]
+
+    # Replay from before sub_node in 2nd invocation (newest match)
+    before_sub_2nd = [s for s in original_history if s.next == ("sub_node",)][0]
+    started_state.clear()
+    replay = graph.invoke(None, before_sub_2nd.config)
+    assert "__interrupt__" in replay
+
+    # Subgraph should see accumulated state from END of 1st invocation
+    assert started_state[0] == {"value": ["a:a1"]}
+
+    # Verify fork was created
+    post_replay_history = list(graph.get_state_history(config))
+    assert [s.next for s in post_replay_history] == [
+        ("sub_node",),  # fork (interrupt pending)
+        (),  # 2nd done
+        ("sub_node",),  # 2nd sub_node
+        ("parent_node",),  # 2nd parent_node
+        ("__start__",),  # 2nd input
+        (),  # 1st done
+        ("sub_node",),  # 1st sub_node
+        ("parent_node",),  # 1st parent_node
+        ("__start__",),  # 1st input
+    ]
+    assert [s.metadata["source"] for s in post_replay_history] == [
+        "fork",
+        "loop",
+        "loop",
+        "loop",
+        "input",
+        "loop",
+        "loop",
+        "loop",
+        "input",
+    ]
+
+    # Resume with a new answer
+    started_state.clear()
+    final = graph.invoke(Command(resume="a3"), config)
+    assert "__interrupt__" not in final
+    assert final["results"] == ["p", "p"]
+
+    # Final history
+    final_history = list(graph.get_state_history(config))
+    assert [s.next for s in final_history] == [
+        (),  # new branch done
+        ("sub_node",),  # fork
+        (),  # 2nd done
+        ("sub_node",),  # 2nd sub_node
+        ("parent_node",),  # 2nd parent_node
+        ("__start__",),  # 2nd input
+        (),  # 1st done
+        ("sub_node",),  # 1st sub_node
+        ("parent_node",),  # 1st parent_node
+        ("__start__",),  # 1st input
+    ]
+    assert [s.metadata["source"] for s in final_history] == [
+        "loop",
+        "fork",
+        "loop",
+        "loop",
+        "loop",
+        "input",
+        "loop",
+        "loop",
+        "loop",
+        "input",
+    ]
+
+
 def test_subgraph_interrupt_full_flow(
     sync_checkpointer: BaseCheckpointSaver,
 ) -> None:
@@ -1290,6 +1692,321 @@ def test_subgraph_time_travel_to_second_interrupt(
     assert "ask_1" not in called
 
 
+def test_subgraph_time_travel_resume_from_first_interrupt(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Time travel to a subgraph checkpoint at the first interrupt, then
+    resume through both interrupts with new answers.
+
+    This verifies the key bug fix: after time-traveling to a subgraph
+    checkpoint with an interrupt, a fork checkpoint is created so that
+    subsequent resumes find the correct state (not the old branch tip).
+
+    Parent:    START --> executor (subgraph, checkpointer=True) --> END
+    Executor:  START --> step_a --> ask_1 (interrupt) --> ask_2 (interrupt) --> END
+
+    Parent history after original run completes:
+      source=input  next=(__start__,)   values=[]
+      source=loop   next=(executor,)    values=[]
+      source=loop   next=()             values=[step_a_done, ask_1:answer_1, ask_2:answer_2]
+
+    After time-traveling to 1st interrupt + resuming with new answers:
+      source=input  next=(__start__,)   values=[]
+      source=loop   next=(executor,)    values=[]          <-- branch point
+      source=loop   next=()             values=[..., ask_2:answer_2]  (old branch)
+      source=fork   next=(executor,)    values=[]          <-- fork from time travel
+      source=loop   next=()             values=[..., ask_2:new_answer_2]  (new branch)
+    """
+
+    called: list[str] = []
+
+    def step_a(state: State) -> State:
+        called.append("step_a")
+        return {"value": ["step_a_done"]}
+
+    def ask_1(state: State) -> State:
+        called.append("ask_1")
+        answer = interrupt("Question 1?")
+        return {"value": [f"ask_1:{answer}"]}
+
+    def ask_2(state: State) -> State:
+        called.append("ask_2")
+        answer = interrupt("Question 2?")
+        return {"value": [f"ask_2:{answer}"]}
+
+    executor = (
+        StateGraph(State)
+        .add_node("step_a", step_a)
+        .add_node("ask_1", ask_1)
+        .add_node("ask_2", ask_2)
+        .add_edge(START, "step_a")
+        .add_edge("step_a", "ask_1")
+        .add_edge("ask_1", "ask_2")
+        .add_edge("ask_2", "__end__")
+        .compile(checkpointer=True)
+    )
+
+    graph = (
+        StateGraph(State)
+        .add_node("executor", executor)
+        .add_edge(START, "executor")
+        .compile(checkpointer=sync_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # --- Original run: hit both interrupts and resume ---
+    graph.invoke({"value": []}, config)
+    sub_config_at_first = graph.get_state(config, subgraphs=True).tasks[0].state.config
+    graph.invoke(Command(resume="answer_1"), config)
+    graph.invoke(Command(resume="answer_2"), config)
+
+    original = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # --- Time travel to first interrupt's subgraph checkpoint ---
+    called.clear()
+    replay_result = graph.invoke(None, sub_config_at_first)
+    assert replay_result["__interrupt__"][0].value == "Question 1?"
+    assert "step_a" not in called  # before interrupt, not re-executed
+
+    # Fork is now the latest parent checkpoint
+    post_tt = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"]) for s in post_tt] == [
+        ("fork", ("executor",)),  # <-- new fork (latest)
+        ("loop", ()),  # original done
+        ("loop", ("executor",)),
+        ("input", ("__start__",)),
+    ]
+
+    # --- Resume both interrupts with new answers ---
+    called.clear()
+    resume_1 = graph.invoke(Command(resume="new_answer_1"), config)
+    assert resume_1["__interrupt__"][0].value == "Question 2?"
+    assert "ask_1" in called
+
+    called.clear()
+    resume_2 = graph.invoke(Command(resume="new_answer_2"), config)
+    assert resume_2["value"] == [
+        "step_a_done",
+        "ask_1:new_answer_1",
+        "ask_2:new_answer_2",
+    ]
+
+    # Verify final history: original branch preserved, new branch appended
+    final = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch (from time travel fork)
+        (
+            "loop",
+            (),
+            {"value": ["step_a_done", "ask_1:new_answer_1", "ask_2:new_answer_2"]},
+        ),
+        ("fork", ("executor",), {"value": []}),
+        # Original branch (preserved)
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
+def test_subgraph_time_travel_resume_from_second_interrupt(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Time travel to a subgraph checkpoint at the second interrupt, then
+    resume with a new answer. The first interrupt's answer should be preserved.
+
+    Parent:    START --> executor (subgraph, checkpointer=True) --> END
+    Executor:  START --> step_a --> ask_1 (interrupt) --> ask_2 (interrupt) --> END
+
+    Key assertion: after resuming from a time-travel to the 2nd interrupt,
+    the final state keeps ask_1's original answer but uses the new ask_2 answer.
+    """
+
+    called: list[str] = []
+
+    def step_a(state: State) -> State:
+        called.append("step_a")
+        return {"value": ["step_a_done"]}
+
+    def ask_1(state: State) -> State:
+        called.append("ask_1")
+        answer = interrupt("Question 1?")
+        return {"value": [f"ask_1:{answer}"]}
+
+    def ask_2(state: State) -> State:
+        called.append("ask_2")
+        answer = interrupt("Question 2?")
+        return {"value": [f"ask_2:{answer}"]}
+
+    executor = (
+        StateGraph(State)
+        .add_node("step_a", step_a)
+        .add_node("ask_1", ask_1)
+        .add_node("ask_2", ask_2)
+        .add_edge(START, "step_a")
+        .add_edge("step_a", "ask_1")
+        .add_edge("ask_1", "ask_2")
+        .add_edge("ask_2", "__end__")
+        .compile(checkpointer=True)
+    )
+
+    graph = (
+        StateGraph(State)
+        .add_node("executor", executor)
+        .add_edge(START, "executor")
+        .compile(checkpointer=sync_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # --- Original run: hit both interrupts and resume ---
+    graph.invoke({"value": []}, config)
+    graph.invoke(Command(resume="answer_1"), config)
+    sub_config_at_second = graph.get_state(config, subgraphs=True).tasks[0].state.config
+    graph.invoke(Command(resume="answer_2"), config)
+
+    original = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # --- Time travel to second interrupt ---
+    called.clear()
+    replay_result = graph.invoke(None, sub_config_at_second)
+    assert replay_result["__interrupt__"][0].value == "Question 2?"
+    assert "step_a" not in called
+    assert "ask_1" not in called  # already resolved, not re-executed
+
+    # Fork is now the latest parent checkpoint
+    post_tt = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"]) for s in post_tt] == [
+        ("fork", ("executor",)),  # <-- new fork (latest)
+        ("loop", ()),  # original done
+        ("loop", ("executor",)),
+        ("input", ("__start__",)),
+    ]
+
+    # --- Resume with a new answer for ask_2 only ---
+    called.clear()
+    resume_result = graph.invoke(Command(resume="new_answer_2"), config)
+    # ask_1's original answer preserved, ask_2 uses the new answer
+    assert resume_result["value"] == [
+        "step_a_done",
+        "ask_1:answer_1",
+        "ask_2:new_answer_2",
+    ]
+
+    # Verify final history: original branch preserved, new branch appended
+    final = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch (from time travel fork)
+        (
+            "loop",
+            (),
+            {"value": ["step_a_done", "ask_1:answer_1", "ask_2:new_answer_2"]},
+        ),
+        ("fork", ("executor",), {"value": []}),
+        # Original branch (preserved)
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
+def test_subgraph_time_travel_checkpoint_pattern(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Verify the checkpoint pattern created by time travel to a subgraph
+    interrupt. A fork checkpoint should branch from the replay point and
+    become the latest parent checkpoint.
+
+    Parent:    START --> executor (subgraph, checkpointer=True) --> END
+    Executor:  START --> ask (interrupt) --> END
+
+    Original run (after completing):
+      source=input  next=(__start__,)  values=[]
+      source=loop   next=(executor,)   values=[]          <-- replay point
+      source=loop   next=()            values=[a:first]
+
+    After time travel to interrupt + resume with "second":
+      source=input  next=(__start__,)  values=[]
+      source=loop   next=(executor,)   values=[]          <-- branch point
+      source=loop   next=()            values=[a:first]   (old branch)
+      source=fork   next=(executor,)   values=[]          <-- fork
+      source=loop   next=()            values=[a:second]  (new branch)
+    """
+
+    def ask(state: State) -> State:
+        answer = interrupt("Q?")
+        return {"value": [f"a:{answer}"]}
+
+    executor = (
+        StateGraph(State)
+        .add_node("ask", ask)
+        .add_edge(START, "ask")
+        .compile(checkpointer=True)
+    )
+
+    graph = (
+        StateGraph(State)
+        .add_node("executor", executor)
+        .add_edge(START, "executor")
+        .compile(checkpointer=sync_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # Run until interrupt, then complete
+    graph.invoke({"value": []}, config)
+    sub_config = graph.get_state(config, subgraphs=True).tasks[0].state.config
+    graph.invoke(Command(resume="first"), config)
+
+    original = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["a:first"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # Time travel to the interrupt
+    graph.invoke(None, sub_config)
+
+    # Fork is now the latest, branching from the original replay point
+    post_tt = list(graph.get_state_history(config))
+    post_tt_summary = _checkpoint_summary(post_tt)
+    assert [(s["source"], s["next"]) for s in post_tt_summary] == [
+        ("fork", ("executor",)),  # <-- new fork (latest)
+        ("loop", ()),
+        ("loop", ("executor",)),  # <-- replay point / fork parent
+        ("input", ("__start__",)),
+    ]
+    # Verify the fork's parent is the original replay point
+    replay_point_id = sub_config["configurable"]["checkpoint_map"][""]
+    assert post_tt[0].parent_config["configurable"]["checkpoint_id"] == replay_point_id
+
+    # Resume from the fork — graph completes with new answer
+    result = graph.invoke(Command(resume="second"), config)
+    assert result["value"] == ["a:second"]
+
+    final = _checkpoint_summary(list(graph.get_state_history(config)))
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch
+        ("loop", (), {"value": ["a:second"]}),
+        ("fork", ("executor",), {"value": []}),
+        # Original branch
+        ("loop", (), {"value": ["a:first"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
 def test_subgraph_time_travel_after_completion(
     sync_checkpointer: BaseCheckpointSaver,
 ) -> None:
@@ -2283,14 +3000,16 @@ def test_replay_creates_branch_preserving_old_checkpoints(
     # -- Post-replay checkpoint history (newest first) --
     post_replay_history = list(graph.get_state_history(config))
     post_summary = _checkpoint_summary(post_replay_history)
-    assert len(post_summary) == 7  # 5 original + 2 new branch checkpoints
+    # 5 original + 1 fork + 2 new branch checkpoints = 8
+    assert len(post_summary) == 8
 
