HTTP 505 HTTP Version Not Supported

Server error code, RFC 9110, section 15.6.6

What it means

The server does not support the version of HTTP that the request used.

A real response

This is the request and the answer our test server gave when we asked it to return 505.

GET /c/505 HTTP/1.1
Host: example.com

HTTP/1.1 505 HTTP Version Not Supported
Date: Sat, 14 Nov 2026 10:00:00 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 11

<p>demo</p>

If you run the site

Rare today. It can appear when a client speaks a much newer or older version than the server supports, or when a proxy is badly configured.

If you are a visitor

Try another browser or update your software.

A common mistake

Treating 505 as a general error.

Registered in the IANA HTTP Status Code Registry as HTTP Version Not Supported, defined in RFC 9110, section 15.6.6.

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.