AssertionError: Unexpected message while using middleware

[JonZeolla]

Description

We are using FastMCP v2.8.1 and seeing the same as modelcontextprotocol/python-sdk#883

Here is my error:

ERROR:    Exception in ASGI application
  + Exception Group Traceback (most recent call last):
  |   File "/app/.venv/lib/python3.13/site-packages/starlette/_utils.py", line 77, in collapse_excgroups
  |     yield
  |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 183, in __call__
  |     async with anyio.create_task_group() as task_group:
  |                ~~~~~~~~~~~~~~~~~~~~~~~^^
  |   File "/app/.venv/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
  |     raise BaseExceptionGroup(
  |         "unhandled errors in a TaskGroup", self._exceptions
  |     ) from None
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/app/.venv/lib/python3.13/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
    |     result = await app(  # type: ignore[func-returns-value]
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |         self.scope, self.receive, self.send
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |     )
    |     ^
    |   File "/app/.venv/lib/python3.13/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    |     return await self.app(scope, receive, send)
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/applications.py", line 113, in __call__
    |     await self.middleware_stack(scope, receive, send)
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/errors.py", line 187, in __call__
    |     raise exc
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/errors.py", line 165, in __call__
    |     await self.app(scope, receive, _send)
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 182, in __call__
    |     with recv_stream, send_stream, collapse_excgroups():
    |                                    ~~~~~~~~~~~~~~~~~~^^
    |   File "/usr/local/lib/python3.13/contextlib.py", line 162, in __exit__
    |     self.gen.throw(value)
    |     ~~~~~~~~~~~~~~^^^^^^^
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
    |     raise exc
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 185, in __call__
    |     await response(scope, wrapped_receive, send)
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 223, in __call__
    |     async for chunk in self.body_iterator:
    |     ...<5 lines>...
    |         await send({"type": "http.response.body", "body": chunk, "more_body": True})
    |   File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 169, in body_stream
    |     assert message["type"] == "http.response.body", f"Unexpected message: {message}"
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | AssertionError: Unexpected message: {'type': 'http.response.start', 'status': 200, 'headers': [(b'content-length', b'0')]}
    +------------------------------------
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/app/.venv/lib/python3.13/site-packages/uvicorn/protocols/http/httptools_impl.py", line 409, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        self.scope, self.receive, self.send
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/app/.venv/lib/python3.13/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/.venv/lib/python3.13/site-packages/starlette/applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/errors.py", line 187, in __call__
    raise exc
  File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/errors.py", line 165, in __call__
    await self.app(scope, receive, _send)
  File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 182, in __call__
    with recv_stream, send_stream, collapse_excgroups():
                                   ~~~~~~~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/contextlib.py", line 162, in __exit__
    self.gen.throw(value)
    ~~~~~~~~~~~~~~^^^^^^^
  File "/app/.venv/lib/python3.13/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
    raise exc
  File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 185, in __call__
    await response(scope, wrapped_receive, send)
  File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 223, in __call__
    async for chunk in self.body_iterator:
    ...<5 lines>...
        await send({"type": "http.response.body", "body": chunk, "more_body": True})
  File "/app/.venv/lib/python3.13/site-packages/starlette/middleware/base.py", line 169, in body_stream
    assert message["type"] == "http.response.body", f"Unexpected message: {message}"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Unexpected message: {'type': 'http.response.start', 'status': 200, 'headers': [(b'content-length', b'0')]}

I just wanted to port the issue over here since it appears to have the same root cause. Note, I did not test the example code, it came from the upstream issue; if it's not enough I'm already working on a separate MRE

Example Code

class CustomHeaderMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        return await call_next(request)


middleware = [
    Middleware(CustomHeaderMiddleware)
]

app = Starlette(middleware=middleware)
app.mount("/", mcp_server.sse_app())

def main():
    uvicorn.run(
        app="atlantis_mcp_server:app",
        host="0.0.0.0",
        port=8000,
        workers=1,
        log_level="info"
    )

__all__ = ["main"]

Version Information

FastMCP version:                                                                                    2.8.1
MCP version:                                                                                        1.9.4
Python version:                                                                                    3.13.5
Platform:                                                               macOS-15.5-arm64-arm-64bit-Mach-O
FastMCP root path: /Users/jonzeolla/.venv/lib/python3.13…

Additional Context

No response

[jlowin]

Thanks @JonZeolla -- to be clear, you're seeing the same error as the upstream issue, but with different code (presumably from a FastMCP server)?

[JonZeolla]

@jlowin right; it is from a FastMCP server, and we're also using a middleware. Our env is somewhat complex, so working on my own MRE, but everything else seems to line up with the upstream ticket.

[JonZeolla]

I wonder if this is something that should be caught and suppressed when message["type"] is http.response.start? Just a guess but this seems seems like it's just noise from the middleware during session setup.

[Kludex]

There's probably an issue on the mcp side, not in this repository. They are wrongly sending two http.response.start messages.

If an MRE that I can copy/paste is provided, I'm happy to try to debug it.

[icyriverwolf]

me too

[icyriverwolf]

@jlowin right; it is from a FastMCP server, and we're also using a middleware. Our env is somewhat complex, so working on my own MRE, but everything else seems to line up with the upstream ticket.确实如此；它来自一个 FastMCP 服务器，而且我们也在使用一个中间件。我们的环境有些复杂，所以我正在处理自己的最小化重现案例（MRE），但其他方面似乎与上游工单一致。

i had the same problem, is any solution？