     # Verify the full shape after replay
     assert [s["next"] for s in post_summary] == [
-        (),  # new branch tip (C6)
-        ("node_c",),  # new branch (C5)
-        (),  # old branch tip (C4)
-        ("node_c",),  # old (C3)
+        (),  # new branch tip
+        ("node_c",),  # new branch
+        ("node_b",),  # fork from replay point
+        (),  # old branch tip
+        ("node_c",),  # old
         ("node_b",),  # branch point (C2)
         ("node_a",),  # old (C1)
         ("__start__",),  # old (C0)
@@ -2298,6 +3017,7 @@ def test_replay_creates_branch_preserving_old_checkpoints(
     assert [s["values"] for s in post_summary] == [
         {"value": ["a", "b2", "c"]},  # new branch tip
         {"value": ["a", "b2"]},  # new: node_b re-ran with call_count=2
+        {"value": ["a"]},  # fork from replay point
         {"value": ["a", "b1", "c"]},  # old branch tip preserved
         {"value": ["a", "b1"]},  # old
         {"value": ["a"]},  # branch point

libs/langgraph/tests/test_time_travel_async.py

@@ -46,6 +46,7 @@ def _checkpoint_summary(history: list) -> list[dict]:
     Returns a list of dicts (newest-first, matching get_state_history order) with:
       - id: short checkpoint id suffix (last 6 chars)
       - parent_id: short parent checkpoint id suffix or None
+      - source: checkpoint metadata source (input, loop, fork, update)
       - next: tuple of next node names
       - values: channel values snapshot
     """
@@ -61,6 +62,7 @@ def _checkpoint_summary(history: list) -> list[dict]:
             {
                 "id": cid[-6:],
                 "parent_id": pid[-6:] if pid else None,
+                "source": s.metadata.get("source"),
                 "next": s.next,
                 "values": s.values,
             }
@@ -335,8 +337,14 @@ async def test_replay_interrupt_stable_across_replays(
         r = await graph.ainvoke(None, before_ask.config)
         results.append(r)
 
-    assert all(r == results[0] for r in results)
-    assert "__interrupt__" in results[0]
+    # Each replay creates a fork with a unique interrupt ID, so we compare
+    # interrupt values and state values rather than full equality.
+    assert all("__interrupt__" in r for r in results)
+    assert all(
+        r["__interrupt__"][0].value == results[0]["__interrupt__"][0].value
+        for r in results
+    )
+    assert all(r["value"] == results[0]["value"] for r in results)
 
 
 @NEEDS_CONTEXTVARS
@@ -1261,6 +1269,391 @@ async def test_subgraph_time_travel_after_completion_async(
     assert "ask_2:answer_2" in replay_result["value"]
 
