fix(langgraph): give a DeltaChannel message inside an Overwrite an id

ensure_message_ids skipped an Overwrite, so the messages inside it were
saved with no id.

libs/langgraph/langgraph/pregel/_messages.py

@@ -17,6 +17,7 @@ from pydantic import BaseModel
 
 from langgraph._internal._config import filter_to_user_tags
 from langgraph._internal._constants import NS_SEP
+from langgraph.channels.binop import _get_overwrite
 from langgraph.constants import TAG_HIDDEN, TAG_NOSTREAM
 from langgraph.pregel.protocol import StreamChunk
 from langgraph.types import Command
@@ -431,7 +432,7 @@ def ensure_message_ids(value: Any) -> None:
     BaseMessages; every get_state() replay then produces a different UUID and
     the same message appears with a different ID in each LangSmith trace.
 
-    Handles three input shapes:
+    Handles four input shapes:
     - BaseMessage objects: assign a UUID if id is None.
     - Dicts with a known "role" (OpenAI-style) or "type" (LangChain format) at
       the root level: stamp "id" into the dict in-place. The reducer's
@@ -439,6 +440,7 @@ def ensure_message_ids(value: Any) -> None:
     - Lists of the above: apply the same logic to each element, replacing dict
       items with coerced BaseMessages so the shared list reference seen by
       checkpoint_pending_writes and the background thread both get typed messages.
+    - An `Overwrite` of any of the above: apply the same logic to its value.
 
     Mutating synchronously here (before the background thread is submitted) is
     safe: the serialised bytes always reflect the post-coercion state.
@@ -459,3 +461,7 @@ def ensure_message_ids(value: Any) -> None:
                 if msg.id is None:
                     msg.id = str(uuid4())
                 value[i] = msg
+    else:
+        is_overwrite, overwrite_value = _get_overwrite(value)
+        if is_overwrite:
+            ensure_message_ids(overwrite_value)

libs/langgraph/tests/test_delta_channel_id_stability.py

@@ -17,6 +17,7 @@ from typing_extensions import TypedDict
 
 from langgraph.channels.delta import DeltaChannel
 from langgraph.graph import END, START, StateGraph
+from langgraph.types import Overwrite
 
 pytestmark = pytest.mark.anyio
 
@@ -138,3 +139,24 @@ def test_delta_channel_dict_style_message_gets_stable_id() -> None:
     assert len(set(ids)) == 1, (
         f"dict-style HumanMessage id must be stable across get_state() calls; got {ids}"
     )
+
+
+def test_delta_channel_message_in_an_overwrite_gets_stable_id() -> None:
+    graph = _build_graph(InMemorySaver())
+    config = {"configurable": {"thread_id": "overwrite-id-stability"}}
+
+    graph.invoke({"messages": Overwrite([HumanMessage(content="hello")])}, config)
+
+    ids = [
+        next(
+            m.id
+            for m in graph.get_state(config).values["messages"]
+            if isinstance(m, HumanMessage)
+        )
+        for _ in range(3)
+    ]
+
+    assert ids[0] is not None, "a message inside an Overwrite should get a UUID"
+    assert len(set(ids)) == 1, (
+        f"its id must be stable across get_state() calls; got {ids}"
+    )

libs/langgraph/tests/test_delta_channel_update_state.py

@@ -31,7 +31,7 @@ from langgraph.channels.last_value import LastValue
 from langgraph.graph import START, StateGraph
 from langgraph.graph.message import _messages_delta_reducer
 from langgraph.pregel import NodeBuilder, Pregel
-from langgraph.types import StateSnapshot, StateUpdate
+from langgraph.types import Overwrite, StateSnapshot, StateUpdate
 
 pytestmark = pytest.mark.anyio
 
@@ -688,6 +688,20 @@ async def test_aupdate_state_gives_a_message_an_id_that_every_read_keeps() -> No
     assert first == second
 
 
+def test_update_state_gives_a_message_in_an_overwrite_an_id_that_every_read_keeps() -> (
+    None
+):
+    graph = _build_graph(InMemorySaver())
+    config = {"configurable": {"thread_id": "t"}}
+    graph.invoke({"messages": [HumanMessage("a", id="a")]}, config)
+
+    graph.update_state(config, {"messages": Overwrite([HumanMessage("b")])})
+
+    first, second = _message_ids(graph, config), _message_ids(graph, config)
+    assert first[-1] is not None
+    assert first == second
+
+
 def test_update_state_on_an_older_checkpoint_gives_a_message_an_id() -> None:
     graph = _build_graph(InMemorySaver())
     config = {"configurable": {"thread_id": "t"}}