T02 / LOCAL WORKBENCHNO ACCOUNT
CORS preflight request simulator
Model a browser preflight without calling a remote server. Compare the requested origin, method, headers, and credentials against the response policy.
- Input
- Origin, method, request headers, and server CORS headers
- Output
- Preflight requirement, exact failure reasons, and corrected response policy
The modeled preflight policy admits this origin, method, header set, and credential mode.
OPERATING NOTES01–03
What this tool checks—and what it does not.
- Local analysis
The supplied value is processed in the active tab. Remote endpoint behavior is not inferred when the evidence does not contain it.
- Evidence before advice
The report separates observed fields from implementation guidance. Preserve request IDs and provider error codes for production incidents.
- Version-aware verification
Provider behavior changes. Each related field note links to the official source used for the current rule.
THE RULES BEHIND THIS TOOL03 NOTES
Why it decided that.
Each note states the rule this workbench applies and cites the provider or standards document it came from.
Debug CORS preflight failures caused by origin mismatches, missing methods or headers, credentials with wildcard origins, and redirect behavior.
Read the note →CORS header referenceAccess-Control-Allow-Origin: exact origins, wildcard, Vary, and credentialsConfigure Access-Control-Allow-Origin safely for public APIs, credentialed requests, allowlists, CDN caches, and preflight responses.
Read the note →Authentication field noteAPI 401 vs 403: credential failure or permission failure?Use the status, WWW-Authenticate challenge, token state, and resource policy to separate API 401 Unauthorized from 403 Forbidden.
Read the note →FAQ
About this workbench
Does the cors preflight lab upload my data?
No. The value you supply is parsed and evaluated in the active browser tab. No account is required and no remote endpoint is called on your behalf.
What does this tool read, and what does it return?
It reads origin, method, request headers, and server cors headers, and returns preflight requirement, exact failure reasons, and corrected response policy.
How current are the rules it applies?
Each rule links to the field note that cites its primary provider or standards source, with the review date shown on that note. Provider behaviour changes, so verify against your pinned version before deploying.