 
+@NEEDS_CONTEXTVARS
+async def test_replay_from_before_interrupt_then_resume_async(
+    async_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Replay from checkpoint before interrupt node, then resume with a new
+    answer and verify the graph completes with the new value.
+
+    Graph: START --> node_a --> ask_human (interrupt) --> node_b --> END
+    """
+
+    called: list[str] = []
+
+    async def node_a(state: State) -> State:
+        called.append("node_a")
+        return {"value": ["a"]}
+
+    async def ask_human(state: State) -> State:
+        called.append("ask_human")
+        answer = interrupt("What is your input?")
+        return {"value": [f"human:{answer}"]}
+
+    async def node_b(state: State) -> State:
+        called.append("node_b")
+        return {"value": ["b"]}
+
+    graph = (
+        StateGraph(State)
+        .add_node("node_a", node_a)
+        .add_node("ask_human", ask_human)
+        .add_node("node_b", node_b)
+        .add_edge(START, "node_a")
+        .add_edge("node_a", "ask_human")
+        .add_edge("ask_human", "node_b")
+        .compile(checkpointer=async_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # --- Original run: invoke until interrupt, then resume to complete ---
+    await graph.ainvoke({"value": []}, config)
+    await graph.ainvoke(Command(resume="old_answer"), config)
+
+    original_history = [s async for s in graph.aget_state_history(config)]
+    original = _checkpoint_summary(original_history)
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["a", "human:old_answer", "b"]}),
+        ("loop", ("node_b",), {"value": ["a", "human:old_answer"]}),
+        ("loop", ("ask_human",), {"value": ["a"]}),
+        ("loop", ("node_a",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # --- Replay from checkpoint before ask_human ---
+    before_ask = next(s for s in original_history if s.next == ("ask_human",))
+
+    called.clear()
+    replay_result = await graph.ainvoke(None, before_ask.config)
+    assert replay_result["__interrupt__"][0].value == "What is your input?"
+    assert "ask_human" in called
+    assert "node_a" not in called
+
+    # A fork checkpoint is now the latest
+    post_replay = _checkpoint_summary(
+        [s async for s in graph.aget_state_history(config)]
+    )
+    assert [(s["source"], s["next"]) for s in post_replay] == [
+        ("fork", ("ask_human",)),
+        ("loop", ()),
+        ("loop", ("node_b",)),
+        ("loop", ("ask_human",)),
+        ("loop", ("node_a",)),
+        ("input", ("__start__",)),
+    ]
+
+    # --- Resume with a new answer ---
+    called.clear()
+    final_result = await graph.ainvoke(Command(resume="new_answer"), config)
+    assert final_result["value"] == ["a", "human:new_answer", "b"]
+    assert "ask_human" in called
+    assert "node_b" in called
+
+    final = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch (from fork)
+        ("loop", (), {"value": ["a", "human:new_answer", "b"]}),
+        ("loop", ("node_b",), {"value": ["a", "human:new_answer"]}),
+        ("fork", ("ask_human",), {"value": ["a"]}),
+        # Original branch (preserved)
+        ("loop", (), {"value": ["a", "human:old_answer", "b"]}),
+        ("loop", ("node_b",), {"value": ["a", "human:old_answer"]}),
+        ("loop", ("ask_human",), {"value": ["a"]}),
+        ("loop", ("node_a",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
+@NEEDS_CONTEXTVARS
+async def test_subgraph_time_travel_resume_from_first_interrupt_async(
+    async_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Time travel to a subgraph checkpoint at the first interrupt, then
+    resume through both interrupts with new answers.
+
+    Parent:    START --> executor (subgraph, checkpointer=True) --> END
+    Executor:  START --> step_a --> ask_1 (interrupt) --> ask_2 (interrupt) --> END
+    """
+
+    called: list[str] = []
+
+    async def step_a(state: State) -> State:
+        called.append("step_a")
+        return {"value": ["step_a_done"]}
+
+    async def ask_1(state: State) -> State:
+        called.append("ask_1")
+        answer = interrupt("Question 1?")
+        return {"value": [f"ask_1:{answer}"]}
+
+    async def ask_2(state: State) -> State:
+        called.append("ask_2")
+        answer = interrupt("Question 2?")
+        return {"value": [f"ask_2:{answer}"]}
+
+    executor = (
+        StateGraph(State)
+        .add_node("step_a", step_a)
+        .add_node("ask_1", ask_1)
+        .add_node("ask_2", ask_2)
+        .add_edge(START, "step_a")
+        .add_edge("step_a", "ask_1")
+        .add_edge("ask_1", "ask_2")
+        .add_edge("ask_2", "__end__")
+        .compile(checkpointer=True)
+    )
+
+    graph = (
+        StateGraph(State)
+        .add_node("executor", executor)
+        .add_edge(START, "executor")
+        .compile(checkpointer=async_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # --- Original run: hit both interrupts and resume ---
+    await graph.ainvoke({"value": []}, config)
+    sub_config_at_first = (
+        (await graph.aget_state(config, subgraphs=True)).tasks[0].state.config
+    )
+    await graph.ainvoke(Command(resume="answer_1"), config)
+    await graph.ainvoke(Command(resume="answer_2"), config)
+
+    original = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # --- Time travel to first interrupt's subgraph checkpoint ---
+    called.clear()
+    replay_result = await graph.ainvoke(None, sub_config_at_first)
+    assert replay_result["__interrupt__"][0].value == "Question 1?"
+    assert "step_a" not in called
+
+    # Fork is now the latest parent checkpoint
+    post_tt = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"]) for s in post_tt] == [
+        ("fork", ("executor",)),  # <-- new fork (latest)
+        ("loop", ()),  # original done
+        ("loop", ("executor",)),
+        ("input", ("__start__",)),
+    ]
+
+    # --- Resume both interrupts with new answers ---
+    called.clear()
+    resume_1 = await graph.ainvoke(Command(resume="new_answer_1"), config)
+    assert resume_1["__interrupt__"][0].value == "Question 2?"
+    assert "ask_1" in called
+
+    called.clear()
+    resume_2 = await graph.ainvoke(Command(resume="new_answer_2"), config)
+    assert resume_2["value"] == [
+        "step_a_done",
+        "ask_1:new_answer_1",
+        "ask_2:new_answer_2",
+    ]
+
+    # Verify final history: original branch preserved, new branch appended
+    final = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch (from time travel fork)
+        (
+            "loop",
+            (),
+            {"value": ["step_a_done", "ask_1:new_answer_1", "ask_2:new_answer_2"]},
+        ),
+        ("fork", ("executor",), {"value": []}),
+        # Original branch (preserved)
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
+@NEEDS_CONTEXTVARS
+async def test_subgraph_time_travel_resume_from_second_interrupt_async(
+    async_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Time travel to a subgraph checkpoint at the second interrupt, then
+    resume with a new answer. The first interrupt's answer should be preserved.
+
+    Parent:    START --> executor (subgraph, checkpointer=True) --> END
+    Executor:  START --> step_a --> ask_1 (interrupt) --> ask_2 (interrupt) --> END
+    """
+
+    called: list[str] = []
+
+    async def step_a(state: State) -> State:
+        called.append("step_a")
+        return {"value": ["step_a_done"]}
+
+    async def ask_1(state: State) -> State:
+        called.append("ask_1")
+        answer = interrupt("Question 1?")
+        return {"value": [f"ask_1:{answer}"]}
+
+    async def ask_2(state: State) -> State:
+        called.append("ask_2")
+        answer = interrupt("Question 2?")
+        return {"value": [f"ask_2:{answer}"]}
+
+    executor = (
+        StateGraph(State)
+        .add_node("step_a", step_a)
+        .add_node("ask_1", ask_1)
+        .add_node("ask_2", ask_2)
+        .add_edge(START, "step_a")
+        .add_edge("step_a", "ask_1")
+        .add_edge("ask_1", "ask_2")
+        .add_edge("ask_2", "__end__")
+        .compile(checkpointer=True)
+    )
+
+    graph = (
+        StateGraph(State)
+        .add_node("executor", executor)
+        .add_edge(START, "executor")
+        .compile(checkpointer=async_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # --- Original run: hit both interrupts and resume ---
+    await graph.ainvoke({"value": []}, config)
+    await graph.ainvoke(Command(resume="answer_1"), config)
+    sub_config_at_second = (
+        (await graph.aget_state(config, subgraphs=True)).tasks[0].state.config
+    )
+    await graph.ainvoke(Command(resume="answer_2"), config)
+
+    original = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # --- Time travel to second interrupt ---
+    called.clear()
+    replay_result = await graph.ainvoke(None, sub_config_at_second)
+    assert replay_result["__interrupt__"][0].value == "Question 2?"
+    assert "step_a" not in called
+    assert "ask_1" not in called
+
+    # Fork is now the latest parent checkpoint
+    post_tt = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"]) for s in post_tt] == [
+        ("fork", ("executor",)),  # <-- new fork (latest)
+        ("loop", ()),  # original done
+        ("loop", ("executor",)),
+        ("input", ("__start__",)),
+    ]
+
+    # --- Resume with a new answer for ask_2 only ---
+    called.clear()
+    resume_result = await graph.ainvoke(Command(resume="new_answer_2"), config)
+    assert resume_result["value"] == [
+        "step_a_done",
+        "ask_1:answer_1",
+        "ask_2:new_answer_2",
+    ]
+
+    # Verify final history
+    final = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch (from time travel fork)
+        (
+            "loop",
+            (),
+            {"value": ["step_a_done", "ask_1:answer_1", "ask_2:new_answer_2"]},
+        ),
+        ("fork", ("executor",), {"value": []}),
+        # Original branch (preserved)
+        ("loop", (), {"value": ["step_a_done", "ask_1:answer_1", "ask_2:answer_2"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
+@NEEDS_CONTEXTVARS
+async def test_subgraph_time_travel_checkpoint_pattern_async(
+    async_checkpointer: BaseCheckpointSaver,
+) -> None:
+    """Verify the checkpoint pattern created by time travel to a subgraph
+    interrupt. A fork checkpoint should branch from the replay point.
+
+    Parent:    START --> executor (subgraph, checkpointer=True) --> END
+    Executor:  START --> ask (interrupt) --> END
+    """
+
+    async def ask(state: State) -> State:
+        answer = interrupt("Q?")
+        return {"value": [f"a:{answer}"]}
+
+    executor = (
+        StateGraph(State)
+        .add_node("ask", ask)
+        .add_edge(START, "ask")
+        .compile(checkpointer=True)
+    )
+
+    graph = (
+        StateGraph(State)
+        .add_node("executor", executor)
+        .add_edge(START, "executor")
+        .compile(checkpointer=async_checkpointer)
+    )
+
+    config = {"configurable": {"thread_id": "1"}}
+
+    # Run until interrupt, then complete
+    await graph.ainvoke({"value": []}, config)
+    sub_config = (await graph.aget_state(config, subgraphs=True)).tasks[0].state.config
+    await graph.ainvoke(Command(resume="first"), config)
+
+    original = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in original] == [
+        ("loop", (), {"value": ["a:first"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+    # Time travel to the interrupt
+    await graph.ainvoke(None, sub_config)
+
+    # Fork is now the latest, branching from the original replay point
+    post_tt = [s async for s in graph.aget_state_history(config)]
+    post_tt_summary = _checkpoint_summary(post_tt)
+    assert [(s["source"], s["next"]) for s in post_tt_summary] == [
+        ("fork", ("executor",)),  # <-- new fork (latest)
+        ("loop", ()),
+        ("loop", ("executor",)),  # <-- replay point / fork parent
+        ("input", ("__start__",)),
+    ]
+    # Verify the fork's parent is the original replay point
+    replay_point_id = sub_config["configurable"]["checkpoint_map"][""]
+    assert post_tt[0].parent_config["configurable"]["checkpoint_id"] == replay_point_id
+
+    # Resume from the fork
+    result = await graph.ainvoke(Command(resume="second"), config)
+    assert result["value"] == ["a:second"]
+
+    final = _checkpoint_summary([s async for s in graph.aget_state_history(config)])
+    assert [(s["source"], s["next"], s["values"]) for s in final] == [
+        # New branch
+        ("loop", (), {"value": ["a:second"]}),
+        ("fork", ("executor",), {"value": []}),
+        # Original branch
+        ("loop", (), {"value": ["a:first"]}),
+        ("loop", ("executor",), {"value": []}),
+        ("input", ("__start__",), {"value": []}),
+    ]
+
+
 @NEEDS_CONTEXTVARS
 async def test_3_levels_deep_time_travel_to_first_interrupt_async(
     async_checkpointer: BaseCheckpointSaver,
@@ -2088,13 +2481,15 @@ async def test_replay_creates_branch_preserving_old_checkpoints(
     # -- Post-replay checkpoint history (newest first) --
     post_replay_history = [s async for s in graph.aget_state_history(config)]
     post_summary = _checkpoint_summary(post_replay_history)
-    assert len(post_summary) == 7  # 5 original + 2 new branch checkpoints
+    # 5 original + 1 fork + 2 new branch checkpoints = 8
+    assert len(post_summary) == 8
 
     assert [s["next"] for s in post_summary] == [
-        (),  # new branch tip (C6)
-        ("node_c",),  # new branch (C5)
-        (),  # old branch tip (C4)
-        ("node_c",),  # old (C3)
+        (),  # new branch tip
+        ("node_c",),  # new branch
+        ("node_b",),  # fork from replay point
+        (),  # old branch tip
+        ("node_c",),  # old
         ("node_b",),  # branch point (C2)
         ("node_a",),  # old (C1)
         ("__start__",),  # old (C0)
@@ -2102,6 +2497,7 @@ async def test_replay_creates_branch_preserving_old_checkpoints(
     assert [s["values"] for s in post_summary] == [
         {"value": ["a", "b2", "c"]},  # new branch tip
         {"value": ["a", "b2"]},  # new: node_b re-ran with call_count=2
+        {"value": ["a"]},  # fork from replay point
         {"value": ["a", "b1", "c"]},  # old branch tip preserved
         {"value": ["a", "b1"]},  # old
         {"value": ["a"]},  # branch point

