CORS Tester & Simulator

The 30-Second CORS Tester Fix

Enter your target API URL and origin domain, select the HTTP method, and click test to instantly see the actual CORS response headers. The tool directly mimics a browser fetch, verifying if the server's Access-Control-Allow-Origin configuration actually permits your cross-origin request.

Ideal Use Cases for the CORS Tester

Use this testing utility to diagnose and resolve cross-origin access blocks between frontends and APIs.

• Debugging preflight failures (HTTP OPTIONS) when your browser blocks complex requests containing custom headers or authentication tokens.
• Testing API from different origins to ensure your backend whitelist rules correctly accept staging URLs and reject unauthorized domains.
• Verifying CDN CORS configuration when attempting to load web fonts, images, or scripts across different subdomains.

Fixing Common CORS Tester Issues

Issue: 'null' origin issues or wildcard (*) rejection
Fix: If you are testing from a local file (file://), the origin is often 'null'. Use a local dev server (like localhost:3000). Additionally, you cannot use a wildcard * for the origin if your request includes credentials.

Issue: Credentials mode conflicts (The value of the 'Access-Control-Allow-Origin' header must not be the wildcard '*')
Fix: When sending cookies or authorization headers (credentials: 'include'), the server must explicitly return the exact origin domain, and must also include the header Access-Control-Allow-Credentials: true.

Deep Dive: Preflight OPTIONS Mechanics, Credentials & Header Visibility

The browser's Same-Origin Policy (SOP) restricts scripts from reading HTTP responses across differing protocols, domains, or ports. Cross-Origin Resource Sharing (CORS) is the W3C/WHATWG mechanism that relaxes this sandbox safely. A common misconception is that CORS blocks requests from reaching the server; in reality, the browser dispatches the network request, but unilaterally withholds the response payload from client-side JavaScript unless the server includes the correct response headers.

Modern API gateways and microservice backends must handle three specific preflight and credentialing requirements:

• Preflight Handshake Triggers: Any request utilizing non-simple methods (PUT, DELETE, PATCH), custom request headers (such as Authorization or X-API-Key), or a Content-Type of application/json forces the browser to issue an automatic OPTIONS preflight request. The server must respond with HTTP 200 or 204 and include Access-Control-Allow-Methods and Access-Control-Allow-Headers before the actual request is dispatched.
• Preflight Caching Optimization: To prevent redundant round-trip network latency on every client API call, include the Access-Control-Max-Age: 86400 header. This instructs browsers to cache the preflight approval locally for up to 24 hours (clamped by browser-specific maximums, such as 7200s in Chromium).
• The Credentials Wildcard Prohibition: If the client sends cookies or HTTP authentication (credentials: 'include'), the server must not return a wildcard (*) for Access-Control-Allow-Origin. Browsers reject wildcards with credentials; the server must dynamically reflect the caller's validated Origin header and return Access-Control-Allow-Credentials: true.
• Header Exposure via Expose-Headers: By default, JavaScript can only inspect basic CORS-safelisted response headers (Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, Pragma). To make custom metadata accessible (like X-Request-Id or X-RateLimit-Remaining), the server must explicitly enumerate them in Access-Control-Expose-Headers.

Diagnosing CORS issues directly within real browser contexts ensures API headers satisfy strict client enforcement before frontend code reaches production.

Browser-Based CORS Testing Without Proxies

Most CORS testing tools rely on server-side proxies that bypass the browser's security model entirely — which inherently defeats the purpose of testing CORS in the first place. This tool works differently: it executes a real fetch() request directly from your web browser, ensuring you see the exact same CORS behavior and enforcement that your end-users will experience. If the browser blocks the request due to policy violations, the tool captures the failure and clearly explains why it occurred, bridging the gap between theory and real-world implementation.

How It Works: Testing Cross-Origin Requests

Testing an API for CORS compatibility is straightforward with our built-in simulator. Follow these steps to diagnose your connection:

1. Input the Target API URL: Enter the absolute URL of the API endpoint you wish to test (e.g., https://api.example.com/data).
2. Define the Origin Domain: Provide the origin URL of your frontend application (e.g., https://myapp.com) to simulate the cross-origin request.
3. Select the HTTP Method: Choose the appropriate method for your request, such as GET, POST, PUT, DELETE, or OPTIONS (for preflight simulation).
4. Execute the Test: Click the "Test CORS" button to dispatch the request directly from your browser engine.
5. Analyze the Results: Review the returned headers, the pass/fail status, and read our actionable fix recommendations to resolve any configuration issues.

5 Common Use Cases for the CORS Tester

CORS configuration is a frequent stumbling block in modern web development. This testing utility is invaluable for:

• Verifying Frontend-to-Backend Connectivity: Ensure that your newly deployed single-page application (SPA) can successfully fetch data from a separate microservice domain without being blocked.
• Debugging Local Development Errors: Troubleshoot frustrating "CORS policy blocked" errors when running a frontend server on localhost:3000 and a backend API on localhost:8080.
• Validating Preflight (OPTIONS) Requests: Simulate complex cross-origin requests that require custom headers or non-GET methods to verify that the server correctly responds to OPTIONS preflight checks.
• Testing Third-Party API Integration: Confirm whether a public third-party API supports client-side requests from a browser or if you need to build a server-side proxy to access their data.
• Auditing Server Header Configurations: After deploying updates to your Nginx or Apache server configurations, run a quick check to guarantee that the Access-Control-Allow-Origin headers are correctly applied.

Understanding Cross-Origin Resource Sharing (CORS)

CORS errors are among the most notoriously frustrating issues developers face. By default, web browsers enforce the Same-Origin Policy, which prevents a web page from making requests to a different domain. When your frontend at https://myapp.com attempts to fetch data from https://api.example.com, the browser automatically attaches an Origin header. It then inspects the server's response for an Access-Control-Allow-Origin header that matches the requesting origin. If this header is missing or mismatched, the browser silently intercepts and blocks the response data, even if the backend successfully processed the request.

For complex requests involving non-GET methods, custom headers, or authentication credentials, the browser first dispatches an HTTP OPTIONS request known as a "preflight". The server must explicitly approve the operation before the actual request is sent. Read our complete guide to fixing common CORS errors for an in-depth breakdown of these mechanisms.

Why Privacy Matters: Local Execution

When debugging API endpoints, you often deal with sensitive URLs, internal routing, and proprietary data structures. Our CORS Tester strictly adheres to a zero-data philosophy. 100% private — your API requests never leave your browser. We do not log your target URLs, proxy your traffic through our backend, or store any response data. Every test is executed locally on your machine, ensuring complete confidentiality for your development workflow.

Where the CORS Tester Runs

This testing tool relies on the native fetch() API and modern browser security implementations to provide accurate CORS simulations. It is fully supported on all major evergreen browsers, including Google Chrome (version 42+), Mozilla Firefox (version 39+), Apple Safari (version 10.1+), and Microsoft Edge. Because it uses standard web APIs, it accurately reflects the exact network behavior your users will encounter across both desktop and mobile platforms.

Next Steps: Fixing CORS Issues

Need to generate the correct CORS headers for your server? Use our CORS Header Generator to create ready-to-paste configurations for Nginx, Apache, and Express.js. You can also format your configs with our ENV File Formatter, and debug JSON API responses with our JSON Formatter. Together, these tools form a complete CORS debugging workflow: test the endpoint first, identify the failure, and then generate the proper server fix.