4×× CLIENT ERROR
426 UPGRADE REQUIRED
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). 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.
Upgrade RFC7230 Section 6.7
426 UPGRADE REQUIRED Source: RFC7231 Section 6.5.15
426 CODE REFERENCES
Rails HTTP Status Symbol :upgrade_required
Symfony HTTP Status Constant Response::HTTP_UPGRADE_REQUIRED
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