feat(serde): diff type encodes prev_checkpoint_id; loads_typed returns DeltaValue

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -263,7 +263,7 @@ class JsonPlusSerializer(SerializerProtocol):
         elif isinstance(obj, bytearray):
             return "bytearray", obj
         elif _is_diff_delta(obj):
-            return "diff", _msgpack_enc({"d": obj.delta, "p": obj.prev_version})
+            return "diff", _msgpack_enc({"d": obj.delta, "c": obj.prev_checkpoint_id})
         else:
             try:
                 return "msgpack", _msgpack_enc(obj)
@@ -287,9 +287,11 @@ class JsonPlusSerializer(SerializerProtocol):
                 data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
             )
         elif type_ == "diff":
-            return ormsgpack.unpackb(
+            from langgraph.checkpoint.base import DeltaValue  # lazy import
+            raw = ormsgpack.unpackb(
                 data_, ext_hook=self._unpack_ext_hook, option=ormsgpack.OPT_NON_STR_KEYS
             )
+            return DeltaValue(delta=raw["d"], prev_checkpoint_id=raw.get("c"))
         elif self.pickle_fallback and type_ == "pickle":
             return pickle.loads(data_)
         else:

libs/checkpoint/tests/test_jsonplus.py

@@ -999,33 +999,27 @@ def test_msgpack_nested_pydantic_serializes_as_dict(
     assert result == obj
 
 
-def test_diff_delta_serde_round_trip() -> None:
+def test_delta_value_serde_round_trip() -> None:
     from langgraph.checkpoint.base import DeltaValue
+    from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
 
     serde = JsonPlusSerializer()
-    prev = "00000000000000000000000000000001.1234567890123456"
-    delta = DeltaValue(
-        delta=[HumanMessage(content="hello", id="msg-1")],
-        prev_version=prev,
-    )
-    type_tag, blob = serde.dumps_typed(delta)
+    original = DeltaValue(delta=[{"type": "human", "content": "hi"}], prev_checkpoint_id="abc-123")
+    type_tag, blob = serde.dumps_typed(original)
     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"
+    loaded = serde.loads_typed((type_tag, blob))
+    assert isinstance(loaded, DeltaValue)
+    assert loaded.delta == original.delta
+    assert loaded.prev_checkpoint_id == "abc-123"
 
 
-def test_diff_delta_serde_root_blob() -> None:
+def test_delta_value_serde_chain_root() -> None:
     from langgraph.checkpoint.base import DeltaValue
+    from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
 
     serde = JsonPlusSerializer()
-    delta = DeltaValue(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"] == []
+    original = DeltaValue(delta=[], prev_checkpoint_id=None)
+    type_tag, blob = serde.dumps_typed(original)
+    loaded = serde.loads_typed((type_tag, blob))
+    assert isinstance(loaded, DeltaValue)
+    assert loaded.prev_checkpoint_id is None