refactor(langgraph): drop the deferred fork-snapshot queue

Now that a forced snapshot mints a version for a never-written channel,
the first checkpoint a forked run writes seals every delta channel, so
_delta_channels_awaiting_fork_snapshot never outlived it. Seed
_delta_channels_forced_snapshot directly at loop construction instead.

Also asserts that forking by invoke leaves the abandoned branch's reads
intact, and trims comments and test docstrings.

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -80,12 +80,14 @@ def get_updated_channels_from_tasks(
 
 def get_delta_channels_from_all_channels(
     channels: Mapping[str, BaseChannel],
+    *,
+    include_unavailable: bool = False,
 ) -> set[str]:
-    """DeltaChannels to snapshot on the first update_state of a fresh thread."""
+    """DeltaChannels to snapshot on the first update_state of a fresh thread or fork."""
     return {
         k
         for k, ch in channels.items()
-        if isinstance(ch, DeltaChannel) and ch.is_available()
+        if isinstance(ch, DeltaChannel) and (include_unavailable or ch.is_available())
     }
 
 
@@ -122,15 +124,22 @@ def create_checkpoint_plan_for_update_state_api(
     parents: dict[str, Any],
     saved_metadata: Mapping[str, Any] | None,
     is_fresh_thread: bool,
+    is_fork: bool,
 ) -> tuple[set[str], dict[str, Any]]:
-    """Return ``(channels_to_snapshot, metadata)`` for an update_state head."""
+    """Return ``(channels_to_snapshot, metadata)`` for an update_state head.
+
+    A fork snapshots everything, like a fresh thread: its base also holds the
+    writes of the branch it abandons, so the ancestor walk must stop here.
+    """
     metadata: dict[str, Any] = {
         "source": "update",
         "step": step,
         "parents": parents,
     }
-    if is_fresh_thread:
-        return get_delta_channels_from_all_channels(channels), metadata
+    if is_fresh_thread or is_fork:
+        return get_delta_channels_from_all_channels(
+            channels, include_unavailable=is_fork
+        ), metadata
 
     new_counters = create_metadata_for_update_state_api(
         channels,
@@ -146,6 +155,34 @@ def create_checkpoint_plan_for_update_state_api(
     return channels_to_snapshot, metadata
 
 
+def create_fork_checkpoint(
+    checkpoint: Checkpoint,
+    channels: Mapping[str, BaseChannel],
+    step: int,
+    *,
+    is_fork: bool,
+    get_next_version: GetNextVersion,
+) -> Checkpoint:
+    """``create_checkpoint`` for the update_state paths that skip the plan.
+
+    The fork has to be sealed by its first checkpoint: any later superstep
+    has already rebuilt its delta channels through the shared base. These
+    paths never write the delta channel, so its version must be bumped here
+    or ``put`` drops the blob; derive ``new_versions`` from the result.
+    """
+    if not is_fork:
+        return create_checkpoint(checkpoint, channels, step)
+    return create_checkpoint(
+        checkpoint,
+        channels,
+        step,
+        get_next_version=get_next_version,
+        channels_to_snapshot=get_delta_channels_from_all_channels(
+            channels, include_unavailable=True
+        ),
+    )
+
+
 def create_checkpoint(
     checkpoint: Checkpoint,
     channels: Mapping[str, BaseChannel] | None,
@@ -174,14 +211,23 @@ 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)
+                    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.
+                # delta channel reaches its snapshot cadence, update_state on
+                # a fresh thread (no ancestor to replay writes from), and a
+                # fork. 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

libs/langgraph/langgraph/pregel/_loop.py

@@ -222,10 +222,16 @@ 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;
+    # * this run forked off an explicitly addressed checkpoint. That base also
+    #   holds the writes of the branch the fork abandons, and nothing records
+    #   which child consumed which, so the ancestor walk must stop inside the
+    #   fork. Any addressed checkpoint counts, because telling a real fork
+    #   apart would mean trusting the base's `pending_writes` to be complete.
+    _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
@@ -369,6 +375,13 @@ class PregelLoop:
             if self.config[CONF].get(CONFIG_KEY_CHECKPOINT_NS)
             else ()
         )
+        # Value, not key presence like `is_replaying`: subgraph task configs
+        # always carry an explicit `None` checkpoint_id.
+        self._delta_channels_forced_snapshot = (
+            {k for k, spec in specs.items() if isinstance(spec, DeltaChannel)}
+            if self.checkpoint_config[CONF].get(CONFIG_KEY_CHECKPOINT_ID)
+            else set()
+        )
         self.prev_checkpoint_config = None
         runtime = self.config[CONF].get(CONFIG_KEY_RUNTIME)
         self.control = runtime.control if isinstance(runtime, Runtime) else None
@@ -683,7 +696,7 @@ 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]
@@ -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

