The HTTP 426 status code means a server doesn’t want to accept the current protocol and wants the client to switch to a different protocol (for example, HTTP/1.1 -> WebSocket) or a newer version of the same protocol (for example, HTTP/2 -> HTTP/3).
The server responds with the
Upgrade header indicating the protocol they are willing to accept:
HTTP/1.1 426 Upgrade Required Connection: Upgrade Upgrade: WebSocket
A server is supposed to return
426 Upgrade Required when the WebSocket handshake fails due to the incorrect protocol version
(as presented in the
Sec-WebSocket-Version HTTP header).
Typically, a WebSocket server will respond with the supported version(s) of the protocol:
Sec-WebSocket-Version: 13, 8, 7
- 101 Switching Protocols - when a server accepts protocol upgrade requested by a client.
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