Does this fetch my API?

No. It only parses pasted headers in your browser, so private endpoints and preview URLs are not requested by allthevalidators.com.

Can wildcard origins be okay?

Sometimes for public, unauthenticated, cacheable reads. Do not combine '*' with credentials=true, and avoid wildcards for login, account, checkout, or user-data APIs.

Why does my server show 200 while the browser fails?

CORS is enforced by the browser. A successful server response can still be hidden from frontend code if origin, credentials, methods, or requested headers do not match the browser's CORS rules.

Should I test OPTIONS separately?

Yes. POST, Authorization, JSON bodies, custom headers, and non-simple methods often trigger OPTIONS preflights that can use different headers than the final API response.

web validator

CORS SEO Validator — API Origin Trust Checker

Validate Access-Control-Allow-* response headers for public API, app, CDN, and browser trust QA before frontend launches ship.

Results

Paste input and validate locally.

Status: Ready when you validate.
Details: Ready when you validate.
Allow-Origin: Ready when you validate.
Allow-Methods: Ready when you validate.
Allow-Headers: Ready when you validate.
Allow-Credentials: Ready when you validate.
Max-Age: Ready when you validate.
Issues: Ready when you validate.

How to use this validator

Capture headers with DevTools Network, curl -I, or an OPTIONS preflight request against the exact API route and origin pair you are shipping.
Paste only the response headers and run the validator to check origin, credentials, methods, allowed headers, and max-age coherence.
Fix flagged conflicts in your framework route, serverless function, reverse proxy, or CDN rule, then retest the final redirected production/preflight response.

Rules & checks

Access-Control-Allow-Origin is required on cross-origin API responses; production should normally use an explicit origin or reviewed allowlist, not a blind wildcard.

Access-Control-Allow-Credentials: true cannot be combined with Access-Control-Allow-Origin: * because browsers reject credentialed requests with wildcard origins.

Access-Control-Allow-Methods should include the verbs your frontend actually sends, especially OPTIONS preflight coverage for POST, PUT, PATCH, DELETE, and authenticated flows.

Access-Control-Allow-Headers should include requested headers such as Content-Type, Authorization, X-CSRF-Token, and custom analytics or tenant headers used by the app.

Access-Control-Max-Age must be numeric when present; keep cache windows intentional so stale preflight decisions do not mask rollback or CDN changes.

This is a pasted-header validator: it does not fetch URLs, perform browser preflights, verify cookies, or test live CDN/header inheritance.

Inputs explained

HTTP response headers
Paste raw Access-Control-Allow-* lines from the actual response path you care about: local preview, Vercel branch deployment, production API route, CDN cache hit, or OPTIONS preflight. Avoid secrets; parsing stays in the browser.

When to use it

QA public API responses before launching a new app shell, account flow, checkout, lead-capture form, webhook debugger, or validator page.
Compare Vercel preview and production CORS behavior before merging frontend changes that introduce new origins or headers.
Debug browser fetch failures where server logs show 200s but DevTools reports origin, credentials, or preflight rejection.
Pair CORS QA with security headers, CSP, compression, performance budget, manifest, favicon, Open Graph, JSON-LD, canonical, sitemap, and robots validation.

Common errors

Using Access-Control-Allow-Origin: * together with Access-Control-Allow-Credentials: true on login, checkout, dashboard, or saved-account APIs.
Testing only GET while the real browser failure is the OPTIONS preflight for POST, PATCH, DELETE, Authorization, or Content-Type: application/json.
Forgetting Authorization, Content-Type, X-CSRF-Token, tenant, or analytics headers in Access-Control-Allow-Headers after a frontend release.
Different CORS headers between localhost, Vercel preview, production, CDN cache hits, and API route/middleware layers.
Returning CORS headers on success responses but not errors, redirects, 204 OPTIONS responses, or edge/runtime exceptions.

Limitations

Does not fetch a URL, issue a real preflight, test cookies, verify SameSite/Secure flags, or reproduce browser cache state.
Cannot prove every route variant is covered; test success, failure, redirect, OPTIONS, cached CDN, and authenticated API paths separately.
Does not replace a browser DevTools run, integration test, or CDN/platform configuration review for complex multi-origin apps.

Tips

Keep production origins explicit for credentialed requests, and review wildcard usage separately for public, unauthenticated, cacheable APIs.
Test OPTIONS responses as well as the final GET/POST response; browsers can fail before the application handler runs.
Include every header your frontend sends, especially Authorization, Content-Type, X-CSRF-Token, and custom tenant/source headers.
Check the final CDN/edge layer after redirects and rewrites; framework defaults, middleware, and cache rules can overwrite route-level headers.
Use a short staged max-age during rollout, then raise it only after production and preview origins have been verified.

Examples

Credentialed app API

Access-Control-Allow-Origin: https://app.example.com
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Content-Type, Authorization
Access-Control-Allow-Credentials: true

Public read-only API

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, OPTIONS
No credentials header for public cacheable reads

Rejected browser request

Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true
Browser blocks credentialed wildcard origins

Deep dive

This CORS SEO validator checks the browser trust layer behind public web apps and API-powered SEO surfaces. Clean title tags and schema still lose conversions if the app's API is blocked by an origin, credentials, or preflight mismatch.

Paste Access-Control-Allow-* headers from curl, DevTools, Vercel previews, or CDN responses to catch wildcard credential conflicts, missing methods, missing Authorization/Content-Type allowlists, and max-age mistakes locally before launch.

Use it alongside security headers and CSP audits when shipping marketing pages, validator tools, dashboards, checkout flows, and lead forms that depend on same-site or cross-origin API routes.

FAQs

Does this fetch my API?: No. It only parses pasted headers in your browser, so private endpoints and preview URLs are not requested by allthevalidators.com.
Can wildcard origins be okay?: Sometimes for public, unauthenticated, cacheable reads. Do not combine '*' with credentials=true, and avoid wildcards for login, account, checkout, or user-data APIs.
Why does my server show 200 while the browser fails?: CORS is enforced by the browser. A successful server response can still be hidden from frontend code if origin, credentials, methods, or requested headers do not match the browser's CORS rules.
Should I test OPTIONS separately?: Yes. POST, Authorization, JSON bodies, custom headers, and non-simple methods often trigger OPTIONS preflights that can use different headers than the final API response.

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All CORS parsing happens in your browser. Headers are not uploaded, fetched, logged, or shared.

Header-level validation only; verify real browser preflights, cookies, redirects, CDN cache behavior, and authenticated flows separately before production launch.