fix(message): assign UUIDs to ID-less messages in _messages_delta_reducer

Without UUID assignment, message eviction and RemoveMessage tombstoning
fail for messages created without explicit IDs. Matches the behavior of
add_messages. IDs are now assigned inline within the existing iterations
over state_msgs and msgs to avoid an extra pass.

libs/langgraph/langgraph/graph/message.py

@@ -257,9 +257,10 @@ def _messages_delta_reducer(
 
     Raw dict / string / tuple inputs are coerced to typed `BaseMessage`
     objects so that HTTP-driven graphs work without a separate coercion
-    step. This is not full `add_messages` parity — `REMOVE_ALL_MESSAGES`,
-    unknown-id `RemoveMessage` errors, missing-id UUID assignment, and
-    `BaseMessageChunk` conversion are not handled here.
+    step. Messages without IDs are assigned UUIDs (matching `add_messages`
+    behavior) so that message eviction and `RemoveMessage` tombstoning work
+    correctly. `REMOVE_ALL_MESSAGES` and `BaseMessageChunk` conversion are
+    not handled here.
 
     Example::
 
@@ -289,15 +290,19 @@ def _messages_delta_reducer(
         state_msgs = cast("list[AnyMessage]", convert_to_messages(state))
     msgs = cast("list[AnyMessage]", convert_to_messages(flat))
 
-    index: dict[str, int] = {
-        m.id: i for i, m in enumerate(state_msgs) if m.id is not None
-    }
+    # Build index and assign missing IDs in one pass (parity with add_messages
+    # so that eviction and RemoveMessage tombstoning work on ID-less messages).
+    index: dict[str, int] = {}
+    for i, m in enumerate(state_msgs):
+        if m.id is None:
+            m.id = str(uuid.uuid4())
+        index[m.id] = i
     result: list[AnyMessage | None] = list(state_msgs)
     for msg in msgs:
+        if msg.id is None:
+            msg.id = str(uuid.uuid4())
         mid = msg.id
-        if mid is None:
-            result.append(msg)
-        elif isinstance(msg, RemoveMessage):
+        if isinstance(msg, RemoveMessage):
             if mid in index:
                 result[index[mid]] = None
                 del index[mid]

libs/langgraph/tests/test_channels.py

@@ -284,6 +284,28 @@ def test_messages_delta_reducer_tuple_write_is_one_message() -> None:
     assert result[0].content == "hi"
 
 
+def test_messages_delta_reducer_assigns_uuid_to_id_less_messages() -> None:
+    """Messages without IDs get UUIDs assigned, matching add_messages behavior.
+
+    Without UUID assignment, RemoveMessage tombstoning fails on messages that
+    were created without explicit IDs.
+    """
+    m1 = HumanMessage(content="hi")
+    m2 = AIMessage(content="hello")
+    assert m1.id is None
+    assert m2.id is None
+
+    result = _messages_delta_reducer([], [[m1, m2]])
+    assert len(result) == 2
+    assert result[0].id is not None
+    assert result[1].id is not None
+
+    # RemoveMessage tombstoning must work on the now-assigned IDs.
+    result2 = _messages_delta_reducer(result, [RemoveMessage(id=result[1].id)])
+    assert len(result2) == 1
+    assert result2[0].content == "hi"
+
+
 def test_delta_channel_checkpoint_returns_missing() -> None:
     """checkpoint() always returns MISSING regardless of state.