MCP server in the HTTP Streamable mode broken

Starting from v1.12.0 (checked with v1.12.0 and v1.12.1) any request to the MCP server in the HTTP Streamable mode produces an error:

INFO:     127.0.0.1:32926 - "POST /mcp/ HTTP/1.1" 307 Temporary Redirect
[07/23/25 16:34:18] INFO     Processing request of type ListToolsRequest                                        server.py:625
INFO:     127.0.0.1:32926 - "POST /mcp HTTP/1.1" 200 OK
                    INFO     Terminating session: None                                                 streamable_http.py:630
                    ERROR    Error in message router                                                   streamable_http.py:880
                             ╭────────────────── Traceback (most recent call last) ──────────────────╮                       
                             │ /usr/local/lib/python3.12/site-packages/mcp/server/streamable_http.py │                       
                             │ :831 in message_router                                                │                       
                             │                                                                       │                       
                             │   828 │   │   │   # Create a message router that distributes messages │                       
                             │   829 │   │   │   async def message_router():                         │                       
                             │   830 │   │   │   │   try:                                            │                       
                             │ ❱ 831 │   │   │   │   │   async for session_message in write_stream_r │                       
                             │   832 │   │   │   │   │   │   # Determine which request stream(s) sho │                       
                             │   833 │   │   │   │   │   │   message = session_message.message       │                       
                             │   834 │   │   │   │   │   │   target_request_id = None                │                       
                             │                                                                       │                       
                             │ /usr/local/lib/python3.12/site-packages/anyio/abc/_streams.py:35 in   │                       
                             │ __anext__                                                             │                       
                             │                                                                       │                       
                             │    32 │                                                               │                       
                             │    33 │   async def __anext__(self) -> T_co:                          │                       
                             │    34 │   │   try:                                                    │                       
                             │ ❱  35 │   │   │   return await self.receive()                         │                       
                             │    36 │   │   except EndOfStream:                                     │                       
                             │    37 │   │   │   raise StopAsyncIteration                            │                       
                             │    38                                                                 │                       
                             │                                                                       │                       
                             │ /usr/local/lib/python3.12/site-packages/anyio/streams/memory.py:111   │                       
                             │ in receive                                                            │                       
                             │                                                                       │                       
                             │   108 │   async def receive(self) -> T_co:                            │                       
                             │   109 │   │   await checkpoint()                                      │                       
                             │   110 │   │   try:                                                    │                       
                             │ ❱ 111 │   │   │   return self.receive_nowait()                        │                       
                             │   112 │   │   except WouldBlock:                                      │                       
                             │   113 │   │   │   # Add ourselves in the queue                        │                       
                             │   114 │   │   │   receive_event = Event()                             │                       
                             │                                                                       │                       
                             │ /usr/local/lib/python3.12/site-packages/anyio/streams/memory.py:93 in │                       
                             │ receive_nowait                                                        │                       
                             │                                                                       │                       
                             │    90 │   │                                                           │                       
                             │    91 │   │   """                                                     │                       
                             │    92 │   │   if self._closed:                                        │                       
                             │ ❱  93 │   │   │   raise ClosedResourceError                           │                       
                             │    94 │   │                                                           │                       
                             │    95 │   │   if self._state.waiting_senders:                         │                       
                             │    96 │   │   │   # Get the item from the next sender                 │                       
                             ╰───────────────────────────────────────────────────────────────────────╯                       
                             ClosedResourceError

Rollback to 1.11.0 fix the issue.

Hi thank you for reporting this - could you please provide a full code snippet that demonstrates the issue so we can debug on our machines?

Hi thank you for reporting this - could you please provide a full code snippet that demonstrates the issue so we can debug on our machines?

This is what I used:

mcp = FastMCP("My Test Server", 
    stateless_http=True,
    json_response=True,
    port=settings.SERVER_PORT, 
    token_verifier=SimpleTokenVerifier(),
    auth=AuthSettings(
        issuer_url=AnyHttpUrl(settings.API_BASE_URL),
        resource_server_url=AnyHttpUrl(settings.SERVER_URL),
        required_scopes=["email"],
    )
)

If I drop the json_response the error is gone.

I still have issues with this streaming http server. how can I send you debug info? https://docs.ahrefs.com/docs/mcp/reference/introduction

I still have issues with this streaming http server. how can I send you debug info? https://docs.ahrefs.com/docs/mcp/reference/introduction

Is your issue with the ahrefs server specifically or something you built? Are you running a server yourself or you mean you have trouble connecting to the ahrefs mcp server?

