[BUG] Exception occurs during client session disconnection

[cuspymd]

Describe the bug

When adding a middleware, the following exception always occurs during client session disconnection. While there is no issue with calling the tool, I'm curious if there is a way to prevent this exception from occurring.

[2025-06-13 12:15:28,591][DEBUG][sse_starlette.sse][line=230][49972] - Got event: http.disconnect. Stop streaming.
[2025-06-13 12:15:28,591][DEBUG][root][line=159][49972] - Client session disconnected 04bbc672-f124-4291-b1fa-3b8a923ffe35
     ERROR   Exception in ASGI application
  + Exception Group Traceback (most recent call last):
  |   File "C:\...\Lib\site-packages\starlette\_utils.py", line 76, in collapse_excgroups
  |     yield
  |   File "C:\...\Lib\site-packages\starlette\middleware\base.py", line 177, in __call__
  |     async with anyio.create_task_group() as task_group:
  |                ^^^^^^^^^^^^^^^^^^^^^^^^^
  |   File "C:\...\Lib\site-packages\anyio\_backends\_asyncio.py", line 772, in __aexit__
  |     raise BaseExceptionGroup(
  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "C:\...\Lib\site-packages\uvicorn\protocols\http\httptools_impl.py", line 409, in run_asgi
    |     result = await app(  # type: ignore[func-returns-value]
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\...\Lib\site-packages\uvicorn\middleware\proxy_headers.py", line 60, in __call__
    |     return await self.app(scope, receive, send)
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "C:\...\Lib\site-packages\fastapi\applications.py", line 1054, in __call__
    |     await super().__call__(scope, receive, send)
    |   File "C:\...\Lib\site-packages\starlette\applications.py", line 112, in __call__
    |     await self.middleware_stack(scope, receive, send)
    |   File "C:\...\Lib\site-packages\starlette\middleware\errors.py", line 187, in __call__
    |     raise exc
    |   File "C:\...\Lib\site-packages\starlette\middleware\errors.py", line 165, in __call__
    |     await self.app(scope, receive, _send)
    |   File "C:\...\Lib\site-packages\starlette\middleware\base.py", line 176, in __call__
    |     with recv_stream, send_stream, collapse_excgroups():
    |                                    ^^^^^^^^^^^^^^^^^^^^
    |   File "C:\...\Lib\contextlib.py", line 158, in __exit__
    |     self.gen.throw(value)
    |   File "C:\...\Lib\site-packages\starlette\_utils.py", line 82, in collapse_excgroups
    |     raise exc
    |   File "C:\...\Lib\site-packages\starlette\middleware\base.py", line 179, in __call__
    |     await response(scope, wrapped_receive, send)
    |   File "C:\...\Lib\site-packages\starlette\middleware\base.py", line 216, in __call__
    |     async for chunk in self.body_iterator:
    |   File "C:\...\Lib\site-packages\starlette\middleware\base.py", line 163, in body_stream
    |     assert message["type"] == "http.response.body"
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    | AssertionError
    +------------------------------------

To Reproduce

1. After adding the middleware below to the code, start the server
2. Run Cline and connect to the MCP server
3. Terminate the Cline (VS Code)

@app.middleware("http")
async def some_middleware(request: Request, call_next):
    return await call_next(request)

System Info

fastapi : 0.115.12
fastapi-mcp : 0.3.4
MCP host : Cline (VS Code)

[satheler]

This is also happening here.
I commented out all the middleware in my application and it stopped giving this exception.

[satheler]

I had to rewrite my application's middlewares to be in the format of other Starlet middlewares (such as CORSMiddleware).

Previously, I created my middleware inheriting from BaseHTTPMiddleware. Now that I applied the standard, this error stopped happening.

In any case, I believe this error is related to the SSE return of the /mcp path.