505 HTTP Version Not Supported

The server does not support the major HTTP version used in the request.

505 at a glance

Code: 505
Name: HTTP Version Not Supported
Class: 5xx Server error
Status: IANA-registered standard code
Defined in: RFC 9110, Section 15.6.6
Cacheable by default: No by default. Caches store it only when the response carries explicit freshness (Cache-Control max-age, s-maxage or Expires).

What 505 means

505 HTTP Version Not Supported means the server refuses the protocol version in the request line. It is rare because clients and servers negotiate versions (ALPN for HTTP/2 and 3). When it happens it usually means a malformed request line that the server parsed as an unknown version.

What typically causes a 505

Hand-written clients or scripts sending a bad request line (e.g. spaces in the path, so the "version" token is garbage).
Legacy embedded servers that only speak HTTP/1.0.

Fix it on the server

Return a body that says which versions are supported.

Fix it as a client or visitor

Percent-encode spaces and special characters in the URL; use a standard HTTP library.

Example response

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain

Only HTTP/1.0 and HTTP/1.1 are supported