HTTP Status Codes
The three-digit codes an HTTP response carries — 2xx success, 3xx redirect, 4xx client error, 5xx server error.
HTTP status codes are the three-digit numbers every HTTP response carries to signal the outcome, grouped by first digit: 2xx success (200 OK, 201 Created), 3xx redirection (301 moved, 304 not modified), 4xx client error (400 bad request, 401 unauthorized, 404 not found, 429 too many requests), 5xx server error (500 internal, 503 unavailable).
Worked example: a rate-limited client gets 429 with a Retry-After header; a missing page gets 404; a crash gets 500 — the class of the code tells the caller whether to retry, fix its request, or give up. Gotcha: the wrong code breaks clients and tooling — returning 200 with an error body hides failures from monitoring and retries, and confusing 401 (unauthenticated) with 403 (authenticated but forbidden) muddles auth debugging; the code is a contract, not decoration.