Move terminate() after task group exit for deterministic cancellation

Cancel the request-scoped task group first so the Cancelled exception
deterministically reaches the server task before terminate() closes the
streams. This avoids a race between Cancelled and ClosedResourceError
in the message router.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wiggzz

src/mcp/server/streamable_http.py

@@ -1019,7 +1019,7 @@ async def message_router():
                             )
                 except anyio.ClosedResourceError:
                     if self._terminated:
-                        logger.debug("Read stream closed by client")  # pragma: no cover
+                        logger.debug("Read stream closed by client")  # pragma: lax no cover
                     else:
                         logger.exception("Unexpected closure of read stream in message router")
                 except Exception:  # pragma: lax no cover

src/mcp/server/streamable_http_manager.py

@@ -191,14 +191,20 @@ async def run_request_handler(*, task_status: TaskStatus[None] = anyio.TASK_STAT
                 task_status.started()
                 # Handle the HTTP request and return the response
                 await http_transport.handle_request(scope, receive, send)
-                # Terminate the transport after the request is handled
-                await http_transport.terminate()
-                # Cancel the request-scoped task group to stop the server task
+                # Cancel the request-scoped task group to stop the server task.
+                # This ensures the Cancelled exception reaches the server task
+                # before terminate() closes the streams, avoiding a race between
+                # Cancelled and ClosedResourceError in the message router.
                 request_tg.cancel_scope.cancel()
 
             await request_tg.start(run_stateless_server)
             await request_tg.start(run_request_handler)
 
+        # Terminate after the task group exits — the server task is already
+        # cancelled at this point, so this is just cleanup (sets _terminated
+        # flag and closes any remaining streams).
+        await http_transport.terminate()
+
     async def _handle_stateful_request(self, scope: Scope, receive: Receive, send: Send) -> None:
         """Process request in stateful mode - maintaining session state between requests."""
         request = Request(scope, receive)