libs/langgraph/tests/test_tool_stream_handler.py

@@ -1,290 +0,0 @@
-"""Tests for StreamToolCallHandler and emit_tool_output_delta.
-
-These tests exercise the langgraph-core piece in isolation — the prebuilt
-`ToolCallTransformer` has its own test file. Here we feed real graphs
-through `Pregel.stream(stream_mode=["tools", ...])` and inspect the raw
-`(ns, mode, payload)` tuples on the `tools` channel.
-"""
-
-from __future__ import annotations
-
-from typing import Annotated, Any
-
-import pytest
-from langchain_core.messages import AIMessage
-from langchain_core.tools import tool
-from langgraph.prebuilt import ToolNode
-from typing_extensions import TypedDict
-
-from langgraph.config import emit_tool_output_delta
-from langgraph.constants import END, START
-from langgraph.graph import StateGraph
-from langgraph.graph.message import add_messages
-
-
-class _State(TypedDict):
-    messages: Annotated[list, add_messages]
-
-
-def _caller_sync(tool_name: str, tool_args: dict[str, Any], tc_id: str = "tc1"):
-    def caller(state: _State) -> dict:
-        return {
-            "messages": [
-                AIMessage(
-                    content="",
-                    tool_calls=[{"name": tool_name, "args": tool_args, "id": tc_id}],
-                )
-            ]
-        }
-
-    return caller
-
-
-def _caller_async(tool_name: str, tool_args: dict[str, Any], tc_id: str = "tc1"):
-    async def caller(state: _State) -> dict:
-        return {
-            "messages": [
-                AIMessage(
-                    content="",
-                    tool_calls=[{"name": tool_name, "args": tool_args, "id": tc_id}],
-                )
-            ]
-        }
-
-    return caller
-
-
-def _build_graph(caller, tools) -> Any:
-    sg = StateGraph(_State)
-    sg.add_node("caller", caller)
-    sg.add_node("tools", ToolNode(tools))
-    sg.add_edge(START, "caller")
-    sg.add_edge("caller", "tools")
-    sg.add_edge("tools", END)
-    return sg.compile()
-
-
-def _tool_events(stream) -> list[tuple[tuple[str, ...], dict]]:
-    """Collect `(ns, payload)` for every `tools`-mode chunk."""
-    out: list[tuple[tuple[str, ...], dict]] = []
-    for ns, mode, payload in stream:
-        if mode == "tools":
-            out.append((tuple(ns), payload))
-    return out
-
-
-class TestSyncGraphSyncTool:
-    def test_started_finished_cycle(self) -> None:
-        @tool
-        def echo(text: str) -> str:
-            """echo."""
-            return f"echoed:{text}"
-
-        graph = _build_graph(_caller_sync("echo", {"text": "hi"}), [echo])
-        events = _tool_events(
-            graph.stream(
-                {"messages": []},
-                stream_mode=["tools"],
-                subgraphs=True,
-            )
-        )
-
-        assert [p["event"] for _, p in events] == [
-            "tool-started",
-            "tool-finished",
-        ]
-        assert events[0][1]["tool_call_id"] == "tc1"
-        assert events[0][1]["tool_name"] == "echo"
-        assert events[0][1]["input"] == {"text": "hi"}
-        # ToolNode wraps the return in a ToolMessage.
-        assert events[1][1]["tool_call_id"] == "tc1"
-
-    def test_emit_tool_output_delta_produces_delta_events(self) -> None:
-        @tool
-        def streaming_echo(text: str) -> str:
-            """stream chunks."""
-            for chunk in ("a", "b", "c"):
-                emit_tool_output_delta(chunk)
-            return text
-
-        graph = _build_graph(
-            _caller_sync("streaming_echo", {"text": "x"}), [streaming_echo]
-        )
-        events = _tool_events(
-            graph.stream(
-                {"messages": []},
-                stream_mode=["tools"],
-                subgraphs=True,
-            )
-        )
-
-        deltas = [p["delta"] for _, p in events if p["event"] == "tool-output-delta"]
-        assert deltas == ["a", "b", "c"]
-        # The deltas must be bracketed by started and finished.
-        ordered = [p["event"] for _, p in events]
-        assert ordered[0] == "tool-started"
-        assert ordered[-1] == "tool-finished"
-
-    def test_tool_error_event(self) -> None:
-        @tool
-        def boom() -> str:
-            """raises."""
-            raise ValueError("nope")
-
-        graph = _build_graph(_caller_sync("boom", {}), [boom])
-        events: list[tuple[tuple[str, ...], dict]] = []
-        with pytest.raises(ValueError, match="nope"):
-            for ns, mode, payload in graph.stream(
-                {"messages": []},
-                stream_mode=["tools"],
-                subgraphs=True,
-            ):
-                if mode == "tools":
-                    events.append((tuple(ns), payload))
-
-        kinds = [p["event"] for _, p in events]
-        assert kinds == ["tool-started", "tool-error"]
-        assert events[1][1]["message"] == "nope"
-
-    def test_emit_outside_tool_is_noop(self) -> None:
-        # Called at import time (outside any tool body) — must not raise.
-        emit_tool_output_delta("ignored")
-        emit_tool_output_delta({"any": "payload"})
-
-    def test_no_events_without_tools_mode(self) -> None:
-        @tool
-        def echo(text: str) -> str:
-            """echo."""
-            return text
-
-        graph = _build_graph(_caller_sync("echo", {"text": "hi"}), [echo])
-        # No "tools" in stream_mode — handler is not attached and zero
-        # `tools`-method events fire.
-        chunks = list(
-            graph.stream(
-                {"messages": []},
-                stream_mode=["values"],
-                subgraphs=True,
-            )
-        )
-        assert all(
-            not (isinstance(c, tuple) and len(c) == 3 and c[1] == "tools")
-            for c in chunks
-        )
-
-
-class TestAsyncGraphAsyncTool:
-    @pytest.mark.anyio
-    async def test_async_tool_produces_events(self) -> None:
-        @tool
-        async def aecho(text: str) -> str:
-            """async echo."""
-            emit_tool_output_delta(text)
-            return f"got:{text}"
-
-        graph = _build_graph(_caller_async("aecho", {"text": "hi"}), [aecho])
-        events: list[tuple[tuple[str, ...], dict]] = []
-        async for ns, mode, payload in graph.astream(
-            {"messages": []},
-            stream_mode=["tools"],
-            subgraphs=True,
-        ):
-            if mode == "tools":
-                events.append((tuple(ns), payload))
-
-        kinds = [p["event"] for _, p in events]
-        assert kinds == ["tool-started", "tool-output-delta", "tool-finished"]
-        assert events[1][1]["delta"] == "hi"
-
-
-class TestConcurrentToolCalls:
-    def test_parallel_tool_calls_do_not_bleed(self) -> None:
-        @tool
-        def streamer(marker: str) -> str:
-            """emits marker twice."""
-            emit_tool_output_delta(f"{marker}-1")
-            emit_tool_output_delta(f"{marker}-2")
-            return marker
-
-        def caller(state: _State) -> dict:
-            return {
-                "messages": [
-                    AIMessage(
-                        content="",
-                        tool_calls=[
-                            {"name": "streamer", "args": {"marker": "A"}, "id": "a"},
-                            {"name": "streamer", "args": {"marker": "B"}, "id": "b"},
-                        ],
-                    )
-                ]
-            }
-
-        graph = _build_graph(caller, [streamer])
-        events = _tool_events(
-            graph.stream(
-                {"messages": []},
-                stream_mode=["tools"],
-                subgraphs=True,
-            )
-        )
-
-        # Group deltas by tool_call_id.
-        by_id: dict[str, list[str]] = {}
-        for _, p in events:
-            if p["event"] == "tool-output-delta":
-                by_id.setdefault(p["tool_call_id"], []).append(p["delta"])
-        assert by_id["a"] == ["A-1", "A-2"]
-        assert by_id["b"] == ["B-1", "B-2"]
-
-
-class TestSubgraphNamespacePropagation:
-    def test_tool_inside_subgraph_emits_with_subgraph_ns(self) -> None:
-        @tool
-        def inner_tool(text: str) -> str:
-            """inner tool."""
-            return text
-
-        def sub_caller(state: _State) -> dict:
-            return {
-                "messages": [
-                    AIMessage(
-                        content="",
-                        tool_calls=[
-                            {
-                                "name": "inner_tool",
-                                "args": {"text": "x"},
-                                "id": "tc1",
-                            }
-                        ],
-                    )
-                ]
-            }
-
-        inner = StateGraph(_State)
-        inner.add_node("sub_caller", sub_caller)
-        inner.add_node("sub_tools", ToolNode([inner_tool]))
-        inner.add_edge(START, "sub_caller")
-        inner.add_edge("sub_caller", "sub_tools")
-        inner.add_edge("sub_tools", END)
-        inner_graph = inner.compile()
-
-        outer = StateGraph(_State)
-        outer.add_node("sub", inner_graph)
-        outer.add_edge(START, "sub")
-        outer.add_edge("sub", END)
-        graph = outer.compile()
-
-        events = _tool_events(
-            graph.stream(
-                {"messages": []},
-                stream_mode=["tools"],
-                subgraphs=True,
-            )
-        )
-
-        # All `tools` events should carry a non-empty namespace rooted
-        # at the `sub` node.
-        assert events, "expected at least one tools event"
-        for ns, _ in events:
-            assert ns  # non-empty
-            assert ns[0].startswith("sub:")

