refactor(langgraph): call create_checkpoint directly for the fork seal

create_fork_checkpoint only forwarded to create_checkpoint, and an empty
fork set bumps nothing there either, so the four update_state call sites
pass the set straight through. The fork tests also had two identical
helpers; keep one.

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -8,13 +8,15 @@ from typing import Any, cast
 from langchain_core.runnables import RunnableConfig
 from langgraph.checkpoint.base import (
     BaseCheckpointSaver,
+    ChannelVersions,
     Checkpoint,
+    PendingWrite,
 )
 from langgraph.checkpoint.base.id import uuid6
 from langgraph.checkpoint.serde.types import _DeltaSnapshot
 
 from langgraph._internal._config import DELTA_MAX_SUPERSTEPS_SINCE_SNAPSHOT
-from langgraph._internal._constants import PUSH
+from langgraph._internal._constants import INTERRUPT, PUSH
 from langgraph._internal._typing import MISSING
 from langgraph.channels.base import BaseChannel
 from langgraph.channels.delta import DeltaChannel
@@ -89,6 +91,23 @@ def get_delta_channels_from_all_channels(
     }
 
 
+def delta_channels_with_pending_writes(
+    specs: Mapping[str, Any],
+    pending_writes: Iterable[PendingWrite] | None,
+) -> set[str]:
+    """DeltaChannels a branch starting from this checkpoint must snapshot.
+
+    A checkpoint's pending writes belong to the child that consumed them, and
+    nothing records which child that was. A new branch snapshots every delta
+    channel they touch, so its ancestor walk never replays them.
+    """
+    return {
+        ch
+        for _, ch, _ in pending_writes or ()
+        if isinstance(specs.get(ch), DeltaChannel)
+    }
+
+
 def create_metadata_for_update_state_api(
     channels: Mapping[str, BaseChannel],
     updated_channels: set[str],
@@ -122,6 +141,7 @@ def create_checkpoint_plan_for_update_state_api(
     parents: dict[str, Any],
     saved_metadata: Mapping[str, Any] | None,
     is_fresh_thread: bool,
+    fork_channels: set[str],
 ) -> tuple[set[str], dict[str, Any]]:
     """Return ``(channels_to_snapshot, metadata)`` for an update_state head."""
     metadata: dict[str, Any] = {
@@ -137,7 +157,9 @@ def create_checkpoint_plan_for_update_state_api(
         updated_channels,
         prev_metadata=saved_metadata,
     )
-    channels_to_snapshot = delta_channels_to_snapshot(channels, new_counters)
+    channels_to_snapshot = (
+        delta_channels_to_snapshot(channels, new_counters) | fork_channels
+    )
     for k in channels_to_snapshot:
         new_counters[k] = (0, 0)
     non_zero = {k: v for k, v in new_counters.items() if v != (0, 0)}
@@ -167,6 +189,7 @@ def create_checkpoint(
     """
     ts = datetime.now(timezone.utc).isoformat()
     channels_to_snapshot = channels_to_snapshot or set()
+    bumped: dict[str, tuple[Any, Any]] = {}
     if channels is None:
         values = checkpoint["channel_values"]
         channel_versions = checkpoint["channel_versions"]
@@ -174,30 +197,29 @@ def create_checkpoint(
         values = {}
         channel_versions = dict(checkpoint["channel_versions"])
         for k in channels:
-            if k not in channel_versions:
-                continue
             ch = channels[k]
+            if k not in channel_versions:
+                # A forced snapshot of a never-written channel still has to
+                # land to stop the ancestor walk, and `put` only stores blobs
+                # for versioned channels.
+                if k in channels_to_snapshot and get_next_version is not None:
+                    channel_versions[k] = get_next_version(None, None)
+                    bumped[k] = (None, channel_versions[k])
+                    values[k] = _DeltaSnapshot(
+                        ch.get() if ch.is_available() else ch.typ()
+                    )
+                continue
             if k in channels_to_snapshot:
-                # Callers force a full snapshot blob here: exit mode when a
-                # delta channel reaches its snapshot cadence, and update_state
-                # on a fresh thread (no ancestor to replay writes from). The
-                # manual version-bump below only applies to the exit-mode case.
-                #
-                # In exit mode, the snapshot decision is deferred to exit
-                # time (intermediate steps have do_checkpoint=False). The
-                # channel's count may have reached snapshot_frequency over
-                # several supersteps, but the LAST superstep may not have
-                # written to this channel. In that case apply_writes()
-                # (in _algo.py) didn't bump this channel's version, so
-                # saver.put() wouldn't include it in new_versions and
-                # the snapshot blob would be silently dropped. The manual
-                # bump below closes the gap. In sync/async durability this
-                # branch is effectively dead code (the step that pushes
-                # the count to freq always writes the channel).
+                # `put` only stores a blob for a channel whose version moved,
+                # so snapshotting a channel this step did not write needs a
+                # bump: exit mode reaching the cadence on a superstep that
+                # skipped the channel, and a fork's first checkpoint.
                 if get_next_version is not None and (
                     updated_channels is None or k not in updated_channels
                 ):
-                    channel_versions[k] = get_next_version(channel_versions[k], None)
+                    old = channel_versions[k]
+                    channel_versions[k] = get_next_version(old, None)
+                    bumped[k] = (old, channel_versions[k])
                 values[k] = _DeltaSnapshot(ch.get())
             else:
                 v = ch.checkpoint()
@@ -209,11 +231,30 @@ def create_checkpoint(
         id=id or str(uuid6(clock_seq=step)),
         channel_values=values,
         channel_versions=channel_versions,
-        versions_seen=checkpoint["versions_seen"],
+        versions_seen=_mark_bumps_seen(checkpoint["versions_seen"], bumped),
         updated_channels=None if updated_channels is None else sorted(updated_channels),
     )
 
