How to Use the CORS Header & Preflight Simulator
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1
Enter your API URL and select the HTTP request method (GET, POST, PUT, DELETE, PATCH, OPTIONS).
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2
Specify your requesting Origin (e.g., https://example.com) and any custom headers (e.g., Authorization, Content-Type).
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3
Click "Analyze CORS Headers" to run live or simulated preflight validation.
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4
Examine Access-Control-Allow-Origin, Allow-Methods, Allow-Headers, and Allow-Credentials compliance.
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5
Copy copy-paste ready backend configuration snippets for Express, Nginx, Apache, or Python FastAPI to fix discovered errors.
Practical Example & Use Case
Common Real-World Scenario
Solving the dreaded 'No Access-Control-Allow-Origin header is present on the requested resource' browser console error when calling a newly deployed backend API.
Technical Notes & Limitations
Browser Engine Architecture
Due to browser security constraints, requests blocked by browser CORS cannot expose raw headers to client scripts; simulator evaluates preflight rules systematically.
Frequently Asked Questions
Why does CORS only happen in browsers?
CORS is a browser security mechanism designed to protect users from malicious websites making unauthorized authenticated requests to other websites on their behalf.