libs/langgraph/uv.lock

@@ -1348,11 +1348,10 @@ wheels = [
 
 [[package]]
 name = "langchain-core"
-version = "1.3.2"
+version = "1.3.0a2"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "jsonpatch" },
-    { name = "langchain-protocol" },
     { name = "langsmith" },
     { name = "packaging" },
     { name = "pydantic" },
@@ -1361,21 +1360,9 @@ dependencies = [
     { name = "typing-extensions" },
     { name = "uuid-utils" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/a8/03/7219502e8ca728d65eb44d7a3eb60239230742a70dbfc9241b9bfd61c4ab/langchain_core-1.3.2.tar.gz", hash = "sha256:fd7a50b2f28ba561fd9d7f5d2760bc9e06cf00cdf820a3ccafe88a94ffa8d5b7", size = 911813, upload-time = "2026-04-24T15:49:23.699Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/af/bc/0bff31fcaff174d86031cc713471a3e85ed4ec8e5cd95ad0217f2aced20e/langchain_core-1.3.0a2.tar.gz", hash = "sha256:52d978c84552b74b9a3f16c1fced84f9e27cc96d7a67c601925ce6cbc4ea3cf9", size = 854580, upload-time = "2026-04-13T14:37:55.745Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/7d/d5/8fa4431007cbb7cfed7590f4d6a5dea3ad724f4174d248f6642ef5ce7d05/langchain_core-1.3.2-py3-none-any.whl", hash = "sha256:d44a66127f9f8db735bdfd0ab9661bccb47a97113cfd3f2d89c74864422b7274", size = 542390, upload-time = "2026-04-24T15:49:21.991Z" },
-]
-
-[[package]]
-name = "langchain-protocol"
-version = "0.0.12"
-source = { registry = "https://pypi.org/simple" }
-dependencies = [
-    { name = "typing-extensions" },
-]
-sdist = { url = "https://files.pythonhosted.org/packages/5c/51/1157009b6f94e6e58be58fa8b620187d657909a8b36a6bf5b0c52a2711f6/langchain_protocol-0.0.12.tar.gz", hash = "sha256:5e14c434290a705c9510fdb1a83ecf7561a5e6e0dfd053930ade80dba069269f", size = 6408, upload-time = "2026-04-25T01:05:01.489Z" }
-wheels = [
-    { url = "https://files.pythonhosted.org/packages/95/82/3431e3061c917439589fa88a6b23c9bc0e154cba0f05d2e895a68c76ff74/langchain_protocol-0.0.12-py3-none-any.whl", hash = "sha256:402b61f42d4139692528cf37226c367bb6efc8ff8165b29380accb0abfece7b2", size = 6639, upload-time = "2026-04-25T01:05:00.487Z" },
+    { url = "https://files.pythonhosted.org/packages/0e/14/03c09686602567059f26af29de0c44546a83af2f2aa29925e61040e43ea2/langchain_core-1.3.0a2-py3-none-any.whl", hash = "sha256:9e929a34f0b0c6c1255e395a1de34f8626893ceb4cdae550a22a0bd18c87be54", size = 510233, upload-time = "2026-04-13T14:37:54.277Z" },
 ]
 
 [[package]]
@@ -1452,7 +1439,7 @@ test = [
 
 [package.metadata]
 requires-dist = [
-    { name = "langchain-core", specifier = ">=1.3.2" },
+    { name = "langchain-core", specifier = "==1.3.0a2" },
     { name = "langgraph-checkpoint", editable = "../checkpoint" },
     { name = "langgraph-prebuilt", editable = "../prebuilt" },
     { name = "langgraph-sdk", editable = "../sdk-py" },
@@ -1561,7 +1548,7 @@ wheels = [
 
 [[package]]
 name = "langgraph-checkpoint"
-version = "4.0.1"
+version = "4.0.2"
 source = { editable = "../checkpoint" }
 dependencies = [
     { name = "langchain-core" },
@@ -1865,7 +1852,7 @@ test = [
 
 [[package]]
 name = "langsmith"
-version = "0.6.4"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -1875,11 +1862,12 @@ dependencies = [
     { name = "requests" },
     { name = "requests-toolbelt" },
     { name = "uuid-utils" },
+    { name = "xxhash" },
     { name = "zstandard" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/e7/85/9c7933052a997da1b85bc5c774f3865e9b1da1c8d71541ea133178b13229/langsmith-0.6.4.tar.gz", hash = "sha256:36f7223a01c218079fbb17da5e536ebbaf5c1468c028abe070aa3ae59bc99ec8", size = 919964, upload-time = "2026-01-15T20:02:28.873Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/e6/11/696019490992db5c87774dc20515529ef42a01e1d770fb754ed6d9b12fb0/langsmith-0.7.31.tar.gz", hash = "sha256:331ee4f7c26bb5be4022b9859b7d7b122cbf8c9d01d9f530114c1914b0349ffb", size = 1178480, upload-time = "2026-04-14T17:55:41.242Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/66/0f/09a6637a7ba777eb307b7c80852d9ee26438e2bdafbad6fcc849ff9d9192/langsmith-0.6.4-py3-none-any.whl", hash = "sha256:ac4835860160be371042c7adbba3cb267bcf8d96a5ea976c33a8a4acad6c5486", size = 283503, upload-time = "2026-01-15T20:02:26.662Z" },
+    { url = "https://files.pythonhosted.org/packages/1d/a1/a013cf458c301cda86a213dd153ce0a01c93f1ab5833f951e6a44c9763ce/langsmith-0.7.31-py3-none-any.whl", hash = "sha256:0291d49203f6e80dda011af1afda61eb0595a4d697adb684590a8805e1d61fb6", size = 373276, upload-time = "2026-04-14T17:55:39.677Z" },
 ]
 
 [package.optional-dependencies]

libs/prebuilt/langgraph/prebuilt/__init__.py

@@ -1,7 +1,5 @@
 """langgraph.prebuilt exposes a higher-level API for creating and executing agents and tools."""
 
-from langgraph.prebuilt._tool_call_stream import ToolCallStream
-from langgraph.prebuilt._tool_call_transformer import ToolCallTransformer
 from langgraph.prebuilt.chat_agent_executor import create_react_agent
 from langgraph.prebuilt.tool_node import (
     InjectedState,
@@ -15,8 +13,6 @@ from langgraph.prebuilt.tool_validator import ValidationNode
 __all__ = [
     "create_react_agent",
     "ToolNode",
-    "ToolCallStream",
-    "ToolCallTransformer",
     "tools_condition",
     "ValidationNode",
     "InjectedState",

libs/prebuilt/langgraph/prebuilt/_tool_call_stream.py

@@ -1,117 +0,0 @@
-"""In-process handle for a single tool call's streaming execution.
-
-Mirrors the shape of `ChatModelStream` from langchain-core but simpler —
-a tool has one output channel, no content-block multiplexing. Populated
-by `ToolCallTransformer` as `tool-started` / `tool-output-delta` /
-`tool-finished` / `tool-error` events flow in on the `tools` channel.
-"""
-
-from __future__ import annotations
-
-from collections.abc import AsyncIterator, Iterator
-from typing import Any
-
-from langgraph.stream._event_log import EventLog
-
-
-class ToolCallStream:
-    """Scoped view of a single tool call's lifecycle.
-
-    Yielded on `run.tool_calls` once per `tool-started` event. Fields
-    are populated as events arrive:
-
-    - `tool_call_id`, `tool_name`, `input`: stable from the start event.
-    - `output_deltas`: an `EventLog` of delta chunks. Iterate (sync or
-      async) to consume partial output in arrival order.
-    - `output`: terminal payload from `tool-finished`, or `None` if the
-      call failed or is still in flight.
-    - `error`: terminal error string from `tool-error`, or `None` if the
-      call succeeded or is still in flight.
-    - `completed`: True once a terminal event (`tool-finished` or
-      `tool-error`) has been observed.
-
-    `ToolCallStream` is not meant to be constructed by end users — it's
-    produced by `ToolCallTransformer` as events flow through the mux.
-    """
-
-    def __init__(
-        self,
-        tool_call_id: str,
-        tool_name: str,
-        input: dict[str, Any] | None = None,
-    ) -> None:
-        """Initialize a fresh handle for a tool call.
-
-        Args:
-            tool_call_id: The `tool_call_id` from the AIMessage.
-            tool_name: The tool's name.
-            input: The tool's input arguments (as reported by
-                `on_tool_start`), or `None` if none were captured.
-        """
-        self.tool_call_id = tool_call_id
-        self.tool_name = tool_name
-        self.input = input
-        self._output_deltas: EventLog[Any] = EventLog()
-        self.output: Any = None
-        self.error: str | None = None
-        self.completed = False
-
-    @property
-    def output_deltas(self) -> EventLog[Any]:
-        """The EventLog of streamed `tool-output-delta` payloads.
-
-        Iterate (sync or async depending on how the run was started)
-        to consume partial output in arrival order. The log closes when
-        the tool finishes or errors.
-        """
-        return self._output_deltas
-
-    def _bind(self, *, is_async: bool) -> None:
-        """Bind the deltas log to sync or async iteration.
-
-        Called by `ToolCallTransformer` when constructing this handle so
-        the log matches the enclosing mux's mode.
-        """
-        self._output_deltas._bind(is_async=is_async)
-
-    def _push_delta(self, delta: Any) -> None:
-        self._output_deltas.push(delta)
-
-    def _finish(self, output: Any) -> None:
-        self.output = output
-        self.completed = True
-        self._output_deltas.close()
-
-    def _fail(self, message: str) -> None:
-        self.error = message
-        self.completed = True
-        self._output_deltas.close()
-
-    def __iter__(self) -> Iterator[Any]:
-        """Iterate delta chunks synchronously.
-
-        Equivalent to `iter(self.output_deltas)`. Raises `TypeError` if
-        the underlying log is bound to async mode.
-        """
-        return iter(self._output_deltas)
-
-    def __aiter__(self) -> AsyncIterator[Any]:
-        """Iterate delta chunks asynchronously.
-
-        Equivalent to `aiter(self.output_deltas)`. Raises `TypeError`
-        if the underlying log is bound to sync mode.
-        """
-        return self._output_deltas.__aiter__()
-
-    def __repr__(self) -> str:
-        status = (
-            "completed"
-            if self.completed and self.error is None
-            else "failed"
-            if self.completed
-            else "running"
-        )
-        return (
-            f"ToolCallStream(tool_call_id={self.tool_call_id!r}, "
-            f"tool_name={self.tool_name!r}, status={status})"
-        )

