406 Not Acceptable

Client error: the problem lies with the request. Repeating it unchanged will normally fail again.

When a server should send it. When the client's Accept, Accept-Language or Accept-Encoding headers rule out every format the server has. Most servers sensibly ignore that and send their default format instead, which the standard allows.

Common causes. An API client asking for XML from a JSON only API, or a security module reacting to unusual headers.

What to do. Loosen the Accept header, for example to */*.

Browsers and HTTP clients. Rarely seen by browsers because they send broad Accept headers. Some web application firewalls return it to block requests, which is a misuse.

Google Search. Like every 4xx except 429, it tells Google the content does not exist: an indexed URL is removed from the index, and a new one is not processed. Crawl frequency for the URL gradually drops. 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/1.1 406 Not Acceptable
Content-Type: text/plain; charset=utf-8

Not Acceptable

location /example {
    return 406;
}

RewriteEngine On
RewriteRule ^example$ - [R=406,L]

http_response_code(406);
echo 'Not Acceptable';
exit;

res.writeHead(406, { 'Content-Type': 'text/plain' });
res.end('Not Acceptable');

# in a view function
return 'Not Acceptable', 406