fix(sdk): handle None state in messages delta reducer (#3636)

Fixes #3564

---

`DeltaChannel.replay_writes` can pass `state=None` when a thread's
earliest checkpoint never seeded `messages: []`. The reducer now treats
`None` as an empty list, preventing `convert_to_messages` from receiving
a `None` argument. Regression tests for langchain-ai/deepagents#3564
included.

---

Note: intentionally diverging from upstream, will port back later

libs/deepagents/deepagents/_messages_reducer.py

@@ -29,7 +29,7 @@ from langgraph.graph.message import REMOVE_ALL_MESSAGES
 
 
 def _messages_delta_reducer(  # noqa: C901, PLR0912
-    state: list[AnyMessage], writes: list[list[AnyMessage]]
+    state: list[AnyMessage] | None, writes: list[list[AnyMessage]]
 ) -> list[AnyMessage]:
     """Batch reducer for use with `DeltaChannel` on the messages key.
 
@@ -51,8 +51,11 @@ def _messages_delta_reducer(  # noqa: C901, PLR0912
             flat.append(w)
     # Steady state: the reducer's own output is already typed BaseMessages,
     # so skip convert_to_messages on the fast path. Only raw input (initial
-    # dicts, deserialized blobs) hits the slow path.
-    state_msgs = state if state and isinstance(state[0], BaseMessage) else cast("list[AnyMessage]", convert_to_messages(state))
+    # dicts, deserialized blobs) hits the slow path. `state` is `None` on
+    # `DeltaChannel.replay_writes` for threads whose earliest checkpoint did
+    # not seed `messages: []`; treat that as the empty list so the slow path
+    # doesn't pass `None` into `convert_to_messages`.
+    state_msgs = state if state and isinstance(state[0], BaseMessage) else cast("list[AnyMessage]", convert_to_messages(state or []))
     msgs = cast("list[AnyMessage]", convert_to_messages(flat))
 
     # REMOVE_ALL_MESSAGES resets everything; find the last sentinel and

libs/deepagents/tests/unit_tests/test_messages_reducer.py

@@ -91,6 +91,31 @@ def test_human_message_id_stable_across_invocations_sync() -> None:
     )
 
 
+def test_reducer_handles_none_base_state() -> None:
+    """`DeltaChannel.replay_writes` passes `state=None` when the earliest
+    checkpoint for a thread did not seed `messages: []`. The reducer must
+    treat that the same as an empty base.
+
+    Regression test for https://github.com/langchain-ai/deepagents/issues/3564.
+    """  # noqa: D205
+    msg = HumanMessage(content="hi", id="h1")
+    result = _messages_delta_reducer(None, [[msg]])
+    assert result == [msg]
+
+    # Empty writes against a None base should also not crash.
+    assert _messages_delta_reducer(None, []) == []
+    assert _messages_delta_reducer(None, [[]]) == []
+
+
+def test_reducer_handles_none_base_state_with_dict_messages() -> None:
+    """None-base replay still coerces raw over-the-wire message payloads."""
+    result = _messages_delta_reducer(None, [[{"role": "user", "content": "hi"}]])
+
+    assert len(result) == 1
+    assert isinstance(result[0], HumanMessage)
+    assert result[0].content == "hi"
+
+
 @pytest.mark.anyio
 async def test_human_message_id_stable_across_invocations_async() -> None:
     """Same check via ainvoke (AsyncPregelLoop path)."""