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

Go HTTP Status Constant http.StatusUpgradeRequired
Python3+ HTTP Status Constant http.client.UPGRADE_REQUIRED
Python3.5+ HTTP Status Constant http.HTTPStatus.UPGRADE_REQUIRED
Rails HTTP Status Symbol :upgrade_required
Symfony HTTP Status Constant Response::HTTP_UPGRADE_REQUIRED

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