Development Tools

CORS Matrix Sandbox

Interactive simulator for Cross-Origin Resource Sharing. Configure client and server headers to diagnose preflight failures instantly.

1. Client Request

Method

Origin (Client Domain)

Custom Headers (comma separated)

Include Credentials (fetch: credentials: 'include')

2. Server Config (Headers)

Access-Control-Allow-Origin

Access-Control-Allow-Methods

Access-Control-Allow-Headers

Allow Credentials

Max-Age (s)

Expose Hdrs

Simulation Diagnostics

REJECTED

> Initiating cross-origin fetch from https://myapp.com

OPTIONSPreflight RequestREQUIRED

Evaluation: FAIL

Cannot use wildcard '*' for Access-Control-Allow-Origin when the request includes credentials.
Header 'authorization' is not allowed by Access-Control-Allow-Headers.

Browser Blocked: The Fetch API or XMLHttpRequest will throw a Network Error here. The response from the server is typically completely hidden from the Javascript code due to the CORS policy violation. Fix the server headers configured above to resolve this.

Guide & Insights

CORS Problem Diagnostic Lab

Tired of the "Blocked by CORS policy" error? Our CORS Debugger lets you simulate client requests and server responses to see exactly why a browser is blocking a request and provides immediate fixes.

🕵️

Preflight Simulation

Automatically identifies if an OPTIONS preflight is needed and shows how the browser evaluates Origins, Methods, and Headers.

🪄

One-Click Auto Fix

Paste your failing environment and click "Auto Fix" to generate the perfect cross-origin response configuration.

🔐

Credential Security Analysis

Explains the traps of Access-Control-Allow-Credentials and helps avoid security risks from using wildcards with credentials.

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