@@ -108,6 +108,7 @@ from langgraph.callbacks import (
     get_sync_graph_callback_manager_for_config,
 )
 from langgraph.channels.base import BaseChannel
+from langgraph.channels.delta import DeltaChannel
 from langgraph.channels.topic import Topic
 from langgraph.config import get_config
 from langgraph.constants import END
@@ -133,6 +134,7 @@ from langgraph.pregel._checkpoint import (
     copy_checkpoint,
     create_checkpoint,
     create_checkpoint_plan_for_update_state_api,
+    create_fork_checkpoint,
     empty_checkpoint,
     get_updated_channels_from_tasks,
 )
@@ -1637,8 +1639,21 @@ class Pregel(
             else:
                 raise ValueError(f"Subgraph {recast} not found")
 
+        # Read once from the caller's config: every later superstep receives
+        # the config of the checkpoint just written, which always names one.
+        # Cleared by the first checkpoint that carries the snapshots, which
+        # `__copy__` does not write.
+        fork_pending: set[str] = (
+            {k for k, v in self.channels.items() if isinstance(v, DeltaChannel)}
+            if config[CONF].get(CONFIG_KEY_CHECKPOINT_ID)
+            else set()
+        )
+
         def perform_superstep(
-            input_config: RunnableConfig, updates: Sequence[StateUpdate]
+            input_config: RunnableConfig,
+            updates: Sequence[StateUpdate],
+            *,
+            is_fork: bool,
         ) -> RunnableConfig:
             # get last checkpoint
             config = ensure_config(self.config, input_config)
@@ -1726,9 +1741,17 @@ class Pregel(
                         self.trigger_to_nodes,
                     )
                 # save checkpoint
+                next_checkpoint = create_fork_checkpoint(
+                    checkpoint,
+                    channels,
+                    step,
+                    is_fork=is_fork,
+                    get_next_version=checkpointer.get_next_version,
+                )
+                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 +1759,7 @@ class Pregel(
                     },
                     get_new_channel_versions(
                         checkpoint_previous_versions,
-                        checkpoint["channel_versions"],
+                        next_checkpoint["channel_versions"],
                     ),
                 )
                 return patch_checkpoint_map(
@@ -1765,9 +1788,17 @@ class Pregel(
                         if saved and saved.metadata.get("step") is not None
                         else -1
                     )
+                    next_checkpoint = create_fork_checkpoint(
+                        checkpoint,
+                        channels,
+                        next_step,
+                        is_fork=is_fork,
+                        get_next_version=checkpointer.get_next_version,
+                    )
+                    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 +1808,7 @@ class Pregel(
                         },
                         get_new_channel_versions(
                             checkpoint_previous_versions,
-                            checkpoint["channel_versions"],
+                            next_checkpoint["channel_versions"],
                         ),
                     )
 
@@ -1873,6 +1904,7 @@ class Pregel(
                     return perform_superstep(
                         patch_checkpoint_map(next_config, saved.metadata),
                         [item for lst in user_group_by.values() for item in lst],
+                        is_fork=is_fork,
                     )
 
                 return patch_checkpoint_map(next_config, saved.metadata)
@@ -2020,6 +2052,7 @@ class Pregel(
                     parents=saved.metadata.get("parents", {}) if saved else {},
                     saved_metadata=saved.metadata if saved else None,
                     is_fresh_thread=saved is None,
+                    is_fork=is_fork,
                 )
             )
             checkpoint = create_checkpoint(
@@ -2032,6 +2065,8 @@ class Pregel(
                 else None,
                 channels_to_snapshot=channels_to_snapshot,
             )
+            if is_fork:
+                fork_pending.difference_update(checkpoint["channel_values"])
             next_config = checkpointer.put(
                 checkpoint_config,
                 checkpoint,
@@ -2050,7 +2085,9 @@ class Pregel(
             config, {CONFIG_KEY_THREAD_ID: str(config[CONF][CONFIG_KEY_THREAD_ID])}
         )
         for superstep in supersteps:
-            current_config = perform_superstep(current_config, superstep)
+            current_config = perform_superstep(
+                current_config, superstep, is_fork=bool(fork_pending)
+            )
         return current_config
 
     async def abulk_update_state(
@@ -2103,8 +2140,21 @@ class Pregel(
             else:
                 raise ValueError(f"Subgraph {recast} not found")
 
