fix: use ASGI callable for SSE endpoint to avoid BaseHTTPMiddleware AssertionError (#883) - #2029
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…ssertionError (modelcontextprotocol#883) When Starlette's BaseHTTPMiddleware wraps the SSE endpoint, it expects http.response.body messages. However, the SSE handler sends raw ASGI events, causing 'AssertionError: Unexpected message'. Fix: Replace the Starlette endpoint wrapper (sse_endpoint function) with a callable ASGI class (HandleSseAsgi) so the SSE handler bypasses middleware response body wrapping. Also remove the unnecessary 'return Response()' from handle_sse since it's a raw ASGI callable. Closes modelcontextprotocol#883
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maxisbey
marked this pull request as draft
February 11, 2026 14:20
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Marking as draft until CI is fixed |
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Thanks for the PR. Tracking this in #1222 instead. Closing this as part of a wider backlog cleanup following the v2 launch. Feel free to reopen if this is still relevant. |
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When Starlette's BaseHTTPMiddleware wraps the SSE endpoint, it expects http.response.body messages. However, the SSE handler sends raw ASGI events, causing 'AssertionError: Unexpected message'.
This PR fixes the issue by replacing the Starlette endpoint wrapper (sse_endpoint function) with a callable ASGI class (HandleSseAsgi). This ensures the SSE handler bypasses middleware response body wrapping, resolving the AssertionError.
Also removes the unnecessary 'return Response()' from handle_sse since it's a raw ASGI callable.
Closes #883