fix: send notifications/cancelled on request timeout and cancellation#2518
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weiguangli-io wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
fix: send notifications/cancelled on request timeout and cancellation#2518weiguangli-io wants to merge 1 commit intomodelcontextprotocol:mainfrom
weiguangli-io wants to merge 1 commit intomodelcontextprotocol:mainfrom
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Fixes modelcontextprotocol#2507. BaseSession.send_request() never emits a notifications/cancelled message when its in-flight await is interrupted, whether by the SDK's own timeout or by external cancellation. The server never learns the request was abandoned, leaving coroutines suspended holding resources until the session ends. Add a _send_cancelled_notification helper that does best-effort delivery, and call it from both the TimeoutError and CancelledError handlers. The cancellation path uses anyio.CancelScope(shield=True) to ensure the notification is sent even during teardown.
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Fixes #2507
Root Cause
BaseSession.send_request()never emits anotifications/cancelledmessage when its in-flightawaitis interrupted:anyio.fail_after): raisesMCPErrorwithout notifying the serverCancelledError): propagates without notifying the serverThe server never learns the request was abandoned, so server-side coroutines remain suspended holding resources (DB connections, cursors, locks) until the session ends.
Fix
_send_cancelled_notificationhelper onBaseSessionthat does best-effort delivery (catchesExceptionto avoid masking the original error if the transport is already closed)TimeoutErrorhandler, send the notification before raisingMCPErrorexcept anyio.get_cancelled_exc_class()handler withanyio.CancelScope(shield=True)to ensure the notification is sent even during cancellation teardown, then re-raiseCancelledNotificationis already part of bothClientNotificationandServerNotificationunions, so this works for both client and server sessions.