 
+def _mark_bumps_seen(
+    versions_seen: dict[str, ChannelVersions],
+    bumped: Mapping[str, tuple[Any, Any]],
+) -> dict[str, ChannelVersions]:
+    """Advance whoever had seen a bumped channel's old version to the new one.
+
+    A bump that only stores a snapshot is not a write. Left unseen, it would
+    re-fire `interrupt_before` and rerun the channel's subscribers.
+    """
+    if not bumped:
+        return versions_seen
+    out: dict[str, ChannelVersions] = {}
+    for node, seen in {INTERRUPT: {}, **versions_seen}.items():
+        advanced = {k: new for k, (old, new) in bumped.items() if seen.get(k) == old}
+        if advanced or node in versions_seen:
+            out[node] = {**seen, **advanced}
+    return out
+
+
 def _needs_replay(spec: BaseChannel, stored: object) -> bool:
     """True if `spec` is a `DeltaChannel` and no value is stored at this
     checkpoint, requiring an ancestor walk to reconstruct.

libs/langgraph/langgraph/pregel/_loop.py

@@ -102,6 +102,7 @@ from langgraph.pregel._checkpoint import (
     copy_checkpoint,
     create_checkpoint,
     delta_channels_to_snapshot,
+    delta_channels_with_pending_writes,
     empty_checkpoint,
     exit_delta_task_id,
 )
@@ -222,10 +223,13 @@ class PregelLoop:
     # under the saver's `ORDER BY task_id, idx` sorting.
     _exit_delta_writes: list[tuple[int, str, str, Any]] | None = None
 
-    # Delta channels that saw an Overwrite since the last checkpoint. These
-    # channels must snapshot after live update applies overwrite semantics so
-    # sparse replay starts from the same post-overwrite value.
-    _delta_channels_with_overwrite: set[str]
+    # Delta channels that must snapshot at the next checkpoint, whatever their
+    # cadence counters say:
+    # * an Overwrite arrived since the last checkpoint, so sparse replay has to
+    #   start from the post-overwrite value;
+    # * the checkpoint this run starts from has pending writes to them; see
+    #   `delta_channels_with_pending_writes`.
+    _delta_channels_forced_snapshot: set[str]
 
     # The checkpoint_config that points at the parent loaded at `__enter__`
     # (or the synthetic-empty checkpoint, on first run). We capture it
@@ -683,7 +687,7 @@ class PregelLoop:
     def after_tick(self) -> None:
         # finish superstep
         writes = [w for t in self.tasks.values() for w in t.writes]
-        self._delta_channels_with_overwrite.update(
+        self._delta_channels_forced_snapshot.update(
             ch
             for ch, v in writes
             if isinstance(self.specs.get(ch), DeltaChannel) and _get_overwrite(v)[0]
@@ -898,6 +902,15 @@ class PregelLoop:
             self.checkpoint_pending_writes = [
                 w for w in self.checkpoint_pending_writes if w[1] != RESUME
             ]
+        # A resume that is not replaying reuses the head's pending writes
+        # instead of rerunning their tasks, so none of them can leak.
+        self._delta_channels_forced_snapshot = (
+            set()
+            if is_resuming and not self.is_replaying
+            else delta_channels_with_pending_writes(
+                self.specs, self.checkpoint_pending_writes
+            )
+        )
 
         # map command to writes
         if input_is_command:
@@ -991,7 +1004,7 @@ class PregelLoop:
                 manager=None,
                 updated_channels=updated_channels,
             )
-            self._delta_channels_with_overwrite.update(
+            self._delta_channels_forced_snapshot.update(
                 c
                 for c, v in input_writes
                 if isinstance(self.specs.get(c), DeltaChannel) and _get_overwrite(v)[0]
@@ -1136,7 +1149,7 @@ class PregelLoop:
         # create new checkpoint
         channels_to_snapshot = (
             delta_channels_to_snapshot(self.channels, new_counters)
-            | self._delta_channels_with_overwrite
+            | self._delta_channels_forced_snapshot
             if do_checkpoint
             else set()
         )
@@ -1154,7 +1167,7 @@ class PregelLoop:
         for k in channels_to_snapshot:
             new_counters[k] = (0, 0)
         if do_checkpoint:
-            self._delta_channels_with_overwrite.difference_update(channels_to_snapshot)
+            self._delta_channels_forced_snapshot.difference_update(channels_to_snapshot)
         non_zero = {k: v for k, v in new_counters.items() if v != (0, 0)}
         if non_zero:
             self.checkpoint_metadata["counters_since_delta_snapshot"] = non_zero
@@ -1239,7 +1252,7 @@ class PregelLoop:
         )
         channels_to_snapshot = (
             delta_channels_to_snapshot(self.channels, counters)
-            | self._delta_channels_with_overwrite
+            | self._delta_channels_forced_snapshot
         )
 
