AssertionError: Unexpected message while using middlewares

Commencez par le montage de l’application Starlette mcp_server.sse_app(), l’exemple de CustomHeaderMiddleware et le chemin d’appel du middleware indiqué dans la traceback. Reproduisez l’échec en appelant un outil via l’application SSE montée, puis suivez les messages de réponse jusqu’à comprendre le message inattendu. Le travail est terminé lorsque l’application enveloppée par le middleware termine la requête sans AssertionError.

Describe the bug
when I mount the mcp to starlette, and I use middleware to do some prechecks, I met this errors.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/app/.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 "/app/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in call
return await self.app(scope, receive, send)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/app/.venv/lib/python3.12/site-packages/starlette/applications.py", line 113, in call
await self.middleware_stack(scope, receive, send)
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 187, in call
raise exc
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py", line 165, in call
await self.app(scope, receive, _send)
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 182, in call
with recv_stream, send_stream, collapse_excgroups():
^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/contextlib.py", line 158, in exit
self.gen.throw(value)
File "/app/.venv/lib/python3.12/site-packages/starlette/_utils.py", line 83, in collapse_excgroups
raise exc
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 185, in call
await response(scope, wrapped_receive, send)
File "/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py", line 223, in call
async for chunk in self.body_iterator:
File "/app/.venv/lib/python3.12/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')]}

To Reproduce
Steps to reproduce the behavior:

1. mount mcp to starlette
2. use middlewares
3. call tools

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"]

Expected behavior
Should not raise type error

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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