The Python MCP server hangs when the /messages endpoint gives an unexpected response

[samuelcolvin]

When the /messages endpoint returns say 404 or 405, a client hangs rather than raise the exception I would expect.

I get the error summary printed:

Error in post_writer: Client error '404 Not Found' for url 'http://localhost:8000/wrong?sessionId=d9e6a07d-e96f-420a-99f0-90f5444ddd43'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/404

Then nothing until I ctrl+c, when I get the traceback:

Traceback

^CTraceback (most recent call last):
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/anyio/streams/memory.py", line 111, in receive
    return self.receive_nowait()
           ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/anyio/streams/memory.py", line 106, in receive_nowait
    raise WouldBlock
anyio.WouldBlock

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/samuel/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/samuel/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/samuel/code/pydantic-ai/mcp_client_error.py", line 13, in run
    async with ClientSession(read, write) as session:
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/mcp/shared/session.py", line 210, in __aexit__
    return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 776, in __aexit__
    raise exc_val
  File "/Users/samuel/code/pydantic-ai/mcp_client_error.py", line 14, in run
    await session.initialize()
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/mcp/client/session.py", line 122, in initialize
    result = await self.send_request(
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/mcp/shared/session.py", line 252, in send_request
    response_or_error = await response_stream_reader.receive()
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/anyio/streams/memory.py", line 119, in receive
    await receive_event.wait()
  File "/Users/samuel/Library/Caches/uv/environments-v2/mcp-client-error-5fc93589004e4f10/lib/python3.12/site-packages/anyio/_backends/_asyncio.py", line 1774, in wait
    await self._event.wait()
  File "/Users/samuel/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/locks.py", line 212, in wait
    await fut
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/samuel/code/pydantic-ai/mcp_client_error.py", line 26, in <module>
    asyncio.run(run())
  File "/Users/samuel/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 194, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/Users/samuel/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/runners.py", line 123, in run
    raise KeyboardInterrupt()
KeyboardInterrupt

MRE code:

Client:

# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "mcp==1.6.0",
# ]
# ///
from mcp import ClientSession
from mcp.client.sse import sse_client


async def run():
    async with sse_client('http://localhost:8000/sse') as (read, write):
        async with ClientSession(read, write) as session:
            await session.initialize()

            prompts = await session.list_prompts()
            print('prompts:', prompts)
            resources = await session.list_resources()
            print('resources:', resources)
            tools = await session.list_tools()
            print('tools:', tools)


if __name__ == "__main__":
    import asyncio
    asyncio.run(run())

Server

# /// script
# requires-python = ">=3.12"
# dependencies = [
#     "fastapi",
#     "uvicorn",
# ]
# ///
import json
from fastapi import FastAPI, Response

app = FastAPI()

sse_connect_response = json.dumps(
    {
        'result': {
            'protocolVersion': '2024-11-05',
            'capabilities': {'logging': {}, 'tools': {}},
            'serverInfo': {'name': 'Foobar', 'version': '0.0.1'},
        },
        'jsonrpc': '2.0',
        'id': 0,
    }
)

response = f"""\
event: endpoint
data: /wrong?sessionId=d9e6a07d-e96f-420a-99f0-90f5444ddd43

event: message
data: {sse_connect_response}
"""


@app.get('/sse')
def sse():
    return Response(content=response, media_type='text/event-stream')


if __name__ == '__main__':
    import uvicorn
    uvicorn.run(app, port=8000)

[kavinkumar807]

I want to contribute to MCP actively, I’m happy to help with this issue and excited to start looking into it. Shall I begin investigating this ??

[gdoctor]

I also experienced this hang for a similar but different reason. When the sse_client context manager raises the exception in line 81, the code will hang when you get to line 107 of sse_client (send call).

I suspect it's because there is nothing consuming the data we send yet. Of course nothing could be consuming the read_stream because we haven't yielded it yet since the above failure comes during startup.