The server rejected the request before processing it.

Authentication is required or the credentials were rejected.

Access denied — the server understood the request but will not fulfill it.

Nothing exists at this URL.

The endpoint exists but does not accept this HTTP method.

The client took too long to send the complete request.

This resource was intentionally removed and is not coming back.

An intentional novelty response — not a real server error.

Rate limiting is active — slow down.

The server hit an unhandled error while processing the request.

A proxy or gateway got an invalid response from the upstream service.

The service cannot handle requests right now.

A gateway or proxy gave up waiting for the upstream to respond.