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CORS Preflight Predictor

Predict whether a cross-origin request triggers OPTIONS from its method, headers, and Content-Type.

Compare a planned request with the Fetch CORS safelist before debugging an unexpected OPTIONS request.

Everything runs in this browser. The tool never sends a request to either URL.

Calling page URL
Request URL
HTTP method
Request headers (one Name: value per line)

Prediction

Enter request conditions to begin.

    How to use

    1. 1

      Enter both origins

      Enter the full calling-page and target URLs.

    2. 2

      Describe the request

      Enter the method and JavaScript-supplied headers.

    3. 3

      Inspect each reason

      See which method, header, or Content-Type triggers preflight.

    Scope

    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.

    FAQ

    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.