I have trouble connecting through chatmcp. The MCP server itself works when using it through my ide with the same setup. It's a streaming http server.

Is there a way I can provide you with logs

I have trouble connecting through chatmcp. The MCP server itself works when using it through my ide with the same setup. It's a streaming http server.

Is there a way I can provide you with logs

Just paste them here in this issue or share a gist if it's a lot

@felixweinberger

@felixweinberger This is bloating our logs and can be easily reproduced as follows:

Install PyCLI MCP
```
pip install pycli-mcp
```
Install MCP Inspector
```
npm i @modelcontextprotocol/inspector
```
In one shell session run the server using its own CLI:
```
pycli-mcp pycli_mcp.cli:pycli_mcp
```

In another shell session query the server using the inspector:

mcp-inspector --cli --transport http http://localhost:8000/mcp/ --method tools/list

Notice output from the server that is similar to the following:

❯ pycli-mcp pycli_mcp.cli:pycli_mcp
Serving: pycli-mcp
INFO:     Started server process [62884]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     127.0.0.1:57660 - "POST /mcp/ HTTP/1.1" 200 OK
Error in message router
Traceback (most recent call last):
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\mcp\server\streamable_http.py", line 831, in message_router
    async for session_message in write_stream_reader:
    ...<45 lines>...
            )
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\anyio\abc\_streams.py", line 41, in __anext__
    return await self.receive()
          ^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\anyio\streams\memory.py", line 111, in receive
    return self.receive_nowait()
          ~~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\anyio\streams\memory.py", line 93, in receive_nowait
    raise ClosedResourceError
anyio.ClosedResourceError
INFO:     127.0.0.1:57662 - "POST /mcp/ HTTP/1.1" 202 Accepted
INFO:     127.0.0.1:57660 - "GET /mcp/ HTTP/1.1" 200 OK
INFO:     127.0.0.1:57664 - "POST /mcp/ HTTP/1.1" 200 OK
ERROR:mcp.server.streamable_http:Error in message router
Traceback (most recent call last):
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\mcp\server\streamable_http.py", line 831, in message_router
    async for session_message in write_stream_reader:
    ...<45 lines>...
            )
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\anyio\abc\_streams.py", line 41, in __anext__
    return await self.receive()
          ^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\anyio\streams\memory.py", line 111, in receive
    return self.receive_nowait()
          ~~~~~~~~~~~~~~~~~~~^^
  File "C:\Users\ofek\AppData\Roaming\uv\tools\pycli-mcp\Lib\site-packages\anyio\streams\memory.py", line 93, in receive_nowait
    raise ClosedResourceError
anyio.ClosedResourceError

Consolidating duplicate issues:

The following issues have been closed as duplicates of this one:

_handle_stateless_request ClosedResourceError #1219 - _handle_stateless_request ClosedResourceError
Race Condition in StreamableHTTP Transport Causes ClosedResourceError #1363 - Race Condition in StreamableHTTP Transport
ClosedResourceError when using FastMCP(..., stateless_http=True, json_response=True) with MCP Inspector #1658 - ClosedResourceError with FastMCP stateless_http=True

Root Cause Summary (from #1363):

This is a race condition in message_router() at streamable_http.py:959:

message_router enters async for write_stream_reader loop
The iterator calls checkpoint(), yielding control
Request completes/fails early and calls terminate(), closing the stream
message_router resumes and encounters closed stream → ClosedResourceError

Proposed Fix: PR #1384 adds explicit ClosedResourceError handling before the generic except Exception block.

Workaround (from #1363): Suppress logs via uvicorn config:

mcp.server.streamable_http:
  level: CRITICAL

_{AI Disclaimer}

@maxisbey

@maxisbey What's blocking the proposed fix that you mentioned from being merged?

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MCP server in the HTTP Streamable mode broken #1190

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MCP server in the HTTP Streamable mode broken #1190

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felixweinberger commented on Oct 3, 2025

elioengcomp commented on Oct 3, 2025

christoph88 commented on Oct 17, 2025

felixweinberger commented on Oct 17, 2025

christoph88 commented on Oct 17, 2025

felixweinberger commented on Oct 17, 2025

ofek commented on Oct 21, 2025

maxisbey commented on Dec 2, 2025

ofek commented on Dec 2, 2025

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