[v1.x] fix: handle ClosedResourceError when transport closes mid-request (#2334)

Co-authored-by: Owen Devereaux <owendevereaux@users.noreply.github.com>
Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

src/mcp/server/lowlevel/server.py

@@ -671,16 +671,23 @@ class Server(Generic[LifespanResultT, RequestT]):
                 await stack.enter_async_context(task_support.run())
 
             async with anyio.create_task_group() as tg:
-                async for message in session.incoming_messages:
-                    logger.debug("Received message: %s", message)
+                try:
+                    async for message in session.incoming_messages:
+                        logger.debug("Received message: %s", message)
 
-                    tg.start_soon(
-                        self._handle_message,
-                        message,
-                        session,
-                        lifespan_context,
-                        raise_exceptions,
-                    )
+                        tg.start_soon(
+                            self._handle_message,
+                            message,
+                            session,
+                            lifespan_context,
+                            raise_exceptions,
+                        )
+                finally:
+                    # Transport closed: cancel in-flight handlers. Without this the
+                    # TG join waits for them, and when they eventually try to
+                    # respond they hit a closed write stream (the session's
+                    # _receive_loop closed it when the read stream ended).
+                    tg.cancel_scope.cancel()
 
     async def _handle_message(
         self,
@@ -763,12 +770,18 @@ class Server(Generic[LifespanResultT, RequestT]):
                 response = await handler(req)
             except McpError as err:  # pragma: no cover
                 response = err.error
-            except anyio.get_cancelled_exc_class():  # pragma: no cover
-                logger.info(
-                    "Request %s cancelled - duplicate response suppressed",
-                    message.request_id,
-                )
-                return
+            except anyio.get_cancelled_exc_class():
+                if message.cancelled:
+                    # Client sent CancelledNotification; responder.cancel() already
+                    # sent an error response, so skip the duplicate.
+                    logger.info(
+                        "Request %s cancelled - duplicate response suppressed",
+                        message.request_id,
+                    )
+                    return
+                # Transport-close cancellation from the TG in run(); re-raise so the
+                # TG swallows its own cancellation.
+                raise
             except Exception as err:  # pragma: no cover
                 if raise_exceptions:
                     raise err
@@ -777,16 +790,24 @@ class Server(Generic[LifespanResultT, RequestT]):
                 # Reset the global state after we are done
                 if token is not None:  # pragma: no branch
                     request_ctx.reset(token)
-
-            await message.respond(response)
         else:  # pragma: no cover
-            await message.respond(
-                types.ErrorData(
-                    code=types.METHOD_NOT_FOUND,
-                    message="Method not found",
-                )
+            response = types.ErrorData(
+                code=types.METHOD_NOT_FOUND,
+                message="Method not found",
             )
 
+        try:
+            await message.respond(response)
+        except (anyio.BrokenResourceError, anyio.ClosedResourceError):
+            # Transport closed between handler unblocking and respond. Happens
+            # when _receive_loop's finally wakes a handler blocked on
+            # send_request: the handler runs to respond() before run()'s TG
+            # cancel fires, but after the write stream closed. Closed if our
+            # end closed (_receive_loop's async-with exit); Broken if the peer
+            # end closed first (streamable_http terminate()).
+            logger.debug("Response for %s dropped - transport closed", message.request_id)
+            return
+
         logger.debug("Response sent")
 
     async def _handle_notification(self, notify: Any):

src/mcp/shared/session.py

@@ -108,7 +108,7 @@ class RequestResponder(Generic[ReceiveRequestT, SendResultT]):
     ) -> None:
         """Exit the context manager, performing cleanup and notifying completion."""
         try:
-            if self._completed:  # pragma: no branch
+            if self._completed:
                 self._on_complete(self)
         finally:
             self._entered = False
@@ -445,7 +445,9 @@ class BaseSession(
             finally:
                 # after the read stream is closed, we need to send errors
                 # to any pending requests
-                for id, stream in self._response_streams.items():
+                # Snapshot: stream.send() wakes the waiter, whose finally pops
+                # from _response_streams before the next __next__() call.
+                for id, stream in list(self._response_streams.items()):
                     error = ErrorData(code=CONNECTION_CLOSED, message="Connection closed")
                     try:
                         await stream.send(JSONRPCError(jsonrpc="2.0", id=id, error=error))