Browser policy field note
CORS preflight failed: compare the request and response policy
Debug CORS preflight failures caused by origin mismatches, missing methods or headers, credentials with wildcard origins, and redirect behavior.
CORS is enforced by the browser, not fixed by adding a request-side header. Reconstruct the OPTIONS request the browser wants to send, then compare Origin, Access-Control-Request-Method, and Access-Control-Request-Headers with the server’s Access-Control-Allow-* response.
Treat the complete response as an evidence record: status, headers, provider code, request identifier, method, and raw body. The sequence below separates what the response proves from the checks still needed before a safe retry or code change.
Diagnostic procedure
Work from evidence to recovery.
- 01
Reconstruct the preflight
Capture the exact page origin, target URL, method, non-simple headers, credentials mode, and any redirect.
- 02
Compare policy fields
Origin must match, the method and requested headers must be admitted, and credentialed requests cannot use Access-Control-Allow-Origin: *.
- 03
Fix the API boundary
Configure the server or gateway that answers OPTIONS. A browser extension or request-side Allow-Origin header only hides the real deployment issue.
Origin: https://app.example Access-Control-Request-Headers: authorization ← Access-Control-Allow-Origin: * with credentials
Access-Control-Allow-Origin: https://app.example Access-Control-Allow-Headers: authorization Access-Control-Allow-Credentials: true
The modeled preflight policy admits this origin, method, header set, and credential mode.
No account · no upload · no endpoint calledOpen the full cors preflight lab and operating notes →
FAQ
Before you ship
Does APITC send this evidence to an API?
No. The matching and calculations in the linked workbench run in the active browser tab.
Should every CORS response be retried?
No. Retry behavior depends on the method, idempotency protection, provider instructions, and whether the failure is temporary.
Protocol behavior checked against the MDN — Cross-Origin Resource Sharing. Recheck your pinned provider/API version before production deployment.