+        # Read once from the caller's config: every later superstep receives
+        # the config of the checkpoint just written, which always names one.
+        # Cleared by the first checkpoint that carries the snapshots, which
+        # `__copy__` does not write.
+        fork_pending: set[str] = (
+            {k for k, v in self.channels.items() if isinstance(v, DeltaChannel)}
+            if config[CONF].get(CONFIG_KEY_CHECKPOINT_ID)
+            else set()
+        )
+
         async def aperform_superstep(
-            input_config: RunnableConfig, updates: Sequence[StateUpdate]
+            input_config: RunnableConfig,
+            updates: Sequence[StateUpdate],
+            *,
+            is_fork: bool,
         ) -> RunnableConfig:
             # get last checkpoint
             config = ensure_config(self.config, input_config)
@@ -2190,16 +2240,25 @@ class Pregel(
                         self.trigger_to_nodes,
                     )
                 # save checkpoint
+                next_checkpoint = create_fork_checkpoint(
+                    checkpoint,
+                    channels,
+                    step,
+                    is_fork=is_fork,
+                    get_next_version=checkpointer.get_next_version,
+                )
+                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 +2287,17 @@ class Pregel(
                         if saved and saved.metadata.get("step") is not None
                         else -1
                     )
+                    next_checkpoint = create_fork_checkpoint(
+                        checkpoint,
+                        channels,
+                        next_step,
+                        is_fork=is_fork,
+                        get_next_version=checkpointer.get_next_version,
+                    )
+                    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 +2307,7 @@ class Pregel(
                         },
                         get_new_channel_versions(
                             checkpoint_previous_versions,
-                            checkpoint["channel_versions"],
+                            next_checkpoint["channel_versions"],
                         ),
                     )
 
@@ -2335,6 +2402,7 @@ class Pregel(
                     return await aperform_superstep(
                         patch_checkpoint_map(next_config, saved.metadata),
                         [item for lst in user_group_by.values() for item in lst],
+                        is_fork=is_fork,
                     )
 
                 return patch_checkpoint_map(
@@ -2480,6 +2548,7 @@ class Pregel(
                     parents=saved.metadata.get("parents", {}) if saved else {},
                     saved_metadata=saved.metadata if saved else None,
                     is_fresh_thread=saved is None,
+                    is_fork=is_fork,
                 )
             )
             checkpoint = create_checkpoint(
@@ -2492,6 +2561,8 @@ class Pregel(
                 else None,
                 channels_to_snapshot=channels_to_snapshot,
             )
+            if is_fork:
+                fork_pending.difference_update(checkpoint["channel_values"])
             next_config = await checkpointer.aput(
                 checkpoint_config,
                 checkpoint,
@@ -2509,7 +2580,9 @@ class Pregel(
             config, {CONFIG_KEY_THREAD_ID: str(config[CONF][CONFIG_KEY_THREAD_ID])}
         )
         for superstep in supersteps:
-            current_config = await aperform_superstep(current_config, superstep)
+            current_config = await aperform_superstep(
+                current_config, superstep, is_fork=bool(fork_pending)
+            )
         return current_config
 
     def update_state(

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 +1,346 @@
+"""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, StateGraph
+from langgraph.types import Durability, StateSnapshot, StateUpdate
+
+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 _input(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(
+        _input("in-1"), config, durability=durability
+    )
+    graph = _build(sync_checkpointer, "second")
+    graph.invoke(_input("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(
+        _input("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(
+        _input("in-1"), config, durability=durability
+    )
+    graph = _build(async_checkpointer, "second")
+    await graph.ainvoke(_input("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(
+        _input("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(_input("in-1"), config, durability=durability)
+
+    root = list(graph.get_state_history(config))[-1]
+    assert root.values["log"] == []
+
+    _build(sync_checkpointer, "third").invoke(
+        _input("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(_input("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(
+        _input("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(_input("in-1"), config)
+    graph = _build(sync_checkpointer, "second")
+    graph.invoke(_input("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), _input("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(_input("in-1"), config)
+    graph = _build(async_checkpointer, "second")
+    await graph.ainvoke(_input("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), _input("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(_input("in-1"), config, durability=durability)
+    graph.invoke(_input("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(_input("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(_input("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(_input("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(_input("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(_input("in-1"), config)
+    graph = _build(sync_checkpointer, "second")
+    graph.invoke(_input("in-2"), config)
+
+    base = next(
+        snapshot
+        for snapshot in graph.get_state_history(config)
+        if "in-2" not in snapshot.values["log"]
+    )
+    first = (
+        StateUpdate(_input("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(_input("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(_input("in-1"), config)
+
+    graph.bulk_update_state(
+        config,
+        [[StateUpdate(_input(f"u{i}"), "n")] for i in range(4)],
+    )
+
+    assert not _snapshotted_checkpoints(sync_checkpointer, config)