Auth failures leave clues
Authorization, WWW-Authenticate and token-related headers help separate expired credentials from missing permissions or a broken proxy rule.
API reference
HTTP Headers
Search common request, response, caching, CORS and security headers, then connect each header to a practical debugging step.
Reference notes
Headers turn vague failures into concrete evidence
Browser failures need CORS context
A request that works in a server-side client can still fail in a browser when preflight headers do not allow the method, origin or custom headers.
Caching can hide real fixes
Cache-Control, ETag and Last-Modified explain why a client may keep seeing old data after the backend has already changed.