fix(mcp): add timeout to background thread join in stop() to prevent hangs#1787
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giulio-leone wants to merge 1 commit intostrands-agents:mainfrom
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fix(mcp): add timeout to background thread join in stop() to prevent hangs#1787giulio-leone wants to merge 1 commit intostrands-agents:mainfrom
giulio-leone wants to merge 1 commit intostrands-agents:mainfrom
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…hangs When an Agent holding an MCPClient goes out of scope inside a function, the Agent.__del__ finalizer calls MCPClient.stop() which calls _background_thread.join(). If the background thread cannot exit promptly (e.g. transport subprocess teardown is slow), join() blocks indefinitely, causing the entire process to hang on exit. Add a timeout (equal to startup_timeout, default 30s) to the join() call. If the thread does not exit within the timeout, log a warning and proceed with cleanup. The thread is already a daemon thread (daemon=True), so it will be cleaned up by the interpreter on process exit. Fixes strands-agents#1732 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Summary
Fixes #1732
When an
Agentholding anMCPClientgoes out of scope inside a function, theAgent.__del__finalizer callsMCPClient.stop()which calls_background_thread.join(). If the background thread cannot exit promptly (e.g. transport subprocess teardown is slow),join()blocks indefinitely, causing the entire process to hang on exit.Root Cause
_background_thread.join()at line 356 ofmcp_client.pyhas no timeout. When called from the GC finalizer (Agent.__del__→ToolRegistry.cleanup()→MCPClient.remove_consumer()→MCPClient.stop()), the transport subprocess may not shut down quickly, causing the join to block forever.Fix
Add
timeout=self._startup_timeout(default 30s) to thejoin()call. If the thread does not exit within the timeout, log a warning and continue cleanup. The thread is already a daemon thread (daemon=True), so it will be cleaned up by the interpreter on process exit.Changes
src/strands/tools/mcp/mcp_client.pyself._background_thread.join()toself._background_thread.join(timeout=self._startup_timeout)is_alive()check after join to log a warning if the thread didn't exittests/strands/tools/mcp/test_mcp_client.pyjoin(timeout=...)test_stop_does_not_hang_when_join_times_out— verifies cleanup proceeds even when thread is still alive after join timeoutTest Results