libs/prebuilt/langgraph/prebuilt/_tool_call_transformer.py

@@ -1,128 +0,0 @@
-"""Transformer that projects `tools` channel events into `ToolCallStream`s."""
-
-from __future__ import annotations
-
-from collections.abc import Awaitable, Callable
-from typing import Any
-
-from langgraph.stream._event_log import EventLog
-from langgraph.stream._types import ProtocolEvent, StreamTransformer
-
-from langgraph.prebuilt._tool_call_stream import ToolCallStream
-
-
-class ToolCallTransformer(StreamTransformer):
-    """Project `tools` channel events into `ToolCallStream` handles.
-
-    Each `tool-started` event spawns a `ToolCallStream`, pushed onto
-    `run.tool_calls`. Subsequent `tool-output-delta` events append to
-    that stream's deltas log; `tool-finished` and `tool-error` close it.
-
-    Native transformer — the `tool_calls` projection is exposed as a
-    direct attribute on the run stream.
-
-    `EventLog[ToolCallStream]` is used (not `StreamChannel`) because the
-    live handles are not serializable and should not be auto-forwarded
-    onto the main event log. Wire consumers subscribe to the `tools`
-    channel instead, where the raw protocol events flow through
-    untouched by this transformer (`process` returns `True`).
-
-    Registered explicitly by users at compile time via
-    `builder.compile(transformers=[ToolCallTransformer])` — not a
-    default built-in, so the `tools` channel is user-opt-in.
-    """
-
-    _native = True
-    required_stream_modes = ("tools",)
-
-    def __init__(self, scope: tuple[str, ...] = ()) -> None:
-        super().__init__(scope)
-        self._log: EventLog[ToolCallStream] = EventLog()
-        self._active: dict[str, ToolCallStream] = {}
-        self._is_async = False
-        self._pump_fn: Callable[[], bool] | None = None
-        self._apump_fn: Callable[[], Awaitable[bool]] | None = None
-
-    def init(self) -> dict[str, Any]:
-        return {"tool_calls": self._log}
-
-    def _bind_pump(self, fn: Callable[[], bool]) -> None:
-        """Wire the sync pull callback onto this transformer.
-
-        Called by `StreamMux.bind_pump`. Stored so each new
-        `ToolCallStream` created by `process` can wire its deltas log
-        for pump-driven iteration.
-        """
-        self._pump_fn = fn
-        self._is_async = False
-
-    def _bind_apump(self, fn: Callable[[], Awaitable[bool]]) -> None:
-        """Async counterpart to `_bind_pump`."""
-        self._apump_fn = fn
-        self._is_async = True
-
-    def _new_stream(
-        self,
-        tool_call_id: str,
-        tool_name: str,
-        tool_input: dict[str, Any] | None,
-    ) -> ToolCallStream:
-        stream = ToolCallStream(tool_call_id, tool_name, tool_input)
-        stream._bind(is_async=self._is_async)
-        if self._apump_fn is not None:
-            stream._output_deltas._arequest_more = self._apump_fn
-        if self._pump_fn is not None:
-            stream._output_deltas._request_more = self._pump_fn
-        return stream
-
-    def process(self, event: ProtocolEvent) -> bool:
-        # Namespace filtering is handled by the mux via `scope_exact`.
-        if event["method"] != "tools":
-            return True
-
-        data = event["params"]["data"]
-        tool_call_id = data.get("tool_call_id")
-        if tool_call_id is None:
-            return True
-        event_type = data.get("event")
-
-        stream: ToolCallStream | None
-        if event_type == "tool-started":
-            stream = self._new_stream(
-                tool_call_id,
-                data.get("tool_name", ""),
-                data.get("input"),
-            )
-            self._active[tool_call_id] = stream
-            self._log.push(stream)
-        elif event_type == "tool-output-delta":
-            stream = self._active.get(tool_call_id)
-            if stream is not None:
-                stream._push_delta(data.get("delta"))
-        elif event_type == "tool-finished":
-            stream = self._active.pop(tool_call_id, None)
-            if stream is not None:
-                stream._finish(data.get("output"))
-        elif event_type == "tool-error":
-            stream = self._active.pop(tool_call_id, None)
-            if stream is not None:
-                stream._fail(data.get("message", ""))
-
-        # Pass-through — wire consumers subscribe to the `tools` channel
-        # directly and reconstruct handles client-side.
-        return True
-
-    def finalize(self) -> None:
-        """Close any still-active tool streams left open at run end."""
-        for stream in self._active.values():
-            if not stream.completed:
-                stream._finish(None)
-        self._active.clear()
-
-    def fail(self, err: BaseException) -> None:
-        """Fail any still-active tool streams when the run errors."""
-        message = str(err)
-        for stream in self._active.values():
-            if not stream.completed:
-                stream._fail(message)
-        self._active.clear()

libs/prebuilt/langgraph/prebuilt/tool_node.py

@@ -614,6 +614,7 @@ class _InjectedArgs:
     store: str | None
     runtime: str | None
     all_injected_keys: set[str]
+    _optional_state_args: set[str]
 
 
 class ToolNode(RunnableCallable):
@@ -1333,7 +1334,7 @@ class ToolNode(RunnableCallable):
             return tool_call
 
         tool_call_copy: ToolCall = copy(tool_call)
-        injected_args = {}
+        injected_args: dict[str, Any] = {}
 
         # Inject state
         if injected.state:
@@ -1361,14 +1362,20 @@ class ToolNode(RunnableCallable):
             # Extract state values
             if isinstance(state, dict):
                 for tool_arg, state_field in injected.state.items():
-                    injected_args[tool_arg] = (
-                        state[state_field] if state_field else state
-                    )
+                    if not state_field:
+                        injected_args[tool_arg] = state
+                    elif state_field in state:
+                        injected_args[tool_arg] = state[state_field]
+                    elif tool_arg not in injected._optional_state_args:
+                        raise KeyError(state_field)
             else:
                 for tool_arg, state_field in injected.state.items():
-                    injected_args[tool_arg] = (
-                        getattr(state, state_field) if state_field else state
-                    )
+                    if not state_field:
+                        injected_args[tool_arg] = state
+                    elif hasattr(state, state_field):
+                        injected_args[tool_arg] = getattr(state, state_field)
+                    elif tool_arg not in injected._optional_state_args:
+                        raise AttributeError(state_field)
 
         # Inject store
         if injected.store:
@@ -1859,6 +1866,7 @@ def _get_all_injected_args(tool: BaseTool) -> _InjectedArgs:
     store_arg: str | None = None
     runtime_arg: str | None = None
     all_injected_keys: set[str] = set()
+    _optional_state_args: set[str] = set()
 
     for name, type_ in all_annotations.items():
         # Track all InjectedToolArg-annotated params (including custom subclasses)
@@ -1873,6 +1881,9 @@ def _get_all_injected_args(tool: BaseTool) -> _InjectedArgs:
         if state_inj := _get_injection_from_type(type_, InjectedState):
             if isinstance(state_inj, InjectedState) and state_inj.field:
                 state_args[name] = state_inj.field
+                field_info = full_schema.model_fields.get(name)
+                if field_info and not field_info.is_required():
+                    _optional_state_args.add(name)
             else:
                 state_args[name] = None
 
@@ -1889,4 +1900,5 @@ def _get_all_injected_args(tool: BaseTool) -> _InjectedArgs:
         store=store_arg,
         runtime=runtime_arg,
         all_injected_keys=all_injected_keys,
+        _optional_state_args=_optional_state_args,
     )

