feat(checkpoint/serde): serialize DiffDelta as 'diff' type tag

Add serde support for DiffDelta by implementing dump/load for the "diff" type tag.
This allows the checkpoint system to efficiently store delta objects by serializing
them as msgpack-encoded dicts with {"d": delta, "p": prev_version} structure.

The implementation uses runtime type checking to avoid circular imports and
leverages the existing msgpack ext hooks for proper deserialization of complex
types like LangChain messages.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py

@@ -239,6 +239,12 @@ class JsonPlusSerializer(SerializerProtocol):
             return "bytes", obj
         elif isinstance(obj, bytearray):
             return "bytearray", obj
+        elif (
+            type(obj).__name__ == "DiffDelta"
+            and hasattr(obj, "delta")
+            and hasattr(obj, "prev_version")
+        ):
+            return "diff", _msgpack_enc({"d": obj.delta, "p": obj.prev_version})
         else:
             try:
                 return "msgpack", _msgpack_enc(obj)
@@ -261,6 +267,10 @@ class JsonPlusSerializer(SerializerProtocol):
             return ormsgpack.unpackb(
                 data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
             )
+        elif type_ == "diff":
+            return ormsgpack.unpackb(
+                data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
+            )
         elif self.pickle_fallback and type_ == "pickle":
             return pickle.loads(data_)
         else:

libs/checkpoint/tests/test_jsonplus.py

@@ -983,3 +983,35 @@ def test_msgpack_nested_pydantic_serializes_as_dict(
     # No blocking should occur - inner is serialized as dict, not ext
     assert "blocked" not in caplog.text.lower()
     assert result == obj
+
+
+def test_diff_delta_serde_round_trip() -> None:
+    from langgraph.checkpoint.base import DiffDelta
+
+    serde = JsonPlusSerializer()
+    prev = "00000000000000000000000000000001.1234567890123456"
+    delta = DiffDelta(
+        delta=[HumanMessage(content="hello", id="msg-1")],
+        prev_version=prev,
+    )
+    type_tag, blob = serde.dumps_typed(delta)
+    assert type_tag == "diff"
+
+    result = serde.loads_typed(("diff", blob))
+    assert isinstance(result, dict)
+    assert result["p"] == prev
+    assert len(result["d"]) == 1
+    assert result["d"][0].content == "hello"
+
+
+def test_diff_delta_serde_root_blob() -> None:
+    from langgraph.checkpoint.base import DiffDelta
+
+    serde = JsonPlusSerializer()
+    delta = DiffDelta(delta=[], prev_version=None)
+    type_tag, blob = serde.dumps_typed(delta)
+    assert type_tag == "diff"
+
+    result = serde.loads_typed(("diff", blob))
+    assert result["p"] is None
+    assert result["d"] == []