Unable to serialize unknown type: <class 'langgraph.types.Send'>

[higuhigu-lb]

I am building a graph that uses the Send API for interruption and resumption. I've exposed this graph via a FastAPI endpoint and am using ag_ui_langgraph to visualize and interact with it.

However, when I access the UI, I encounter a PydanticSerializationError on the backend. The error message is Unable to serialize unknown type: <class 'langgraph.types.Send'>.

It seems the SSE encoder within ag_ui cannot handle the Send object when processing the event stream.

Here is the anonymized stack trace:

Exception Group Traceback (most recent call last):
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/_utils.py", line 76, in collapse_excgroups
|     yield
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/responses.py", line 263, in call
|     async with anyio.create_task_group() as task_group:
|                ^^^^^^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 772, in aexit
|     raise BaseExceptionGroup(
| ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
+-+---------------- 1 ----------------
| Traceback (most recent call last):
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py", line 403, in run_asgi
|     result = await app(  # type: ignore[func-returns-value]
|              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in call
|     return await self.app(scope, receive, send)
|            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in call
|     await super().call(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/applications.py", line 112, in call
|     await self.middleware_stack(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 187, in call
|     raise exc
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 165, in call
|     await self.app(scope, receive, _send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 62, in call
|     await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
|     raise exc
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
|     await app(scope, receive, sender)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/routing.py", line 714, in call
|     await self.middleware_stack(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/routing.py", line 734, in app
|     await route.handle(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/routing.py", line 288, in handle
|     await self.app(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/routing.py", line 76, in app
|     await wrap_app_handling_exceptions(app, request)(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
|     raise exc
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
|     await app(scope, receive, sender)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/routing.py", line 74, in app
|     await response(scope, receive, send)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/responses.py", line 262, in call
|     with collapse_excgroups():
|          ^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-python-env}/lib/python3.12/contextlib.py", line 158, in exit
|     self.gen.throw(value)
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/_utils.py", line 82, in collapse_excgroups
|     raise exc
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/responses.py", line 266, in wrap
|     await func()
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/starlette/responses.py", line 246, in stream_response
|     async for chunk in self.body_iterator:
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/ag_ui_langgraph/endpoint.py", line 22, in event_generator
|     yield encoder.encode(event)
|           ^^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/ag_ui/encoder/encoder.py", line 26, in encode
|     return self._encode_sse(event)
|            ^^^^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/ag_ui/encoder/encoder.py", line 32, in _encode_sse
|     return f"data: {event.model_dump_json(by_alias=True, exclude_none=True)}\n\n"
|                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|   File "/{path-to-project}/.venv/lib/python3.12/site-packages/pydantic/main.py", line 519, in model_dump_json
|     return self.pydantic_serializer.to_json(
|            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| pydantic_core._pydantic_core.PydanticSerializationError: Unable to serialize unknown type: <class 'langgraph.types.Send'>
+------------------------------------

Is there a recommended way or a known workaround to handle the serialization of the Send type when using it with ag_ui_langgraph?

Any help or guidance would be greatly appreciated. Thank you!

Environment:

• langgraph: 0.6.5
• langchain: 0.3.26
• fastapi: 0.115.14
• pydantic: 2.11.7
• ag_ui_langgraph: 0.0.7
• copilotkit: 0.1.60
• Python: 3.12.11

[higuhigu-lb]

similar discussion
langchain-ai/langgraph#5725