HTTP Status Code 426 refers to Upgrade Required. This status code indicates that the server refuses to perform the requested operation using the current protocol, but it can be upgraded to a different protocol that is supported by the server.
When a client receives a 426 response, it means that the server expects the client to switch to a different protocol, such as upgrading from HTTP/1.1 to HTTP/2 or WebSocket. The server may provide a list of protocols that it supports, prompting the client to initiate the upgrade to successfully complete the request.