         pending = [
@@ -1684,7 +1697,6 @@ class SyncPregelLoop(PregelLoop, AbstractContextManager):
         )
         self._delta_write_futs = []
         self._error_handler_write_futs = []
-        self._delta_channels_with_overwrite = set()
         self._exit_delta_writes = (
             [] if self.durability == "exit" and self.checkpointer is not None else None
         )
@@ -1942,7 +1954,6 @@ class AsyncPregelLoop(PregelLoop, AbstractAsyncContextManager):
         )
         self._delta_write_futs = []
         self._error_handler_write_futs = []
-        self._delta_channels_with_overwrite = set()
         self._exit_delta_writes = (
             [] if self.durability == "exit" and self.checkpointer is not None else None
         )

libs/langgraph/langgraph/pregel/main.py

@@ -133,6 +133,7 @@ from langgraph.pregel._checkpoint import (
     copy_checkpoint,
     create_checkpoint,
     create_checkpoint_plan_for_update_state_api,
+    delta_channels_with_pending_writes,
     empty_checkpoint,
     get_updated_channels_from_tasks,
 )
@@ -1637,12 +1638,21 @@ class Pregel(
             else:
                 raise ValueError(f"Subgraph {recast} not found")
 
+        # Taken from the first superstep's base, and cleared by the first
+        # checkpoint that carries the snapshots, which `__copy__` does not write.
+        fork_pending: set[str] | None = None
+
         def perform_superstep(
             input_config: RunnableConfig, updates: Sequence[StateUpdate]
         ) -> RunnableConfig:
+            nonlocal fork_pending
             # get last checkpoint
             config = ensure_config(self.config, input_config)
             saved = checkpointer.get_tuple(config)
+            if fork_pending is None:
+                fork_pending = delta_channels_with_pending_writes(
+                    self.channels, saved.pending_writes if saved else None
+                )
             if saved is not None:
                 self._migrate_checkpoint(saved.checkpoint)
             checkpoint = (
@@ -1726,9 +1736,17 @@ class Pregel(
                         self.trigger_to_nodes,
                     )
                 # save checkpoint
+                next_checkpoint = create_checkpoint(
+                    checkpoint,
+                    channels,
+                    step,
+                    get_next_version=checkpointer.get_next_version,
+                    channels_to_snapshot=fork_pending,
+                )
+                fork_pending.difference_update(next_checkpoint["channel_values"])
                 next_config = checkpointer.put(
                     checkpoint_config,
-                    create_checkpoint(checkpoint, channels, step),
+                    next_checkpoint,
                     {
                         "source": "update",
                         "step": step + 1,
@@ -1736,7 +1754,7 @@ class Pregel(
                     },
                     get_new_channel_versions(
                         checkpoint_previous_versions,
-                        checkpoint["channel_versions"],
+                        next_checkpoint["channel_versions"],
                     ),
                 )
                 return patch_checkpoint_map(
@@ -1765,9 +1783,17 @@ class Pregel(
                         if saved and saved.metadata.get("step") is not None
                         else -1
                     )
+                    next_checkpoint = create_checkpoint(
+                        checkpoint,
+                        channels,
+                        next_step,
+                        get_next_version=checkpointer.get_next_version,
+                        channels_to_snapshot=fork_pending,
+                    )
+                    fork_pending.difference_update(next_checkpoint["channel_values"])
                     next_config = checkpointer.put(
                         checkpoint_config,
-                        create_checkpoint(checkpoint, channels, next_step),
+                        next_checkpoint,
                         {
                             "source": "input",
                             "step": next_step,
@@ -1777,7 +1803,7 @@ class Pregel(
                         },
                         get_new_channel_versions(
                             checkpoint_previous_versions,
-                            checkpoint["channel_versions"],
+                            next_checkpoint["channel_versions"],
                         ),
                     )
 
@@ -2020,18 +2046,19 @@ class Pregel(
                     parents=saved.metadata.get("parents", {}) if saved else {},
                     saved_metadata=saved.metadata if saved else None,
                     is_fresh_thread=saved is None,
+                    fork_channels=fork_pending,
                 )
             )
             checkpoint = create_checkpoint(
                 checkpoint,
                 channels,
                 step + 1,
-                updated_channels=updated_channels if channels_to_snapshot else None,
                 get_next_version=checkpointer.get_next_version
                 if channels_to_snapshot
                 else None,
                 channels_to_snapshot=channels_to_snapshot,
             )
+            fork_pending.difference_update(checkpoint["channel_values"])
             next_config = checkpointer.put(
                 checkpoint_config,
                 checkpoint,
@@ -2103,12 +2130,21 @@ class Pregel(
             else:
                 raise ValueError(f"Subgraph {recast} not found")
 
