CORS Preflight
Tell whether OPTIONS goes first
Web & HTTP
Prediction
Enter request conditions to begin.
How to use
Enter both origins
Enter the full calling-page and target URLs.
Describe the request
Enter the method and JavaScript-supplied headers.
Inspect each reason
See which method, header, or Content-Type triggers preflight.
Scope of the result
This is a static prediction based on the common Fetch CORS safelist. It does not model Private Network Access, extensions, server responses, or browser-specific behavior.
Frequently asked questions
- Does this send an OPTIONS request?
- No. It only analyzes the entered strings.
- Why does application/json trigger preflight?
- It is not one of the media types allowed for a CORS-safelisted Content-Type.
- Does no preflight mean the request will succeed?
- No. The response still needs suitable Access-Control-Allow-Origin and related headers.