TypeError: Object of type Send is not JSON serializable when using PostgresSaver checkpoint

### Checked other resources

- [x] This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/).
- [x] I added a clear and detailed title that summarizes the issue.
- [x] I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example).
- [x] I included a self-contained, minimal example that demonstrates the issue INCLUDING all the relevant imports. The code run AS IS to reproduce the issue.

### Example Code

```python
import operator
from typing import Annotated, TypedDict, List, Optional
from langgraph.graph import StateGraph, START, END
from langgraph.checkpoint.postgres import PostgresSaver
from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
from langgraph.checkpoint.memory import MemorySaver
from langgraph.constants import Send

class OverallState(TypedDict):
    topics: List[str] = ["fine arts", 'firefighters', 'maths']
    jokes: Annotated[List[str], operator.add]
    all_jokes: str

default_state = OverallState(topics=["fine arts", 'firefighters', 'maths'],
                                   jokes=[], all_jokes="")

class JokeState(TypedDict):
    topic: str

def send_topics(state: OverallState):
    print("Sending joke creation")
    return [Send("create_joke", JokeState(topic=topic)) for topic in state["topics"]]

def create_joke(joke_state: JokeState) -> dict:
    return {"jokes": [f'Joke for topic {joke_state["topic"]}']}

def reduce_jokes(state: OverallState) -> dict:
    print("Merge jokes")
    return {"all_jokes": "\n".join(state['jokes'])} 


def main():
    print("Creating graph with...")
    
    # Create graph
    workflow = StateGraph(OverallState)
    workflow.add_node("create_joke", create_joke)
    workflow.add_node("reduce_jokes", reduce_jokes)
    workflow.add_conditional_edges(START, send_topics, {"create_joke"})
    workflow.add_edge("create_joke", "reduce_jokes")
    workflow.add_edge("reduce_jokes", END)
    

    # Try with Memory checkpointing (this should work)
    print("\nTrying with Memory checkpointing...")
    try:
        checkpointer = MemorySaver()
        app = workflow.compile(checkpointer=checkpointer)
        
        print("Running graph with Memory checkpointing...")
        result = app.invoke(default_state, config={'configurable': {'thread_id': "1"}})
        print(f"Result: {result}")
        
    except Exception as e:
        print(f"Error with Memory checkpointing: {e}")
        raise e


    # Try with Postgres checkpointing (this should fail)
    try:
        with PostgresSaver.from_conn_string(
            "postgresql://test_user:test_password@postgres:9999/test_db") as checkpointer:
            checkpointer.setup()
            app = workflow.compile(checkpointer=checkpointer)
            
            print("\n\nRunning graph with Postgres checkpointing...")
            result = app.invoke(default_state, config={'configurable': {'thread_id': "1"}})
            print(f"Result: {result}")
        
    except Exception as e:
        print(f"Error with Postgres checkpointing: {e}")
        raise e

if __name__ == "__main__":
    main()
```

### Error Message and Stack Trace (if applicable)

```shell
TypeError: Object of type Send is not JSON serializable
 Traceback (most recent call last):
   File "/app/app.py", line 75, in <module>
     main()
   File "/app/app.py", line 72, in main
     raise e
   File "/app/app.py", line 67, in main
     result = app.invoke(default_state, config={'configurable': {'thread_id': "1"}})
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/__init__.py", line 1927, in invoke
     for chunk in self.stream(
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/__init__.py", line 1596, in stream
     with SyncPregelLoop(
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/loop.py", line 874, in __exit__
     return self.stack.__exit__(exc_type, exc_value, traceback)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/contextlib.py", line 601, in __exit__
     raise exc_details[1]
   File "/usr/local/lib/python3.11/contextlib.py", line 586, in __exit__
     if cb(*exc_details):
        ^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/executor.py", line 107, in __exit__
     task.result()
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 449, in result
     return self.__get_result()
            ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
     raise self._exception
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/executor.py", line 70, in done
     task.result()
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 449, in result
     return self.__get_result()
            ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
     raise self._exception
   File "/usr/local/lib/python3.11/concurrent/futures/thread.py", line 58, in run
     result = self.fn(*self.args, **self.kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/loop.py", line 799, in _checkpointer_put_after_previous
     prev.result()
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 456, in result
     return self.__get_result()
            ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
     raise self._exception
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/executor.py", line 70, in done
     task.result()
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 449, in result
     return self.__get_result()
            ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
     raise self._exception
   File "/usr/local/lib/python3.11/concurrent/futures/thread.py", line 58, in run
     result = self.fn(*self.args, **self.kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/loop.py", line 799, in _checkpointer_put_after_previous
     prev.result()
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 456, in result
     return self.__get_result()
            ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
     raise self._exception
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/executor.py", line 70, in done
     task.result()
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 449, in result
     return self.__get_result()
            ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
     raise self._exception
   File "/usr/local/lib/python3.11/concurrent/futures/thread.py", line 58, in run
     result = self.fn(*self.args, **self.kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/langgraph/pregel/loop.py", line 801, in _checkpointer_put_after_previous
     cast(BaseCheckpointSaver, self.checkpointer).put(
   File "/usr/local/lib/python3.11/site-packages/langgraph/checkpoint/postgres/__init__.py", line 321, in put
     cur.execute(
   File "/usr/local/lib/python3.11/site-packages/psycopg/cursor.py", line 93, in execute
     self._conn.wait(
   File "/usr/local/lib/python3.11/site-packages/psycopg/connection.py", line 453, in wait
     return waiting.wait(gen, self.pgconn.socket, interval=interval)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "psycopg_binary/_psycopg/waiting.pyx", line 219, in psycopg_binary._psycopg.wait_c
   File "/usr/local/lib/python3.11/site-packages/psycopg/_cursor_base.py", line 194, in _execute_gen
     pgq = self._convert_query(query, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/site-packages/psycopg/_cursor_base.py", line 453, in _convert_query
     pgq.convert(query, params)
   File "/usr/local/lib/python3.11/site-packages/psycopg/_queries.py", line 95, in convert
     self.dump(vars)
   File "/usr/local/lib/python3.11/site-packages/psycopg/_queries.py", line 106, in dump
     self.params = self._tx.dump_sequence(params, self._want_formats)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "psycopg_binary/_psycopg/transform.pyx", line 353, in psycopg_binary._psycopg.Transformer.dump_sequence
   File "psycopg_binary/_psycopg/transform.pyx", line 404, in psycopg_binary._psycopg.Transformer.dump_sequence
   File "/usr/local/lib/python3.11/site-packages/psycopg/types/json.py", line 218, in dump
     if isinstance((data := dumps(obj)), str):
                            ^^^^^^^^^^
   File "/usr/local/lib/python3.11/json/__init__.py", line 231, in dumps
     return _default_encoder.encode(obj)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/json/encoder.py", line 200, in encode
     chunks = self.iterencode(o, _one_shot=True)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/json/encoder.py", line 258, in iterencode
     return _iterencode(o, 0)
            ^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.11/json/encoder.py", line 180, in default
     raise TypeError(f'Object of type {o.__class__.__name__} '
 TypeError: Object of type Send is not JSON serializable
```