+        # Taken from the first superstep's base, and cleared by the first
+        # checkpoint that carries the snapshots, which `__copy__` does not write.
+        fork_pending: set[str] | None = None
+
         async def aperform_superstep(
             input_config: RunnableConfig, updates: Sequence[StateUpdate]
         ) -> RunnableConfig:
+            nonlocal fork_pending
             # get last checkpoint
             config = ensure_config(self.config, input_config)
             saved = await checkpointer.aget_tuple(config)
+            if fork_pending is None:
+                fork_pending = delta_channels_with_pending_writes(
+                    self.channels, saved.pending_writes if saved else None
+                )
             if saved is not None:
                 self._migrate_checkpoint(saved.checkpoint)
             checkpoint = (
@@ -2190,16 +2226,25 @@ class Pregel(
                         self.trigger_to_nodes,
                     )
                 # save checkpoint
+                next_checkpoint = create_checkpoint(
+                    checkpoint,
+                    channels,
+                    step,
+                    get_next_version=checkpointer.get_next_version,
+                    channels_to_snapshot=fork_pending,
+                )
+                fork_pending.difference_update(next_checkpoint["channel_values"])
                 next_config = await checkpointer.aput(
                     checkpoint_config,
-                    create_checkpoint(checkpoint, channels, step),
+                    next_checkpoint,
                     {
                         "source": "update",
                         "step": step + 1,
                         "parents": saved.metadata.get("parents", {}) if saved else {},
                     },
                     get_new_channel_versions(
-                        checkpoint_previous_versions, checkpoint["channel_versions"]
+                        checkpoint_previous_versions,
+                        next_checkpoint["channel_versions"],
                     ),
                 )
                 return patch_checkpoint_map(
@@ -2228,9 +2273,17 @@ class Pregel(
                         if saved and saved.metadata.get("step") is not None
                         else -1
                     )
+                    next_checkpoint = create_checkpoint(
+                        checkpoint,
+                        channels,
+                        next_step,
+                        get_next_version=checkpointer.get_next_version,
+                        channels_to_snapshot=fork_pending,
+                    )
+                    fork_pending.difference_update(next_checkpoint["channel_values"])
                     next_config = await checkpointer.aput(
                         checkpoint_config,
-                        create_checkpoint(checkpoint, channels, next_step),
+                        next_checkpoint,
                         {
                             "source": "input",
                             "step": next_step,
@@ -2240,7 +2293,7 @@ class Pregel(
                         },
                         get_new_channel_versions(
                             checkpoint_previous_versions,
-                            checkpoint["channel_versions"],
+                            next_checkpoint["channel_versions"],
                         ),
                     )
 
@@ -2480,18 +2533,19 @@ class Pregel(
                     parents=saved.metadata.get("parents", {}) if saved else {},
                     saved_metadata=saved.metadata if saved else None,
                     is_fresh_thread=saved is None,
+                    fork_channels=fork_pending,
                 )
             )
             checkpoint = create_checkpoint(
                 checkpoint,
                 channels,
                 step + 1,
-                updated_channels=updated_channels if channels_to_snapshot else None,
                 get_next_version=checkpointer.get_next_version
                 if channels_to_snapshot
                 else None,
                 channels_to_snapshot=channels_to_snapshot,
             )
+            fork_pending.difference_update(checkpoint["channel_values"])
             next_config = await checkpointer.aput(
                 checkpoint_config,
                 checkpoint,

libs/langgraph/tests/memory_assert.py

@@ -85,11 +85,13 @@ class MemorySaverAssertImmutable(InMemorySaver):
                 )
                 == saved
             ), config["configurable"]["checkpoint_ns"]
+        next_config = super().put(config, checkpoint, metadata, new_versions)
+        # Read back, not the object handed in: a DeltaChannel a step did not
+        # write is refilled on read from the blob its inherited version points at.
         self.storage_for_copies[thread_id][checkpoint_ns][checkpoint["id"]] = (
-            self.serde.dumps_typed(checkpoint)
+            self.serde.dumps_typed(super().get(next_config))
         )
-        # call super to write checkpoint
-        return super().put(config, checkpoint, metadata, new_versions)
+        return next_config
 
 
 class MemorySaverNoPending(InMemorySaver):

