HTTP Status Code Reference

HTTP 404 Not Found means the server could not find the requested resource. The URL may be wrong, the page may have been deleted, or the resource may never have existed. Servers sometimes return 404 instead of 403 to hide the existence of a resource.

301 Moved Permanently tells browsers and search engines the resource has moved forever — browsers cache the redirect and SEO equity transfers to the new URL. 302 Found is a temporary redirect; browsers do not cache it and the original URL retains its SEO value. Use 301 for permanent URL changes and 302 when the move is intentionally temporary.

401 Unauthorized means the client is not authenticated — no credentials were provided or the credentials are invalid. 403 Forbidden means the client is authenticated but does not have permission to access the resource. Re-authenticating may help with 401 but will not help with 403.

HTTP 429 Too Many Requests means the user has sent too many requests in a given time window and has been rate-limited. The response often includes a Retry-After header indicating how long to wait before retrying.

502 Bad Gateway means a proxy or load balancer received an invalid response from the upstream origin server — the origin may be down or returning garbage. 503 Service Unavailable means the server itself is temporarily unable to handle requests due to overload or maintenance. Both are usually temporary, but 503 may include a Retry-After header.