Gracefully terminate active sessions during StreamableHTTP shutdown#2259
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Bortlesboat wants to merge 2 commits intomodelcontextprotocol:mainfrom
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Gracefully terminate active sessions during StreamableHTTP shutdown#2259Bortlesboat wants to merge 2 commits intomodelcontextprotocol:mainfrom
Bortlesboat wants to merge 2 commits intomodelcontextprotocol:mainfrom
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During server shutdown, StreamableHTTPSessionManager.run() cancelled the task group without first terminating active sessions. This killed EventSourceResponse coroutines mid-stream, causing Uvicorn to log "ASGI callable returned without completing response" errors. Two changes fix this: 1. In StreamableHTTPSessionManager.run(), iterate over all active transports and call terminate() on each before cancelling the task group. 2. In StreamableHTTPServerTransport.terminate(), close all SSE stream writers (_sse_stream_writers) so that active EventSourceResponse instances complete gracefully. Also add an early return guard for idempotency since terminate() may now be called from both the manager shutdown path and the per-session cleanup path. Github-Issue: modelcontextprotocol#2150 Reported-by: emmahoggan
Cover the three lines that were missing from code coverage: - streamable_http.py L777: terminate() closing active SSE stream writers - streamable_http_manager.py L139-140: exception handling when transport.terminate() fails during manager shutdown Github-Issue:modelcontextprotocol#2150
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Update the shutdown logic to wrap terminate() calls in try-except blocks, preventing one transport's termination error from affecting others. Changes: - Use logger.exception() instead of logger.debug() for better error visibility - Simplify SSE writer closing by iterating values() directly instead of pop() - Improve code comments to explain why graceful termination is needed This follows the same approach as PR modelcontextprotocol#2259 with minor improvements in error handling. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Fixes #2150
Problem
When shutting down a StreamableHTTP server while clients have active streaming connections, Uvicorn logs:
This happens because
StreamableHTTPSessionManager.run()cancels its task group on shutdown without first terminating active sessions. TheEventSourceResponsecoroutines are killed mid-stream before they can complete their HTTP responses.There are two compounding issues:
run()never callsterminate()on active transports before cancelling the task group — it just clears the dict.terminate()itself doesn't close_sse_stream_writers. It closes_request_streams, but the SSE writers that keepEventSourceResponsealive are stored separately and never touched during termination.Fix
streamable_http_manager.py— Inrun()'sfinallyblock, iterate over all active transports and callterminate()on each before cancelling the task group.streamable_http.py— Interminate():_sse_stream_writersso activeEventSourceResponseinstances complete gracefully.terminate()may now be called from both the manager shutdown path and the per-session cleanup path.