René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"AssertionError: Unexpected message while using middlewares","articleBody":"**Describe the bug**\nwhen I mount the mcp to starlette, and I use middleware to do some prechecks, I met this errors.\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"/app/.venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py\", line 403, in run_asgi\n    result = await app(  # type: ignore[func-returns-value]\n             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/app/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py\", line 60, in __call__\n    return await self.app(scope, receive, send)\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/applications.py\", line 113, in __call__\n    await self.middleware_stack(scope, receive, send)\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py\", line 187, in __call__\n    raise exc\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py\", line 165, in __call__\n    await self.app(scope, receive, _send)\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py\", line 182, in __call__\n    with recv_stream, send_stream, collapse_excgroups():\n                                   ^^^^^^^^^^^^^^^^^^^^\n  File \"/usr/local/lib/python3.12/contextlib.py\", line 158, in __exit__\n    self.gen.throw(value)\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/_utils.py\", line 83, in collapse_excgroups\n    raise exc\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py\", line 185, in __call__\n    await response(scope, wrapped_receive, send)\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py\", line 223, in __call__\n    async for chunk in self.body_iterator:\n  File \"/app/.venv/lib/python3.12/site-packages/starlette/middleware/base.py\", line 169, in body_stream\n    assert message[\"type\"] == \"http.response.body\", f\"Unexpected message: {message}\"\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\nAssertionError: Unexpected message: {'type': 'http.response.start', 'status': 200, 'headers': [(b'content-length', b'0')]}\n\n**To Reproduce**\nSteps to reproduce the behavior:\n1. mount mcp  to starlette\n2. use middlewares\n3. call tools\n```python\nclass CustomHeaderMiddleware(BaseHTTPMiddleware):\n    async def dispatch(self, request, call_next):\n        return await call_next(request)\n\n\nmiddleware = [\n    Middleware(CustomHeaderMiddleware)\n]\n\napp = Starlette(middleware=middleware)\napp.mount(\"/\", mcp_server.sse_app())\n\ndef main():\n    uvicorn.run(\n        app=\"atlantis_mcp_server:app\",\n        host=\"0.0.0.0\",\n        port=8000,\n        workers=1,\n        log_level=\"info\"\n    )\n\n__all__ = [\"main\"]\n```\n\n**Expected behavior**\nShould not raise type error\n\n**Screenshots**\nIf applicable, add screenshots to help explain your problem.\n\n**Desktop (please complete the following information):**\n - OS: [e.g. iOS]\n - Browser [e.g. chrome, safari]\n - Version [e.g. 22]\n\n**Smartphone (please complete the following information):**\n - Device: [e.g. iPhone6]\n - OS: [e.g. iOS8.1]\n - Browser [e.g. stock browser, safari]\n - Version [e.g. 22]\n\n**Additional context**\nAdd any other context about the problem here.\n","author":{"url":"https://github.com/toughnoah","@type":"Person","name":"toughnoah"},"datePublished":"2025-06-04T03:16:59.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":15},"url":"https://github.com/883/python-sdk/issues/883"}