httpstatuses/contents/codes/426.md

set	code	title	references
4	426	Upgrade Required	Rails HTTP Status Symbol :upgrade_required

The 426 Upgrade Required status code indicates that the server refuses to perform the request using the current protocol but might be willing to do so after the client upgrades to a different protocol.

The server MUST send an Upgrade header field in a 426 response to indicate the required protocol(s) (Section 6.7 of RFC7230]2).

Example:

HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain

This service requires use of the HTTP/3.0 protocol.

Source: RFC7231 Section 6.5.15