### Description

Recently I wanted to persist the checkpoint data of my graph. I imported PostgresSaver (actually AsyncPosgresSaver) and figured out how to call this from the documentation: https://docs.langchain.com/oss/python/langgraph/add-memory#example-using-postgres-checkpointer.

However, I found that my implementation using `Send` as specified in many Map-Reduce implementations using LangGraph (e.g. https://www.youtube.com/watch?v=JQznvlSatPQ) raises a `TypeError` due to the `Send` class not being JSON serializable.

I found some other issues regarding serialization, but none are specific to this scenario.


The code shows that whereas with `MemorySaver` it works, with the database and `PostgresSaver` there is a need to serialize data, which turns out to contain the `Send`. 

The output shown at the termina when I perform `docker compose up` for the app container is:

```console
Creating graph with...
 
 Trying with Memory checkpointing...
 Running graph with Memory checkpointing...
 Sending joke creation
 Merge jokes
 Result: {'topics': ['fine arts', 'firefighters', 'maths'], 'jokes': ['Joke for topic fine arts', 'Joke for topic firefighters', 'Joke for topic maths'], 'all_jokes': 'Joke for topic fine arts\nJoke for topic firefighters\nJoke for topic maths'}
 
 
 Running graph with Postgres checkpointing...
 Sending joke creation
 Merge jokes
 Error with Postgres checkpointing: Object of type Send is not JSON serializable
```

### System Info

I am using dockercompose:

```docker-compose.yml
services:

  postgres:
    image: postgres
    restart: always
    environment:
      - POSTGRES_USER=test_user
      - POSTGRES_PASSWORD=test_password
      - POSTGRES_DB=test_db
      - PGPORT=9999
    ports:
      - 9999:9999
    volumes:
      - postgres_data:/var/lib/postgresql/data

  app:
    build: .
    depends_on:
      - postgres
    environment:
      - POSTGRES_HOST=postgres
      - POSTGRES_PORT=9999
      - POSTGRES_DB=test_db
      - POSTGRES_USER=test_user
      - POSTGRES_PASSWORD=test_password
    volumes:
      - .:/app/

volumes:
  postgres_data:
```

Where the Dockerfile for app is:

```DockerFile
FROM python:3.11-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install -r requirements.txt

COPY . .

CMD ["python", "app.py"]
```

The requirements:

```requirements.txt
langchain==0.3.9
langchain-community==0.3.8
langchain-core==0.3.21
langchain-experimental==0.3.3
langgraph==0.2.53
langgraph-checkpoint==2.1.2
langgraph-checkpoint-postgres==3.0.1
langgraph-sdk==0.1.40
psycopg[binary,pool]
psycopg==3.2.12
psycopg-pool==3.2.7
pydantic==2.10.2
```

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

TypeError: Object of type Send is not JSON serializable when using PostgresSaver checkpoint #6456

Checked other resources

Example Code

Error Message and Stack Trace (if applicable)

Description

System Info

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

TypeError: Object of type Send is not JSON serializable when using PostgresSaver checkpoint #6456

Description

Checked other resources

Example Code

Error Message and Stack Trace (if applicable)

Description

System Info

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions