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🌍 CORS Headers Reference
CORS confuses everyone the first ten times. This reference walks through the headers (Access-Control-Allow-Origin, -Methods, -Headers, -Credentials), the preflight OPTIONS request, when it fires, and how to debug the typical "CORS error" message you see in browser DevTools.
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Examples
Simple GET
Output
Browser sends: Origin: example.com · Server replies: Access-Control-Allow-Origin: example.com (or *)Frequently asked questions
Why do I need preflight for some requests?
Requests with custom headers (Authorization, etc.), non-simple Content-Type (application/json), or non-GET/POST methods trigger a preflight OPTIONS request first to check permission.
About this calculator
CORS Headers Reference runs entirely in your browser using standard formulas. No data is sent to any server. We don't share your inputs with lenders, brokers, or anyone else — there's no funnel and no follow-up email.
Related tools
- 📨 HTTP Headers Reference — Searchable reference of HTTP request and response headers.
- 🔧 HTTP Methods Reference — GET, POST, PUT, DELETE, PATCH, OPTIONS, HEAD — when to use each.
- 🌐 HTTP Status Codes — Searchable reference of HTTP status codes (1xx, 2xx, 3xx, 4xx, 5xx).
Why this tool
vs typical free network-tools sites
| NetToolset | Typical free site | |
|---|---|---|
| Price | Free | Free |
| Signup required | No | Often (for advanced features) |
| Ads inside the tool | No | Yes (banner + interstitial) |
| Logs your IP / queries | No | Yes (often resold) |
| Data sent to a server | Math runs in browser; lookups hit Cloudflare DoH directly | Yes (all goes through their server) |
| Open source | Yes | No |
| Bookmarkable URL per tool | Yes | Mixed |
| Multilingual | EN / ES / PT / FR / DE | Usually EN only |
| Loads in under 1s | Yes (static) | Often slow (ad tracking) |