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A WebSocket-only v3.x client is able to reach a v2.x server, as the Engine.IO handshake are compatible, but the v2.x server does not send a sid in the Socket.IO handshake, which resulted in: > Uncaught TypeError: Cannot read property 'sid' of undefined A 'connect_error' event will now be emitted. References: - https://github.com/socketio/engine.io-protocol#difference-between-v3-and-v4 - https://github.com/socketio/socket.io-protocol#difference-between-v5-and-v4 - https://socket.io/docs/v3/migrating-from-2-x-to-3-0/