Indicates an interim response while request processing continues. Allows clients to verify server availability before transmitting payloads (100 Continue) or begin preloading critical subresources (103 Early Hints).

Indicates that the client's request was successfully received, understood, and accepted. Encompasses standard 200 OK deliveries, 201 resource creations, 204 headless actions, and 206 byte-range media streams.

Indicates that further action is needed by the client to fulfill the request. Covers permanent redirects (301, 308) which pass SEO link equity, temporary redirects (302, 307), and cache revalidations (304 Not Modified).

Indicates that the client sent an erroneous request. Includes missing pages (404), unauthenticated access (401), authorization blocks (403), rate-limiting constraints (429), and legal takedowns (451).

Indicates that the server is aware it encountered an unexpected failure or is incapable of completing the request. Features generic backend crashes (500), reverse proxy communication failures (502), and temporary maintenance overloads (503).

Custom status codes implemented by infrastructure providers and edge CDNs, including Cloudflare edge origin timeouts and SSL handshake failures (520–526), Nginx client terminations (499), and anti-bot blocks (999).