505 HTTP Version Not Supported means the server does not support, or refuses to support, the major version of HTTP used in the request. The response should explain why that version is not supported and which protocols the server does support.
It is rare today, because clients and servers agree on HTTP/2 and HTTP/3 during the TLS handshake (ALPN) instead of trying versions. A 505 more often points to a malformed request line: an unencoded space in the URL, for example, makes some servers read part of the path as the protocol version.
Example response
HTTP/1.1 505 HTTP Version Not Supported
Date: Fri, 06 Nov 2026 10:15:32 GMT
Content-Type: text/html
Content-Length: 65
Connection: close
<html><body><h1>505 HTTP Version Not Supported</h1></body></html>SEO and redirect checking
Crawlers use standard HTTP versions, so 505 should not affect them. If you see a 505, look for clients or proxies that send malformed request lines, such as unencoded spaces in URLs, or for an old server that only supports HTTP/1.0.
Common causes and use cases
- A client sends a request line with unencoded spaces in the path, which some servers misread.
- A very old or embedded server supports only HTTP/1.0, or a server refuses HTTP/1.0 clients.
- A misconfigured proxy rewrites the protocol version.