501 Not Implemented

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

When a server should send it. When the server does not support the method at all, for any resource. For a method that works elsewhere but not on this URL, 405 is right instead.

Common causes. An unusual HTTP method sent to a basic server.

What to do. Use a method the server supports.

Browsers and HTTP clients. Clients treat it as a server error; it is cacheable by default, since retrying will not help.

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. Cacheable by default (RFC 9110 section 15.1). With no Cache-Control or Expires header, a cache may still store it and reuse it for a heuristic period, often 10% of the time since Last-Modified, so set Cache-Control explicitly if that is not what you want.

HTTP/1.1 501 Not Implemented
Content-Type: text/plain; charset=utf-8

Not Implemented

location /example {
    return 501;
}

RewriteEngine On
RewriteRule ^example$ - [R=501,L]

http_response_code(501);
echo 'Not Implemented';
exit;

res.writeHead(501, { 'Content-Type': 'text/plain' });
res.end('Not Implemented');

# in a view function
return 'Not Implemented', 501