fix: send CONNECTION_CLOSED error when SSE stream closes without event ID #2028
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…t ID When an SSE stream closes without sending any events with IDs, _handle_sse_response silently returns instead of notifying the session. This leaves send_request waiting forever on a response that will never arrive. Send a JSONRPCError with CONNECTION_CLOSED to the read stream writer so the pending request fails with a clear error instead of hanging. Fixes modelcontextprotocol#1811 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
_handle_sse_responsepreviously returned silently, leavingsend_requesthanging foreverJSONRPCErrorwithCONNECTION_CLOSEDto the read stream writer so the pending request fails with a clear error instead of hanging indefinitelyFixes #1811
Reproduction
When a server closes the SSE stream before sending any events (e.g., server crash, network interruption before first event),
_handle_sse_responsefalls through to the reconnection check. Withlast_event_idstillNone, the function silently returns. The caller (send_request) is left waiting onresponse_stream_reader.receive()which never resolves.Test plan
test_handle_sse_response_sends_error_when_stream_closes_without_event_id— mocks an SSE response with zero events, verifiesCONNECTION_CLOSEDerror is sent to the read stream writertest_streamable_http.pytests pass🤖 Generated with Claude Code