libs/langgraph/tests/test_delta_channel_fork.py

@@ -0,0 +1,533 @@
+"""Forking a thread must not replay the abandoned branch into the fork.
+
+Every graph carries a `DeltaChannel` and a plain reducer channel fed the same
+values; the plain channel needs no replay, so it is the oracle.
+"""
+
+from collections.abc import Sequence
+from operator import add
+from typing import Annotated, Any
+
+import pytest
+from langchain_core.runnables import RunnableConfig
+from langgraph.checkpoint.base import BaseCheckpointSaver
+from langgraph.checkpoint.serde.types import _DeltaSnapshot
+from typing_extensions import TypedDict
+
+from langgraph._internal._constants import INPUT
+from langgraph.channels.delta import DeltaChannel
+from langgraph.graph import END, START, StateGraph
+from langgraph.types import Command, Durability, StateSnapshot, StateUpdate, interrupt
+
+pytestmark = pytest.mark.anyio
+
+
+def _append(current: list | None, writes: Sequence[Any]) -> list:
+    out = list(current or [])
+    for write in writes:
+        out.extend(write if isinstance(write, list) else [write])
+    return out
+
+
+class _State(TypedDict):
+    log: Annotated[list, DeltaChannel(_append, snapshot_frequency=1000)]
+    plain: Annotated[list, add]
+    other: Annotated[list, add]
+
+
+def _build(checkpointer: BaseCheckpointSaver, tag: str) -> Any:
+    def node(state: _State) -> dict:
+        return {"log": [f"{tag}-out"], "plain": [f"{tag}-out"]}
+
+    builder = StateGraph(_State)
+    builder.add_node("n", node)
+    builder.set_entry_point("n")
+    builder.set_finish_point("n")
+    return builder.compile(checkpointer=checkpointer)
+
+
+def _build_without_delta_writes(checkpointer: BaseCheckpointSaver, tag: str) -> Any:
+    def node(state: _State) -> dict:
+        return {"other": [f"{tag}-other"]}
+
+    builder = StateGraph(_State)
+    builder.add_node("n", node)
+    builder.set_entry_point("n")
+    builder.set_finish_point("n")
+    return builder.compile(checkpointer=checkpointer)
+
+
+def _thread(thread_id: str) -> RunnableConfig:
+    return {"configurable": {"thread_id": thread_id}}
+
+
+def _at(config: RunnableConfig, snapshot: StateSnapshot) -> RunnableConfig:
+    return {
+        "configurable": {
+            **config["configurable"],
+            "checkpoint_ns": "",
+            "checkpoint_id": snapshot.config["configurable"]["checkpoint_id"],
+        }
+    }
+
+
+def _both(marker: str) -> dict:
+    return {"log": [marker], "plain": [marker]}
+
+
+def _snapshotted_checkpoints(
+    checkpointer: BaseCheckpointSaver, config: RunnableConfig
+) -> list[str]:
+    return [
+        tuple_.config["configurable"]["checkpoint_id"]
+        for tuple_ in checkpointer.list(config)
+        if isinstance(tuple_.checkpoint["channel_values"].get("log"), _DeltaSnapshot)
+    ]
+
+
+def _assert_fork_is_clean(state: StateSnapshot, abandoned: str) -> None:
+    assert state.values["log"] == state.values["plain"], (
+        f"delta channel diverged from the plain channel: "
+        f"{state.values['log']} != {state.values['plain']}"
+    )
+    assert abandoned not in state.values["log"], (
+        f"{abandoned!r} belongs to the branch the fork replaced, "
+        f"but was replayed into {state.values['log']}"
+    )
+
+
+def test_fork_by_invoke(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    _build(sync_checkpointer, "first").invoke(
+        _both("in-1"), config, durability=durability
+    )
+    graph = _build(sync_checkpointer, "second")
+    graph.invoke(_both("in-2"), config, durability=durability)
+    abandoned_head = graph.get_state(config)
+
+    base = next(
+        snapshot
+        for snapshot in graph.get_state_history(config)
+        if "in-2" not in snapshot.values["log"]
+    )
+    _build(sync_checkpointer, "third").invoke(
+        _both("in-3"), _at(config, base), durability=durability
+    )
+
+    state = graph.get_state(config)
+    _assert_fork_is_clean(state, "in-2")
+    assert state.values["log"] == [*base.values["log"], "in-3", "third-out"]
+
+    abandoned = graph.get_state(abandoned_head.config).values
+    assert abandoned["log"] == abandoned["plain"] == abandoned_head.values["log"]
+
+
+async def test_afork_by_invoke(
+    async_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    await _build(async_checkpointer, "first").ainvoke(
+        _both("in-1"), config, durability=durability
+    )
+    graph = _build(async_checkpointer, "second")
+    await graph.ainvoke(_both("in-2"), config, durability=durability)
+    abandoned_head = await graph.aget_state(config)
+
+    base = await anext(
+        snapshot
+        async for snapshot in graph.aget_state_history(config)
+        if "in-2" not in snapshot.values["log"]
+    )
+    await _build(async_checkpointer, "third").ainvoke(
+        _both("in-3"), _at(config, base), durability=durability
+    )
+
+    state = await graph.aget_state(config)
+    _assert_fork_is_clean(state, "in-2")
+    assert state.values["log"] == [*base.values["log"], "in-3", "third-out"]
+
+    abandoned = (await graph.aget_state(abandoned_head.config)).values
+    assert abandoned["log"] == abandoned["plain"] == abandoned_head.values["log"]
+
+
+def test_fork_off_checkpoint_before_first_input(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build(sync_checkpointer, "first")
+    graph.invoke(_both("in-1"), config, durability=durability)
+
+    root = list(graph.get_state_history(config))[-1]
+    assert root.values["log"] == []
+
+    _build(sync_checkpointer, "third").invoke(
+        _both("in-9"), _at(config, root), durability=durability
+    )
+
+    state = graph.get_state(config)
+    _assert_fork_is_clean(state, "in-1")
+    assert state.values["log"] == ["in-9", "third-out"]
+
+
+async def test_afork_off_checkpoint_before_first_input(
+    async_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build(async_checkpointer, "first")
+    await graph.ainvoke(_both("in-1"), config, durability=durability)
+
+    root = [snapshot async for snapshot in graph.aget_state_history(config)][-1]
