HTTP topic
HTTP diagnostics
Reliable HTTP diagnosis starts with observable messages and the layer that produced them. Use these guides to separate symptoms from causes and move from browser evidence through intermediaries to the origin.
Recommended foundation
Start here
This entry establishes the shared model used by the references, diagnostics, and experiments below.
Learn
Understand the HTTP request-response cycle
Learn the roles, direction, and observable boundaries of one request and its associated responses.
HTTP diagnostics
Troubleshoot
8 reviewed entries
intermediate
A browser does not send a cookie
Diagnose cookie storage and request selection separately from CORS response access.
intermediate
A browser reports a CORS error
Find which CORS flow step failed and preserve the HTTP response evidence the browser received.
intermediate
A conditional request does not return 304
Match the request condition to the current selected representation before expecting 304.
intermediate
A CORS preflight request fails
Compare the browser's requested CORS permission with the exact preflight response.
intermediate
A page still looks stale after deployment
Identify which layer served an old representation and validate the cache key and freshness evidence.
intermediate
A redirect keeps repeating
Find the first repeated target and the layer that generated it before changing redirect rules.
intermediate
A request method changed after a redirect
Trace the redirect status and follow-up request before changing client or server behavior.
advanced
HTTP fields change behind a proxy or CDN
Compare the same exchange at adjacent controlled hops before attributing a field change to the origin.
HTTP diagnostics
Lab
1 reviewed entry