fix: prevent SSE transport crash on concurrent STDIO connections#1130
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MumuTW wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
fix: prevent SSE transport crash on concurrent STDIO connections#1130MumuTW wants to merge 1 commit intomodelcontextprotocol:mainfrom
MumuTW wants to merge 1 commit intomodelcontextprotocol:mainfrom
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Move webAppTransports map registration to after SSEServerTransport.start() completes in both /stdio and /sse endpoints. Previously, the transport was registered before being fully connected, allowing the /message endpoint to access it prematurely and causing "Not connected" errors when multiple STDIO connection requests arrived in quick succession. Also add .catch() handlers to stderr forwarding send() and close() calls to prevent unhandled rejections when the SSE connection drops while the server process is still running. Fixes modelcontextprotocol#1014 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
Fixes #1014
When multiple STDIO connection requests arrive in quick succession (e.g. an MCP server mounting ~20 proxied servers), the SSE transport crashes with a "Not connected" error.
Root cause:
SSEServerTransportwas registered in thewebAppTransportsmap beforeawait transport.start()completed. This allowed the/messageendpoint to look up and use a transport that wasn't fully connected yet, causingsend()to throw.Changes:
webAppTransports.set()to afterawait webAppTransport.start()in both/stdioand/sseendpoints, so only fully-connected transports are discoverable.catch()handlers tosend()andclose()calls in the stderr forwarding handler to prevent unhandled promise rejections when the SSE connection drops while the server process is still runningTest plan
npm run buildpassesnpm run lintpassesnpm test)fastmcp dev server.py, open inspector — no crash