CORS Preflight Checker
Paste CORS request and response headers to diagnose OPTIONS preflight failures, credentials wildcard bugs, missing allowed headers and Vary: Origin issues.
When to use CORS Preflight Doctor
Use this when a browser fetch or XHR call fails before your application code sees the response. The checker turns request method, credentials mode and response headers into a concrete preflight diagnosis.
CORS Preflight Checker - Access-Control Header Debugger
CORS failures are frustrating because the browser blocks JavaScript from inspecting the response that caused the failure. This tool focuses on the exact header relationship the browser evaluates: Origin, method, requested headers, credentials mode and the returned Access-Control-* headers.
The strongest use case is a copied DevTools network entry. Paste the visible request and response headers, then compare the findings against the server framework or edge worker that emits them. The output includes practical Express and Cloudflare Worker patterns because many teams need a fast patch language for pull requests and incident notes.
This is not a security authorization system. A passing CORS response only means the browser is allowed to expose the response to the requesting page. Real API authorization still belongs in authentication, session, token and permission checks on the server.
- Runs in the browser for fast first-pass analysis.
- Does not require login, upload, server storage or build access.
- Works best with redacted snippets copied from DevTools, config files, framework examples or issue reports.
- Findings are practical review signals and should be confirmed with the official toolchain before production changes.
CORS failures are frustrating because the browser blocks JavaScript from inspecting the response that caused the failure. This tool focuses on the exact header relationship the browser evaluates: Origin, method, requested headers, credentials mode and the returned Access-Control-* headers.
The strongest use case is a copied DevTools network entry. Paste the visible request and response headers, then compare the findings against the server framework or edge worker that emits them. The output includes practical Express and Cloudflare Worker patterns because many teams need a fast patch language for pull requests and incident notes.
This is not a security authorization system. A passing CORS response only means the browser is allowed to expose the response to the requesting page. Real API authorization still belongs in authentication, session, token and permission checks on the server.