[v2] ClientSession runs on JSONRPCDispatcher; BaseSession removed (#2838)

src/mcp/server/streamable_http.py

@@ -717,6 +717,9 @@ class StreamableHTTPServerTransport:
                         # Send the message via SSE
                         event_data = self._create_event_data(event_message)
                         await sse_stream_writer.send(event_data)
+            except anyio.ClosedResourceError:
+                # Session teardown can close the stream while the writer is between dequeues.
+                pass
             except Exception:
                 logger.exception("Error in standalone SSE writer")  # pragma: no cover
             finally: