305 Use Proxy was defined in HTTP/1.1 to tell a client that the resource must be accessed through the proxy whose URL is given in the Location header. Letting any website tell a client which proxy to use is dangerous, because an attacker could route the client's traffic through a proxy under their control.
RFC 7231 deprecated 305 in 2014, and RFC 9110 keeps it only as a deprecated registration. Browsers and current HTTP libraries do not act on it: they show the response or treat it as an error.
Example response
HTTP
HTTP/1.1 305 Use Proxy
Date: Fri, 06 Nov 2026 10:15:32 GMT
Location: http://proxy.example.net:8080/
Content-Length: 0SEO and redirect checking
305 has no valid use on the web today. If a redirect check shows a 305, the server or a device in front of it is misconfigured or very old. Crawlers do not follow it, so the content behind it cannot be indexed.