+    assert root.values["log"] == []
+
+    await _build(async_checkpointer, "third").ainvoke(
+        _both("in-9"), _at(config, root), durability=durability
+    )
+
+    state = await graph.aget_state(config)
+    _assert_fork_is_clean(state, "in-1")
+    assert state.values["log"] == ["in-9", "third-out"]
+
+
+def test_fork_by_update_state(sync_checkpointer: BaseCheckpointSaver) -> None:
+    config = _thread("t")
+    _build(sync_checkpointer, "first").invoke(_both("in-1"), config)
+    graph = _build(sync_checkpointer, "second")
+    graph.invoke(_both("in-2"), config)
+
+    base = next(
+        snapshot
+        for snapshot in graph.get_state_history(config)
+        if "in-2" not in snapshot.values["log"]
+    )
+    forked = graph.update_state(_at(config, base), _both("patched"))
+
+    state = graph.get_state(forked)
+    _assert_fork_is_clean(state, "in-2")
+    assert state.values["log"] == [*base.values["log"], "patched"]
+
+
+async def test_afork_by_update_state(
+    async_checkpointer: BaseCheckpointSaver,
+) -> None:
+    config = _thread("t")
+    await _build(async_checkpointer, "first").ainvoke(_both("in-1"), config)
+    graph = _build(async_checkpointer, "second")
+    await graph.ainvoke(_both("in-2"), config)
+
+    base = await anext(
+        snapshot
+        async for snapshot in graph.aget_state_history(config)
+        if "in-2" not in snapshot.values["log"]
+    )
+    forked = await graph.aupdate_state(_at(config, base), _both("patched"))
+
+    state = await graph.aget_state(forked)
+    _assert_fork_is_clean(state, "in-2")
+    assert state.values["log"] == [*base.values["log"], "patched"]
+
+
+def test_unaddressed_run_keeps_snapshot_cadence(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build(sync_checkpointer, "first")
+    graph.invoke(_both("in-1"), config, durability=durability)
+    graph.invoke(_both("in-2"), config, durability=durability)
+
+    assert not _snapshotted_checkpoints(sync_checkpointer, config)
+
+
+def test_fork_before_first_value_when_fork_never_writes_the_channel(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build(sync_checkpointer, "first")
+    graph.invoke(_both("in-1"), config, durability=durability)
+
+    root = list(graph.get_state_history(config))[-1]
+    assert root.values["log"] == []
+
+    _build_without_delta_writes(sync_checkpointer, "third").invoke(
+        {"other": ["in-9"]}, _at(config, root), durability=durability
+    )
+
+    state = graph.get_state(config)
+    _assert_fork_is_clean(state, "in-1")
+    assert state.values["log"] == []
+
+
+async def test_afork_before_first_value_when_fork_never_writes_the_channel(
+    async_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build(async_checkpointer, "first")
+    await graph.ainvoke(_both("in-1"), config, durability=durability)
+
+    root = [snapshot async for snapshot in graph.aget_state_history(config)][-1]
+    assert root.values["log"] == []
+
+    await _build_without_delta_writes(async_checkpointer, "third").ainvoke(
+        {"other": ["in-9"]}, _at(config, root), durability=durability
+    )
+
+    state = await graph.aget_state(config)
+    _assert_fork_is_clean(state, "in-1")
+    assert state.values["log"] == []
+
+
+def test_fork_before_first_value_by_bulk_update(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    config = _thread("t")
+    graph = _build(sync_checkpointer, "first")
+    graph.invoke(_both("in-1"), config)
+
+    root = list(graph.get_state_history(config))[-1]
+    assert root.values["log"] == []
+
+    forked = graph.bulk_update_state(
+        _at(config, root),
+        [
+            [StateUpdate({"other": ["s1"]}, "n")],
+            [StateUpdate(_both("s2"), "n")],
+        ],
+    )
+
+    state = graph.get_state(forked)
+    _assert_fork_is_clean(state, "in-1")
+    assert state.values["log"] == ["s2"]
+
+
+@pytest.mark.parametrize("first_as_node", [INPUT, END, "__copy__"])
+def test_fork_by_bulk_update_whose_first_superstep_skips_the_plan(
+    sync_checkpointer: BaseCheckpointSaver, first_as_node: str
+) -> None:
+    config = _thread("t")
+    _build(sync_checkpointer, "first").invoke(_both("in-1"), config)
+    graph = _build(sync_checkpointer, "second")
+    graph.invoke(_both("in-2"), config)
+
+    base = next(
+        snapshot
+        for snapshot in graph.get_state_history(config)
+        if "in-2" not in snapshot.values["log"]
+    )
+    first = (
+        StateUpdate(_both("first-step"), first_as_node)
+        if first_as_node == INPUT
+        else StateUpdate(None, first_as_node)
+    )
+    forked = graph.bulk_update_state(
+        _at(config, base),
+        [[first], [StateUpdate(_both("second-step"), "n")]],
+    )
+
+    state = graph.get_state(forked)
+    assert state.values["log"] == state.values["plain"], (
+        f"delta channel diverged from the plain channel: "
+        f"{state.values['log']} != {state.values['plain']}"
+    )
+
+
+def test_unaddressed_bulk_update_keeps_snapshot_cadence(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    config = _thread("t")
+    graph = _build(sync_checkpointer, "first")
+    graph.invoke(_both("in-1"), config)
+
+    graph.bulk_update_state(
+        config,
+        [[StateUpdate(_both(f"u{i}"), "n")] for i in range(4)],
+    )
+
+    assert not _snapshotted_checkpoints(sync_checkpointer, config)
+
+
+def _build_paused_before_b(checkpointer: BaseCheckpointSaver) -> Any:
+    builder = StateGraph(_State)
+    builder.add_node("a", lambda state: _both("a"))
+    builder.add_node("b", lambda state: _both("b"))
+    builder.add_edge(START, "a")
+    builder.add_edge("a", "b")
+    builder.add_edge("b", END)
+    return builder.compile(checkpointer=checkpointer, interrupt_before=["b"])
+
+
+def _build_parallel_interrupt(checkpointer: BaseCheckpointSaver) -> Any:
+    def ask(state: _State) -> dict:
+        interrupt("approve?")
+        return {"other": ["q"]}
+
+    builder = StateGraph(_State)
+    builder.add_node("p", lambda state: _both("p"))
