The HTTP 426 Upgrade Required client error response status code indicates that the server refused 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 sends an Upgrade header with this response to indicate the required protocol(s).
Given a GET request to a system:
GET /resources HTTP/1.1
Host: example.com
The origin server expects that requests must be made in HTTP/3:
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.
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