510 Not Extended

The request needs an HTTP extension the server does not have. The code is now obsolete.

Code: 510
Reason phrase: Not Extended
Class: 5xx Server error
Defined in: RFC 2774 section 7
Status: Obsolete: RFC 2774 has been moved to Historic status and the registry marks the code obsoleted.
Cacheable by default: No

What it means

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

Common causes. Very rare.

What to do. Nothing.

How clients and crawlers treat it

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.

Example response

HTTP/1.1 510 Not Extended
Content-Type: text/plain; charset=utf-8

Not Extended

How to send 510

nginx

location /example {
    return 510;
}

Apache (.htaccess)

RewriteEngine On
RewriteRule ^example$ - [R=510,L]

PHP

http_response_code(510);
echo 'Not Extended';
exit;

Node.js

res.writeHead(510, { 'Content-Type': 'text/plain' });
res.end('Not Extended');

Python (Flask)

# in a view function
return 'Not Extended', 510

Change the paths to suit. In nginx, add_header needs always to apply to error responses.

Related codes

All status codes · All 5xx codes

Names and numbers from the IANA HTTP Status Code Registry. Google Search behaviour as documented by Google Search Central.