HTTP 426 Upgrade Required

Client error code, RFC 9110, section 15.5.22

What it means

The server will not do this over the protocol in use but might after the client switches to another, for example from plain HTTP to TLS.

A real response

This is the request and the answer our test server gave when we asked it to return 426.

GET /c/426 HTTP/1.1
Host: example.com

HTTP/1.1 426 Upgrade Required
Date: Sat, 14 Nov 2026 10:00:00 GMT
Content-Type: text/html; charset=utf-8
Upgrade: TLS/1.3
Content-Length: 11

<p>demo</p>

If you run the site

RFC 9110 says a 426 response must carry an Upgrade header with the needed protocol. It is rare in practice: most sites redirect from http to https with a 301 or 308 instead.

If you are a visitor

Try the https version of the address.

A common mistake

Sending 426 with no Upgrade header.

Registered in the IANA HTTP Status Code Registry as Upgrade Required, defined in RFC 9110, section 15.5.22.

This is an independent guide. It is not connected with the IETF, IANA, any browser maker or any company named here. Meanings were checked against the RFC texts on 14 November 2026. Responses shown were recorded on 14 November 2026 from a small test server on the author's own computer.