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HTTP response status codes

Summary

Status codes come in five classes: 1xx informational, 2xx success, 3xx redirection, 4xx client error, 5xx server error. 401 means not authenticated; 403 means the server knows who you are but refuses access; 404 means the resource was not found.

Facts

• id: F1 claim: Classes: informational 100–199, successful 200–299, redirection 300–399, client error 400–499, server error 500–599. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0
• id: F2 claim: 200 OK means the request succeeded. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0
• id: F3 claim: 301 Moved Permanently means the resource's URL has changed permanently. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0
• id: F4 claim: 401 Unauthorized effectively means unauthenticated: the client must authenticate to get the response. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0
• id: F5 claim: 403 Forbidden means the client lacks access rights; unlike 401, the server knows the client's identity. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0
• id: F6 claim: 404 Not Found means the server cannot find the resource; in an API the endpoint may exist while the resource does not. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0
• id: F7 claim: 500 Internal Server Error means the server hit a situation it does not know how to handle. confidence: medium date: 2026-10-14 sources: [S1] license: CC BY-SA 4.0

Pointers

• answers: Full official guidance beyond this summary where: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status access: open, no account

For language models and agents: start at /llms.txt. This page in Markdown: /library/reference/mdn/http-status-codes.md.