+    builder.add_node("q", ask)
+    builder.add_edge(START, "p")
+    builder.add_edge(START, "q")
+    return builder.compile(checkpointer=checkpointer)
+
+
+def test_resume_at_interrupt_before_with_the_head_checkpoint_id_runs_the_node(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build_paused_before_b(sync_checkpointer)
+    graph.invoke(_both("in"), config, durability=durability)
+
+    graph.invoke(None, graph.get_state(config).config, durability=durability)
+
+    state = graph.get_state(config)
+    assert state.next == (), f"resume paused again before {state.next}"
+    assert state.values["log"] == state.values["plain"] == ["in", "a", "b"]
+
+
+async def test_aresume_at_interrupt_before_with_the_head_checkpoint_id_runs_the_node(
+    async_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build_paused_before_b(async_checkpointer)
+    await graph.ainvoke(_both("in"), config, durability=durability)
+
+    await graph.ainvoke(
+        None, (await graph.aget_state(config)).config, durability=durability
+    )
+
+    state = await graph.aget_state(config)
+    assert state.next == (), f"resume paused again before {state.next}"
+    assert state.values["log"] == state.values["plain"] == ["in", "a", "b"]
+
+
+def test_replay_from_a_paused_checkpoint_runs_the_node_once(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    config = _thread("t")
+    graph = _build_paused_before_b(sync_checkpointer)
+    graph.invoke(_both("in"), config)
+    paused = graph.get_state(config).config
+    graph.invoke(None, config)
+
+    graph.invoke(None, paused)
+
+    state = graph.get_state(config)
+    assert state.next == (), f"replay paused again before {state.next}"
+    assert state.values["log"] == state.values["plain"] == ["in", "a", "b"]
+
+
+@pytest.mark.parametrize("addressed", [False, True])
+def test_new_input_on_an_interrupted_head_does_not_replay_its_pending_writes(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability, addressed: bool
+) -> None:
+    config = _thread("t")
+    graph = _build_parallel_interrupt(sync_checkpointer)
+    graph.invoke(_both("in-1"), config, durability=durability)
+    head = graph.get_state(config).config
+
+    graph.invoke(_both("in-2"), head if addressed else config, durability=durability)
+
+    state = graph.get_state(config)
+    assert state.values["log"] == state.values["plain"] == ["in-1", "in-2", "p"]
+
+
+def _build_deferred_after_interrupt(checkpointer: BaseCheckpointSaver) -> Any:
+    builder = StateGraph(_State)
+    builder.add_node("a", lambda state: _both("a"))
+    builder.add_node("b", lambda state: _both("b"), defer=True)
+    builder.add_node("c", lambda state: {})
+    builder.add_edge(START, "a")
+    builder.add_edge("a", "b")
+    builder.add_edge("a", "c")
+    return builder.compile(checkpointer=checkpointer, interrupt_after=["a"])
+
+
+@pytest.mark.parametrize(
+    "durability",
+    [
+        "sync",
+        "async",
+        pytest.param(
+            "exit",
+            marks=pytest.mark.xfail(
+                reason="exit durability stores a resumed run's loaded writes twice",
+                strict=True,
+            ),
+        ),
+    ],
+)
+def test_resume_on_an_interrupted_head_consumes_its_writes_without_a_snapshot(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build_parallel_interrupt(sync_checkpointer)
+    graph.invoke(_both("in-1"), config, durability=durability)
+
+    graph.invoke(Command(resume="yes"), config, durability=durability)
+
+    state = graph.get_state(config)
+    assert state.next == ()
+    assert state.values["log"] == state.values["plain"] == ["in-1", "p"]
+    assert not _snapshotted_checkpoints(sync_checkpointer, config)
+
+
+def test_resume_addressed_at_an_interrupted_head_reruns_its_tasks_once(
+    sync_checkpointer: BaseCheckpointSaver, durability: Durability
+) -> None:
+    config = _thread("t")
+    graph = _build_parallel_interrupt(sync_checkpointer)
+    graph.invoke(_both("in-1"), config, durability=durability)
+
+    graph.invoke(
+        Command(resume="yes"), graph.get_state(config).config, durability=durability
+    )
+
+    state = graph.get_state(config)
+    assert state.next == ()
+    assert state.values["log"] == state.values["plain"] == ["in-1", "p"]
+
+
+def test_update_state_with_the_head_checkpoint_id_keeps_a_deferred_node(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    graph = _build_deferred_after_interrupt(sync_checkpointer)
+    config = _thread("t")
+    graph.invoke(_both("in"), config)
+
+    graph.update_state(graph.get_state(config).config, _both("u"), as_node="c")
+    graph.invoke(None, config)
+
+    state = graph.get_state(config)
+    assert state.next == (), f"deferred node never ran, still pending: {state.next}"
+    assert state.values["log"] == state.values["plain"] == ["in", "a", "u", "b"]
+
+
+async def test_aupdate_state_with_the_head_checkpoint_id_keeps_a_deferred_node(
+    async_checkpointer: BaseCheckpointSaver,
+) -> None:
+    graph = _build_deferred_after_interrupt(async_checkpointer)
+    config = _thread("t")
+    await graph.ainvoke(_both("in"), config)
+
+    await graph.aupdate_state(
+        (await graph.aget_state(config)).config, _both("u"), as_node="c"
+    )
+    await graph.ainvoke(None, config)
+
+    state = await graph.aget_state(config)
+    assert state.next == (), f"deferred node never ran, still pending: {state.next}"
+    assert state.values["log"] == state.values["plain"] == ["in", "a", "u", "b"]
+
+
+def test_turns_addressed_at_the_head_store_no_snapshot(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    config = _thread("t")
+    graph = _build(sync_checkpointer, "turn")
+    graph.invoke(_both("in-1"), config)
+    for turn in range(2, 5):
+        graph.invoke(_both(f"in-{turn}"), graph.get_state(config).config)
+
+    assert not _snapshotted_checkpoints(sync_checkpointer, config)
+    assert (
+        graph.get_state(config).values["log"] == graph.get_state(config).values["plain"]
+    )

