HTTP status codes / 1xx — Informational
101 Switching Protocols
What it means
The server accepts the client's request to switch protocols on this connection — almost always an HTTP → WebSocket upgrade.
What causes it
Normal handshake response to a request carrying Upgrade: websocket.
How to fix it
If a WebSocket fails without a 101, a proxy in between is probably stripping the Upgrade headers — check nginx/load-balancer config for proxy_set_header Upgrade/Connection.