How to Choose CORS Tools

CORS troubleshooting becomes faster once browser error messages are mapped to concrete header checks.

First tool by symptom

• Browser shows a CORS error → CORS Error Troubleshooting
• Need full header consistency check → CORS Diagnostic
• Need directive-level Allow-* inspection → CORS Response Inspect
• Need allowlist boundary validation → Origin Allowlist Check

Recommended practical flow

1. 1) Start from error text and branch with Error Troubleshooting
2. 2) Confirm request/response consistency with CORS Diagnostic
3. 3) Verify ACAO/ACAC/Allow-Headers/Allow-Methods in Response Inspect
4. 4) Finalize prevention with Origin Allowlist and Checklist

Error-to-check mapping

• No Access-Control-Allow-Origin: missing response header or origin mismatch
• CORS preflight channel did not succeed: OPTIONS path failure
• Credential is not supported if ACAO is *: credentials policy conflict
• Request header field X is not allowed: missing Allow-Headers value

Data collection set

• Full browser console error text
• Request/response headers from preflight OPTIONS
• Request/response headers from actual request (GET/POST etc.)
• Observed Origin, Host, and Authority values

Preflight minimum checks

• OPTIONS must return 2xx (avoid 3xx/4xx/5xx)
• Access-Control-Allow-Origin must match request origin
• Access-Control-Allow-Methods must include actual method
• Access-Control-Allow-Headers must include requested headers
• Add Vary: Origin when origin is dynamically allowed

Compared tools

• CORS Error Troubleshooting
• CORS Diagnostic
• CORS Response Inspect
• Origin Allowlist Check
• CORS Checklist
• Host/Authority/Origin Inspect

Frequent cases

• No Access-Control-Allow-Origin: header dropped in response path
• Preflight failed: missing OPTIONS response Allow-* headers
• Credentials conflict: ACAO=* combined with credentials=true

Common mistakes

• Validating only with curl and ignoring browser CORS enforcement
• Forgetting Vary: Origin on dynamic origin reflection
• Blocking preflight OPTIONS in auth middleware

Allowlist design tips

• Prefer explicit environment-specific origins over wildcards
• Include scheme (http/https) and port in matching conditions
• Record reason and expiry for newly allowed origins
• Detect accidental dev-origin inclusion in production via CI

Post-fix verification

• Retest same scenario on major browsers (Chrome/Safari/Firefox)
• Verify both credentialed and non-credentialed requests
• Confirm Vary: Origin works correctly through caches
• Track CORS failure rate as an ongoing monitoring metric

FAQ

How do I choose between CORS Error Troubleshooting and CORS Diagnostic?

Use Troubleshooting for error-text entry, and Diagnostic for request/response header consistency checks.

When should I use Origin Allowlist Check?

Use it when Allow-* looks correct but failures persist, to validate origin matching rules and edge cases.

Referenced specs

• MDN: CORS
• Fetch Standard