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Title: fix: use ASGI callable for SSE endpoint to avoid BaseHTTPMiddleware AssertionError (#883) by Ashutosh0x · Pull Request #2029 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: fix: use ASGI callable for SSE endpoint to avoid BaseHTTPMiddleware AssertionError (#883) by Ashutosh0x · Pull Request #2029 · modelcontextprotocol/python-sdk

X Title: fix: use ASGI callable for SSE endpoint to avoid BaseHTTPMiddleware AssertionError (#883) by Ashutosh0x · Pull Request #2029 · modelcontextprotocol/python-sdk

Description: 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

Open Graph Description: 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 messa...

X Description: When Starlette's BaseHTTPMiddleware wraps the SSE endpoint, it expects http.response.body messages. However, the SSE handler sends raw ASGI events, causing 'AssertionError: Unexpect...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2029

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fix: use ASGI callable for SSE endpoint to avoid BaseHTTPMiddleware AssertionError (#883) https://github.com/modelcontextprotocol/python-sdk/pull/2029/files#top
Show all changes 5 commits https://github.com/modelcontextprotocol/python-sdk/pull/2029/files
c52ff28 fix: use ASGI callable for SSE endpoint to avoid BaseHTTPMiddleware A… Ashutosh0x Feb 10, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2029/commits/c52ff28a8fc119ad97ae7ff737012c6aee17e66b
53d58e6 style: format and lint fix for #883 Ashutosh0x Feb 10, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2029/commits/53d58e6d39d16915222ae46de844acaa1ea7bbd9
597ef40 fix: resolve ruff lint warning and configure pyright for Python 3.10 Ashutosh0x Feb 11, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2029/commits/597ef4074a714df31a17931e6b728e9c030bf088
796358c fix: resolve coverage failure and additional ruff lint warning Ashutosh0x Feb 11, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2029/commits/796358c489d1d66479d12a52c530b8e9070fc773
34dc179 fix: resolve branch coverage in regression test Ashutosh0x Feb 11, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2029/commits/34dc179e7dda6bdeef9bb221266aeca8448d7542
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