A request naming a capability the server doesn't offer failed as a whole parse error, so the client got notRequest with serde's message. RFC 8620 section 3.6.1 says unknownCapability, and a client that checks for it can tell the user what's wrong (an old capability name, a feature this server doesn't have). "using" now parses without failing and keeps the first unknown name; Request::parse and WebSocketMessage::parse then fail with JmapEvent::UnknownCapability, which already maps to the error type, with the name as its detail.