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CORS Debugger

Diagnose a browser CORS error from its message and the response headers, and get the missing Access-Control headers plus an Express fix.

CORS Debugger

Output

Working…

Updated . Provided as is. Check the output before you rely on it in production.

How to use CORS Debugger

  1. 1

    Paste the browser error

    Copy the CORS error from the browser console.

  2. 2

    Add the response headers

    Optionally paste the response headers from the failing request's Network tab.

  3. 3

    Read the diagnosis

    The tool explains what is missing: an Access-Control-Allow-Origin header, a preflight that does not allow the method, or credentials combined with a wildcard origin.

  4. 4

    Apply the fix

    Copy the suggested server headers and adapt the allowed origin to your frontend.

Questions and answers

Can I fix CORS in the frontend?
No. CORS is enforced by the browser based on headers the server sends, so the fix belongs on the server or proxy.
Why can't I use * with credentials?
Browsers reject a wildcard origin on credentialed requests. Return the exact requesting origin and add Vary: Origin.

Use it as an API

CORS Debugger is also callable as a free HTTP JSON API at https://aidevhub.io/api/cors-debugger/ — GET with query parameters or POST with a JSON body, no authentication, CORS enabled, fair use (abusive traffic is throttled at the edge; there is no per-request quota header). Responses return { ok, tool, result, meta }. 

curl -s -X POST https://aidevhub.io/api/cors-debugger/ \
  -H "Content-Type: application/json" \
  -d '{"error":"Access to fetch at https://api.example.com from origin https://app.example.com has been blocked by CORS policy: No Access-Control-Allow-Origin header is present."}'

Machine-readable contract: /api/tool/cors-debugger.json All API endpoints: /agents/ LLM site index: /llms.txt

For AI agents: how to call this tool

Machine-readable contract, endpoints and examples. Humans can ignore this section.

Best Path For Builders

Dedicated API endpoint

Deterministic outputs, machine-safe contracts, and production-ready examples.

Dedicated API

https://aidevhub.io/api/cors-debugger/

OpenAPI: https://aidevhub.io/api/openapi.yaml

GET /api/cors-debugger/ GET cors-debugger
POST /api/cors-debugger/ POST cors-debugger

Unified Runtime API

https://aidevhub.io/api/tools/run/?toolId=cors-debugger&a=Access%20to%20fetch%20at%20'https%3A%2F%2Fapi.example.com'%20from%20origin%20'ht

GET and POST are supported at /api/tools/run/ with identical validation and limits.

Limit: 30 req / 60s, input max 256 KB.