libs/langgraph/tests/test_delta_channel_update_state.py

@@ -338,3 +338,29 @@ def test_state_history_chain_after_fresh_update_state_delta_channel() -> None:
     assert update_snapshot.metadata["step"] == 0
     assert update_snapshot.parent_config is None
     assert [m.content for m in update_snapshot.values["messages"]] == ["hello"]
+
+
+def test_update_state_that_snapshots_keeps_a_deferred_node_pending() -> None:
+    channel = DeltaChannel(_messages_delta_reducer, snapshot_frequency=1)
+
+    class State(TypedDict):
+        messages: Annotated[list, channel]
+
+    builder = StateGraph(State)
+    builder.add_node("a", lambda state: {"messages": [HumanMessage("a", id="a")]})
+    builder.add_node(
+        "b", lambda state: {"messages": [HumanMessage("b", id="b")]}, defer=True
+    )
+    builder.add_node("c", lambda state: {})
+    builder.add_edge(START, "a")
+    builder.add_edge("a", "b")
+    builder.add_edge("a", "c")
+    graph = builder.compile(checkpointer=InMemorySaver(), interrupt_after=["a"])
+    config = {"configurable": {"thread_id": "t"}}
+    graph.invoke({"messages": [HumanMessage("s", id="s")]}, config)
+
+    graph.update_state(config, {"messages": [HumanMessage("u", id="u")]}, as_node="c")
+    final = graph.invoke(None, config)
+
+    assert [m.content for m in final["messages"]] == ["s", "a", "u", "b"]
+    assert graph.get_state(config).next == ()