libs/prebuilt/tests/test_injected_state_not_required.py

@@ -0,0 +1,323 @@
+"""Test InjectedState with NotRequired state fields.
+
+This tests the fix for https://github.com/langchain-ai/langchain/issues/35585
+
+When using InjectedState(<field>) on a tool parameter, and the referenced field is
+declared as NotRequired in the custom state schema, the ToolNode should gracefully
+handle missing fields without raising KeyError so the tool's default can apply.
+"""
+
+import sys
+from typing import Annotated
+
+import pytest
+from langchain_core.messages import AIMessage, AnyMessage, HumanMessage, ToolMessage
+from langchain_core.tools import tool
+from langgraph.graph.message import add_messages
+from pydantic import BaseModel, Field
+from typing_extensions import NotRequired
+
+from langgraph.prebuilt import InjectedState, ToolNode, create_react_agent
+from langgraph.prebuilt.chat_agent_executor import AgentState
+
+from .model import FakeToolCallingModel
+
+
+class CustomAgentStateWithNotRequired(AgentState):
+    """Custom state with a NotRequired field (TypedDict style)."""
+
+    city: NotRequired[str]
+
+
+class CustomAgentStatePydanticWithDefault(BaseModel):
+    """Custom state with Optional field and default (Pydantic style)."""
+
+    messages: Annotated[list[AnyMessage], add_messages]
+    remaining_steps: int = Field(default=10)
+    city: str | None = Field(default=None)
+
+
+@tool
+def get_weather(city: Annotated[str | None, InjectedState("city")] = None) -> str:
+    """Get weather for a given city."""
+    if city is None:
+        return "No city provided"
+    return f"It's always sunny in {city}!"
+
+
+@tool
+def get_weather_with_default(
+    city: Annotated[str, InjectedState("city")] = "Boston",
+) -> str:
+    """Get weather for a given city, defaulting when state omits the field."""
+    return f"It's always sunny in {city}!"
+
+
+def _create_mock_runtime(
+    state: dict | None = None,
+    store=None,
+):
+    """Create a mock Runtime for testing ToolNode directly."""
+    from unittest.mock import Mock
+
+    from langgraph.runtime import Runtime
+
+    mock_runtime = Mock(spec=Runtime)
+    mock_runtime.context = {}
+    return mock_runtime
+
+
+def _create_config_with_runtime(store=None, state=None):
+    """Create a RunnableConfig with mocked runtime for direct ToolNode testing."""
+    from langgraph.prebuilt.tool_node import ToolRuntime
+
+    tool_runtime = ToolRuntime(
+        state=state or {},
+        config={},
+        context={},
+        store=store,
+        stream_writer=None,
+        tool_call_id="test_id",
+    )
+    return {
+        "configurable": {
+            "__pregel_runtime": _create_mock_runtime(),
+            "__tool_runtime__": tool_runtime,
+        }
+    }
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 11),
+    reason="InjectedState field extraction from Optional[Annotated[...]] not supported on Python <3.11",
+)
+def test_injected_state_not_required_field_missing_injects_none():
+    """Test that missing optional InjectedState leaves the tool default in place.
+
+    This verifies the fix for https://github.com/langchain-ai/langchain/issues/35585
+    """
+    tool_node = ToolNode([get_weather])
+
+    tool_call = {
+        "name": "get_weather",
+        "args": {},
+        "id": "call_1",
+        "type": "tool_call",
+    }
+    ai_msg = AIMessage("Let me check the weather", tool_calls=[tool_call])
+
+    # State WITHOUT the "city" field - should inject None instead of raising KeyError
+    state_without_city: CustomAgentStateWithNotRequired = {
+        "messages": [HumanMessage("What's the weather?"), ai_msg],
+    }
+
+    result = tool_node.invoke(
+        state_without_city,
+        config=_create_config_with_runtime(state=state_without_city),
+    )
+
+    assert len(result["messages"]) == 1
+    tool_msg = result["messages"][0]
+    assert isinstance(tool_msg, ToolMessage)
+    assert "No city provided" in tool_msg.content
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 11),
+    reason="InjectedState field extraction from Optional[Annotated[...]] not supported on Python <3.11",
+)
+def test_injected_state_not_required_field_missing_preserves_tool_default():
+    """Test that missing optional InjectedState preserves a non-None tool default."""
+    tool_node = ToolNode([get_weather_with_default])
+
+    tool_call = {
+        "name": "get_weather_with_default",
+        "args": {},
+        "id": "call_1",
+        "type": "tool_call",
+    }
+    ai_msg = AIMessage("Let me check the weather", tool_calls=[tool_call])
+
+    state_without_city: CustomAgentStateWithNotRequired = {
+        "messages": [HumanMessage("What's the weather?"), ai_msg],
+    }
+
+    result = tool_node.invoke(
+        state_without_city,
+        config=_create_config_with_runtime(state=state_without_city),
+    )
+
+    assert len(result["messages"]) == 1
+    tool_msg = result["messages"][0]
+    assert isinstance(tool_msg, ToolMessage)
+    assert "Boston" in tool_msg.content
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 11),
+    reason="InjectedState field extraction from Optional[Annotated[...]] not supported on Python <3.11",
+)
+def test_injected_state_not_required_field_present_works():
+    """Test that InjectedState with NotRequired field works when field IS present."""
+    tool_node = ToolNode([get_weather])
+
+    tool_call = {
+        "name": "get_weather",
+        "args": {},
+        "id": "call_1",
+        "type": "tool_call",
+    }
+    ai_msg = AIMessage("Let me check the weather", tool_calls=[tool_call])
+
+    # State WITH the "city" field - this should work
+    state_with_city: CustomAgentStateWithNotRequired = {
+        "messages": [HumanMessage("What's the weather?"), ai_msg],
+        "city": "San Francisco",
+    }
+
+    result = tool_node.invoke(
+        state_with_city,
+        config=_create_config_with_runtime(state=state_with_city),
+    )
+
+    assert len(result["messages"]) == 1
+    tool_msg = result["messages"][0]
+    assert isinstance(tool_msg, ToolMessage)
+    assert "San Francisco" in tool_msg.content
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 11),
+    reason="InjectedState field extraction from Optional[Annotated[...]] not supported on Python <3.11",
+)
+def test_create_react_agent_injected_state_not_required_field_missing():
+    """Test create_react_agent with InjectedState using NotRequired field that is missing.
+
+    This verifies the fix for https://github.com/langchain-ai/langchain/issues/35585
+    """
+    model = FakeToolCallingModel(
+        tool_calls=[
+            [{"name": "get_weather", "args": {}, "id": "call_1"}],
+            [],  # No more tool calls, agent should stop
+        ]
+    )
+
+    agent = create_react_agent(
+        model,
+        tools=[get_weather],
+        state_schema=CustomAgentStateWithNotRequired,
+    )
+
+    # Invoke WITHOUT the city field - should work, injecting None
+    result = agent.invoke(
+        {"messages": [HumanMessage("What's the weather?")]},
+    )
+
+    # Check that the tool was called successfully with None injected
+    messages = result["messages"]
+    tool_messages = [m for m in messages if isinstance(m, ToolMessage)]
+    assert len(tool_messages) == 1
+    assert "No city provided" in tool_messages[0].content
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 11),
+    reason="InjectedState field extraction from Optional[Annotated[...]] not supported on Python <3.11",
+)
+def test_create_react_agent_injected_state_not_required_field_present():
+    """Test create_react_agent with InjectedState using NotRequired field that IS present."""
+    model = FakeToolCallingModel(
+        tool_calls=[
+            [{"name": "get_weather", "args": {}, "id": "call_1"}],
+            [],  # No more tool calls, agent should stop
+        ]
+    )
+
+    agent = create_react_agent(
+        model,
+        tools=[get_weather],
+        state_schema=CustomAgentStateWithNotRequired,
+    )
+
+    # Invoke WITH the city field
+    result = agent.invoke(
+        {
+            "messages": [HumanMessage("What's the weather?")],
+            "city": "San Francisco",
+        },
+    )
+
+    # Check that the tool was called successfully
+    messages = result["messages"]
+    tool_messages = [m for m in messages if isinstance(m, ToolMessage)]
+    assert len(tool_messages) == 1
+    assert "San Francisco" in tool_messages[0].content
+
+
+@tool
+def get_weather_optional(city: Annotated[str | None, InjectedState("city")]) -> str:
+    """Get weather for a given city (accepts None)."""
+    if city is None:
+        return "Please provide a city!"
+    return f"It's always sunny in {city}!"
+
+
+def test_pydantic_state_with_default_field_missing_works():
+    """Test that Pydantic state with Optional field and default=None works when field is missing.
+
+    This is the workaround suggested in the issue comments - using Pydantic BaseModel
+    with `city: Optional[str] = Field(default=None)` instead of TypedDict with NotRequired.
+    """
+    model = FakeToolCallingModel(
+        tool_calls=[
+            [{"name": "get_weather_optional", "args": {}, "id": "call_1"}],
+            [],  # No more tool calls, agent should stop
+        ]
+    )
+
+    agent = create_react_agent(
+        model,
+        tools=[get_weather_optional],
+        state_schema=CustomAgentStatePydanticWithDefault,
+    )
+
+    # Invoke WITHOUT the city field - should work because Pydantic provides default
+    result = agent.invoke(
+        {"messages": [HumanMessage("What's the weather?")]},
+    )
+
+    # Check that the tool was called successfully with None
+    messages = result["messages"]
+    tool_messages = [m for m in messages if isinstance(m, ToolMessage)]
+    assert len(tool_messages) == 1
+    assert "Please provide a city!" in tool_messages[0].content
+
+
+def test_pydantic_state_with_default_field_present_works():
+    """Test that Pydantic state with Optional field works when field IS present."""
+    model = FakeToolCallingModel(
+        tool_calls=[
+            [{"name": "get_weather_optional", "args": {}, "id": "call_1"}],
+            [],  # No more tool calls, agent should stop
+        ]
+    )
+
+    agent = create_react_agent(
+        model,
+        tools=[get_weather_optional],
+        state_schema=CustomAgentStatePydanticWithDefault,
+    )
+
+    # Invoke WITH the city field
+    result = agent.invoke(
+        {
+            "messages": [HumanMessage("What's the weather?")],
+            "city": "San Francisco",
+        },
+    )
+
+    # Check that the tool was called successfully
+    messages = result["messages"]
+    tool_messages = [m for m in messages if isinstance(m, ToolMessage)]
+    assert len(tool_messages) == 1
+    assert "San Francisco" in tool_messages[0].content

