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)¹

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.

• ¹ Upgrade RFC7230 Section 6.7
• Source: RFC7231 Section 6.5.15

426 Code References

• Rails HTTP Status Symbol: :upgrade_required
• Symfony HTTP Status Constant: Response::HTTP_UPGRADE_REQUIRED