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CORS Tester & Simulator

Test if an API endpoint allows cross-origin requests directly from your browser. Check CORS headers, simulate preflight requests, and get actionable fix recommendations — all without uploading any data.

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CORS Tester
Test if an API endpoint allows cross-origin requests directly from your browser.
Testing CORS...
💡 Fix Recommendations
Show Raw Response Headers 
 
 
   
 
 
     
  
 
 
 
   
How ZeroData protects your privacy

 
 
 
     
  •  ✓ No Uploads: Processing happens entirely via client-side JavaScript. 
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  •  ✓ No Storage: We do not have a database. We physically cannot save your data. 
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  •  ✓ No Tracking: We don't log what you process or track your inputs. 
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  •  ✓ Verifiable: Check your DevTools Network tab. You will see 0 outbound requests. 
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Browser-Based CORS Testing Without Proxies
 

Most CORS testing tools rely on server-side proxies that bypass the browser's security model entirely — which defeats the purpose of testing CORS in the first place. This tool works differently: it makes a real fetch() request directly from your browser, so you see the exact same CORS behavior your users will experience. If the browser blocks the request, the tool catches it and explains why.

 

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. Together, these tools form a complete CORS debugging workflow: test first, then generate the fix.

 
Understanding Cross-Origin Resource Sharing
 

CORS errors are one of the most frustrating issues in web development. When your frontend at https://myapp.com tries to fetch data from https://api.example.com, the browser sends an Origin header and checks whether the server's response includes an Access-Control-Allow-Origin header that matches. Without it, the browser silently blocks the response — even if the server returned valid data. For complex requests (non-GET methods, custom headers, or credentials), the browser first sends an OPTIONS preflight request to check permissions before sending the actual request. Read our complete guide to fixing common CORS errors.

  
  
 
How to Use the CORS Tester & Simulator
 
     
  1. Enter the target API URL you want to test.
  2. Enter the origin domain your frontend app uses.
  3. Select the HTTP method (GET, POST, OPTIONS, etc.).
  4. Click Test CORS to send the request from your browser.
  5. Review the results: header checks, pass/fail status, and fix recommendations.
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Common Use Cases
 
     
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     Testing if your API allows requests from your frontend domain. 
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     Debugging CORS errors during local development. 
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     Verifying CORS configuration after deploying header changes. 
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     Checking preflight response headers for complex requests. 
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     Comparing CORS behavior across different API endpoints. 
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Frequently Asked Questions
 
 
 
What is CORS?
 
CORS (Cross-Origin Resource Sharing) is a browser security mechanism that blocks web pages from making requests to a different domain unless the server explicitly allows it via Access-Control headers. It protects users from malicious cross-site requests.
 
 
How does this CORS tester work?
 
It sends a real HTTP request from your browser to the target URL and inspects the response headers. If the server returns the correct Access-Control-Allow-Origin header matching your origin, CORS is allowed. If the browser blocks the request, the tool detects it and provides fix recommendations.
 
 
Why am I getting a CORS error?
 
The most common cause is that the server's response is missing the Access-Control-Allow-Origin header, or the header value doesn't match your requesting origin. Other causes include missing Access-Control-Allow-Methods for non-GET requests and missing Access-Control-Allow-Headers for custom headers.
 
 
Does this tool send my data to a server?
 
The request is made directly from your browser to the target URL. ZeroData Tools does not proxy, log, or store any request data. Your browser makes the connection directly, just as it would in your actual application.
 
 
Can I test preflight (OPTIONS) requests?
 
Yes. Select OPTIONS as the HTTP method to simulate a preflight request and inspect the server's CORS preflight response. Preflight requests are automatically sent by browsers before complex cross-origin requests.
 
 
 
  
 
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