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

Rust HTTP Status Constant http::StatusCode::UPGRADE_REQUIRED

Symfony HTTP Status Constant Response::HTTP_UPGRADE_REQUIRED

.NET HTTP Status Constant System.Net.HttpStatusCode.UpgradeRequired

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