306 Unused

Reserved. An early draft of HTTP/1.1 used it (as Switch Proxy), and it has had no meaning since.

Code: 306
Reason phrase: Unused
Class: 3xx Redirection
Defined in: RFC 9110 section 15.4.7
Status: Unused: it was used in an early draft and is now reserved.
Cacheable by default: No

What it means

Redirection: the client has to take another step, usually following a Location header, to complete the request.

Common causes. Nothing sends it today.

What to do. Nothing. Treat it as an unknown 3xx.

How clients and crawlers treat it

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/1.1 306 Unused
Content-Type: text/plain; charset=utf-8

Unused

How to send 306

nginx

location /example {
    return 306;
}

PHP

http_response_code(306);
echo 'Unused';
exit;

Node.js

res.writeHead(306, { 'Content-Type': 'text/plain' });
res.end('Unused');

Python (Flask)

# in a view function
return 'Unused', 306

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

Related codes

All status codes · All 3xx codes

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