feat: add str-fallback for custom types in msgpack serialization

Objects whose class defines __str__ (not inherited from object) are now
serialized as EXT_CONSTRUCTOR_SINGLE_ARG with str(obj), enabling
round-trip serde for types like bson.ObjectId without new dependencies.

Co-authored-by: William FH <13333726+hinthornw@users.noreply.github.com>

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

@@ -494,6 +494,13 @@ def _msgpack_default(obj: Any) -> str | ormsgpack.Ext:
 
     elif isinstance(obj, BaseException):
         return repr(obj)
+    elif type(obj).__str__ is not object.__str__:
+        return ormsgpack.Ext(
+            EXT_CONSTRUCTOR_SINGLE_ARG,
+            _msgpack_enc(
+                (obj.__class__.__module__, obj.__class__.__name__, str(obj)),
+            ),
+        )
     else:
         raise TypeError(f"Object of type {obj.__class__.__name__} is not serializable")
 

libs/checkpoint/tests/test_jsonplus.py

@@ -93,6 +93,19 @@ class MyEnum(Enum):
     BAR = "bar"
 
 
+class StrReconstructable:
+    """Mimics types like bson.ObjectId that round-trip via str()."""
+
+    def __init__(self, value: str = "default") -> None:
+        self.value = value
+
+    def __str__(self) -> str:
+        return self.value
+
+    def __eq__(self, other: object) -> bool:
+        return isinstance(other, StrReconstructable) and self.value == other.value
+
+
 @dataclasses_json.dataclass_json
 @dataclasses.dataclass
 class Person:
@@ -580,6 +593,32 @@ def test_msgpack_safe_types_no_warning(caplog: pytest.LogCaptureFixture) -> None
         assert result is not None
 
 
+def test_serde_str_reconstructable_roundtrip() -> None:
+    """Objects with custom __str__ serialize via the str-fallback and round-trip."""
+    serde = JsonPlusSerializer(
+        allowed_msgpack_modules=[
+            (StrReconstructable.__module__, StrReconstructable.__name__)
+        ]
+    )
+    obj = StrReconstructable("abc123")
+    dumped = serde.dumps_typed(obj)
+    assert dumped[0] == "msgpack"
+    result = serde.loads_typed(dumped)
+    assert isinstance(result, StrReconstructable)
+    assert result == obj
+
+
+def test_serde_str_fallback_not_triggered_for_plain_object() -> None:
+    """Objects without custom __str__ still raise TypeError."""
+    serde = JsonPlusSerializer()
+
+    class PlainObj:
+        pass
+
+    with pytest.raises(ormsgpack.MsgpackEncodeError):
+        serde.dumps_typed(PlainObj())
+
+
 def test_msgpack_pydantic_warns_by_default(caplog: pytest.LogCaptureFixture) -> None:
     """Pydantic models not in allowlist should log warning but still deserialize."""
     current = _lg_msgpack.STRICT_MSGPACK_ENABLED