libs/prebuilt/tests/test_tool_call_transformer.py

@@ -1,306 +0,0 @@
-"""Tests for ToolCallTransformer and the ToolCallStream projection."""
-
-from __future__ import annotations
-
-import time
-from typing import Annotated, Any
-
-import pytest
-from langchain_core.messages import AIMessage
-from langchain_core.tools import tool
-from langgraph.config import emit_tool_output_delta
-from langgraph.constants import END, START
-from langgraph.graph import StateGraph
-from langgraph.graph.message import add_messages
-from langgraph.stream._event_log import EventLog
-from langgraph.stream._mux import StreamMux
-from langgraph.stream._types import ProtocolEvent
-from langgraph.stream.transformers import (
-    MessagesTransformer,
-    SubgraphTransformer,
-    ValuesTransformer,
-)
-from typing_extensions import TypedDict
-
-from langgraph.prebuilt import ToolCallStream, ToolCallTransformer, ToolNode
-
-TS = int(time.time() * 1000)
-
-
-def _tool_event(
-    event: str,
-    tool_call_id: str,
-    *,
-    tool_name: str = "",
-    input: dict[str, Any] | None = None,
-    delta: Any = None,
-    output: Any = None,
-    message: str = "",
-    namespace: list[str] | None = None,
-) -> ProtocolEvent:
-    data: dict[str, Any] = {"event": event, "tool_call_id": tool_call_id}
-    if event == "tool-started":
-        data["tool_name"] = tool_name
-        if input is not None:
-            data["input"] = input
-    elif event == "tool-output-delta":
-        data["delta"] = delta
-    elif event == "tool-finished":
-        data["output"] = output
-    elif event == "tool-error":
-        data["message"] = message
-    return {
-        "type": "event",
-        "method": "tools",
-        "params": {
-            "namespace": namespace or [],
-            "timestamp": TS,
-            "data": data,
-        },
-    }
-
-
-def _subscribe(log: EventLog) -> None:
-    log._subscribed = True
-
-
-def _mux() -> tuple[StreamMux, ToolCallTransformer]:
-    mux = StreamMux(
-        factories=[
-            ValuesTransformer,
-            MessagesTransformer,
-            SubgraphTransformer,
-            ToolCallTransformer,
-        ],
-        is_async=False,
-    )
-    transformer = mux.transformer_by_key("tool_calls")
-    assert isinstance(transformer, ToolCallTransformer)
-    _subscribe(transformer._log)
-    return mux, transformer
-
-
-class TestToolCallTransformerUnit:
-    def test_required_stream_modes_declares_tools(self) -> None:
-        assert ToolCallTransformer.required_stream_modes == ("tools",)
-
-    def test_tool_started_yields_handle(self) -> None:
-        mux, transformer = _mux()
-        mux.push(
-            _tool_event(
-                "tool-started",
-                "tc1",
-                tool_name="echo",
-                input={"text": "hi"},
-            )
-        )
-        handles = list(transformer._log._items)
-        assert len(handles) == 1
-        h = handles[0]
-        assert isinstance(h, ToolCallStream)
-        assert h.tool_call_id == "tc1"
-        assert h.tool_name == "echo"
-        assert h.input == {"text": "hi"}
-        assert h.completed is False
-
-    def test_delta_accumulates_on_active_stream(self) -> None:
-        mux, transformer = _mux()
-        mux.push(_tool_event("tool-started", "tc1", tool_name="echo"))
-        _subscribe(transformer._active["tc1"]._output_deltas)
-        mux.push(_tool_event("tool-output-delta", "tc1", delta="a"))
-        mux.push(_tool_event("tool-output-delta", "tc1", delta="b"))
-        stream = transformer._active["tc1"]
-        assert list(stream._output_deltas._items) == ["a", "b"]
-
-    def test_finish_closes_stream(self) -> None:
-        mux, transformer = _mux()
-        mux.push(_tool_event("tool-started", "tc1", tool_name="echo"))
-        stream = transformer._active["tc1"]
-        mux.push(_tool_event("tool-finished", "tc1", output="done"))
-        assert stream.completed is True
-        assert stream.output == "done"
-        assert stream.error is None
-        assert "tc1" not in transformer._active
-
-    def test_error_closes_stream(self) -> None:
-        mux, transformer = _mux()
-        mux.push(_tool_event("tool-started", "tc1", tool_name="boom"))
-        stream = transformer._active["tc1"]
-        mux.push(_tool_event("tool-error", "tc1", message="nope"))
-        assert stream.completed is True
-        assert stream.output is None
-        assert stream.error == "nope"
-        assert "tc1" not in transformer._active
-
-    def test_concurrent_tool_calls_do_not_bleed(self) -> None:
-        mux, transformer = _mux()
-        mux.push(_tool_event("tool-started", "a", tool_name="t"))
-        mux.push(_tool_event("tool-started", "b", tool_name="t"))
-        for tc in ("a", "b"):
-            _subscribe(transformer._active[tc]._output_deltas)
-        mux.push(_tool_event("tool-output-delta", "a", delta="A1"))
-        mux.push(_tool_event("tool-output-delta", "b", delta="B1"))
-        mux.push(_tool_event("tool-output-delta", "a", delta="A2"))
-        assert list(transformer._active["a"]._output_deltas._items) == ["A1", "A2"]
-        assert list(transformer._active["b"]._output_deltas._items) == ["B1"]
-
-    def test_tools_event_passes_through_main_log(self) -> None:
-        mux, transformer = _mux()
-        _subscribe(mux._events)
-        mux.push(_tool_event("tool-started", "tc1", tool_name="echo"))
-        kept = [e for e in mux._events._items if e["method"] == "tools"]
-        assert len(kept) == 1
-
-
-# ---------------------------------------------------------------------------
-# End-to-end tests with a real graph
-# ---------------------------------------------------------------------------
-
-
-class _State(TypedDict):
-    messages: Annotated[list, add_messages]
-
-
-def _build_graph(caller, tools):
-    sg = StateGraph(_State)
-    sg.add_node("caller", caller)
-    sg.add_node("tools", ToolNode(tools))
-    sg.add_edge(START, "caller")
-    sg.add_edge("caller", "tools")
-    sg.add_edge("tools", END)
-    return sg.compile()
-
-
-class TestToolCallTransformerEndToEnd:
-    def test_sync_streaming_tool_populates_tool_calls(self) -> None:
-        @tool
-        def streamer(text: str) -> str:
-            """streams chunks."""
-            for chunk in ("one", "two"):
-                emit_tool_output_delta(chunk)
-            return text
-
-        def caller(state: _State) -> dict:
-            return {
-                "messages": [
-                    AIMessage(
-                        content="",
-                        tool_calls=[
-                            {"name": "streamer", "args": {"text": "x"}, "id": "tc1"}
-                        ],
-                    )
-                ]
-            }
-
-        graph = _build_graph(caller, [streamer])
-        run = graph.stream_v2({"messages": []}, transformers=[ToolCallTransformer])
-
-        tool_calls: list[ToolCallStream] = []
-        for tc in run.tool_calls:
-            tool_calls.append(tc)
-            deltas = list(tc.output_deltas)
-            assert deltas == ["one", "two"]
-        assert len(tool_calls) == 1
-        tc = tool_calls[0]
-        assert tc.tool_call_id == "tc1"
-        assert tc.tool_name == "streamer"
-        assert tc.completed is True
-        assert tc.error is None
-
-    def test_stream_modes_union_includes_tools(self) -> None:
-        @tool
-        def echo(text: str) -> str:
-            """echo."""
-            return text
-
-        def caller(state: _State) -> dict:
-            return {
-                "messages": [
-                    AIMessage(
-                        content="",
-                        tool_calls=[
-                            {"name": "echo", "args": {"text": "x"}, "id": "tc1"}
-                        ],
-                    )
-                ]
-            }
-
-        graph = _build_graph(caller, [echo])
-        # Without ToolCallTransformer, no tool_calls projection is
-        # exposed and no `tools` events flow through (required_stream_modes
-        # omits it).
-        run_no_tc = graph.stream_v2({"messages": []})
-        assert "tool_calls" not in run_no_tc._mux.extensions  # type: ignore[attr-defined]
-
-        # With ToolCallTransformer, the projection is present.
-        run = graph.stream_v2({"messages": []}, transformers=[ToolCallTransformer])
-        assert "tool_calls" in run._mux.extensions  # type: ignore[attr-defined]
-        # Drain so the run closes cleanly.
-        list(run.tool_calls)
-
-    @pytest.mark.anyio
-    async def test_async_streaming_tool_populates_tool_calls(self) -> None:
-        @tool
-        async def astreamer(text: str) -> str:
-            """async streams."""
-            emit_tool_output_delta(text)
-            emit_tool_output_delta(text + "!")
-            return text
-
-        async def caller(state: _State) -> dict:
-            return {
-                "messages": [
-                    AIMessage(
-                        content="",
-                        tool_calls=[
-                            {"name": "astreamer", "args": {"text": "hi"}, "id": "tc1"}
-                        ],
-                    )
-                ]
-            }
-
-        graph = _build_graph(caller, [astreamer])
-        run = await graph.astream_v2(
-            {"messages": []}, transformers=[ToolCallTransformer]
-        )
-
-        collected: list[ToolCallStream] = []
-        async for tc in run.tool_calls:
-            collected.append(tc)
-            deltas = [d async for d in tc.output_deltas]
-            assert deltas == ["hi", "hi!"]
-        assert len(collected) == 1
-        assert collected[0].completed is True
-        assert collected[0].error is None
-
-    def test_tool_error_populates_error_field(self) -> None:
-        @tool
-        def boom() -> str:
-            """raises."""
-            raise ValueError("nope")
-
-        def caller(state: _State) -> dict:
-            return {
-                "messages": [
-                    AIMessage(
-                        content="",
-                        tool_calls=[{"name": "boom", "args": {}, "id": "tc1"}],
-                    )
-                ]
-            }
-
-        graph = _build_graph(caller, [boom])
-        run = graph.stream_v2({"messages": []}, transformers=[ToolCallTransformer])
-
-        collected: list[ToolCallStream] = []
-        with pytest.raises(ValueError, match="nope"):
-            for tc in run.tool_calls:
-                collected.append(tc)
-                # Drain deltas so the error field is populated before we
-                # inspect it below.
-                list(tc.output_deltas)
-
-        assert len(collected) == 1
-        assert collected[0].error == "nope"
-        assert collected[0].output is None
-        assert collected[0].completed is True

libs/prebuilt/uv.lock

@@ -352,7 +352,7 @@ test = [
 
 [[package]]
 name = "langgraph-checkpoint"
-version = "4.0.1"
+version = "4.0.2"
 source = { editable = "../checkpoint" }
 dependencies = [
     { name = "langchain-core" },
@@ -619,7 +619,7 @@ test = [
 
 [[package]]
 name = "langsmith"
-version = "0.6.4"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -629,11 +629,12 @@ dependencies = [
     { name = "requests" },
     { name = "requests-toolbelt" },
     { name = "uuid-utils" },
+    { name = "xxhash" },
     { name = "zstandard" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/e7/85/9c7933052a997da1b85bc5c774f3865e9b1da1c8d71541ea133178b13229/langsmith-0.6.4.tar.gz", hash = "sha256:36f7223a01c218079fbb17da5e536ebbaf5c1468c028abe070aa3ae59bc99ec8", size = 919964, upload-time = "2026-01-15T20:02:28.873Z" }
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libs/sdk-py/uv.lock

@@ -365,7 +365,7 @@ test = [
 
 [[package]]
 name = "langgraph-checkpoint"
-version = "4.0.1"
+version = "4.0.2"
 source = { editable = "../checkpoint" }
 dependencies = [
     { name = "langchain-core" },
@@ -534,7 +534,7 @@ test = [
 
 [[package]]
 name = "langsmith"
-version = "0.7.20"
+version = "0.7.31"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "httpx" },
@@ -547,9 +547,9 @@ dependencies = [
     { name = "xxhash" },
     { name = "zstandard" },
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