I'm encountering a problem in my local development environment when using MSW with Socket.IO. For some strange reason, MSW is able to intercept a plain WebSocket connection when I use:

new WebSocket("ws://localhost:3000/socket.io/?EIO=4&transport=websocket");

However, it fails to intercept when I use the Socket.IO client:

const socket = io("ws://localhost:3000", {
  retries: 1,
  transports: ["websocket"],
  port: 3000,
});
socket.connect();

Note that Socket.IO client automatically appends /socket.io/ to the URL, so both are attempting to connect to the same endpoint.

I have created a minimal reproduction:

• codesandbox
• github

The GitHub repo might be more reliable for reproduction.

Any idea what I might be doing wrong or what modifications are needed to allow MSW to intercept Socket.IO connections as it does with a plain WebSocket?

Thanks in advance for your help!