505 HTTP Version Not Supported

Server error: the request may be fine, but the server failed to carry it out.

When a server should send it. When the server will not use the major HTTP version in the request. Rare in practice: servers negotiate HTTP/2 and HTTP/3 during the TLS handshake and fall back to HTTP/1.1.

Common causes. Very old or very new clients meeting a strict server.

What to do. Use HTTP/1.1, which every server supports.

Browsers and HTTP clients. Clients treat it as a server error.

Google Search. Googlebot treats 5xx responses as a sign of trouble and temporarily slows its crawl rate. Already indexed URLs are kept at first, but are dropped if the errors continue. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.

HTTP/1.1 505 HTTP Version Not Supported
Content-Type: text/plain; charset=utf-8

HTTP Version Not Supported

location /example {
    return 505;
}

RewriteEngine On
RewriteRule ^example$ - [R=505,L]

http_response_code(505);
echo 'HTTP Version Not Supported';
exit;

res.writeHead(505, { 'Content-Type': 'text/plain' });
res.end('HTTP Version Not Supported');

# in a view function
return 'HTTP Version Not Supported', 505