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LV2 · Toolooo Standard
Explore → See → Understand → ActUnderstand it. Feature Level describes the depth of the tool experience.

CORS Doctor

Diagnose exactly which CORS rule is blocking a request, and why.

EngineeringBackendNetworkingSecurityAPI
🔒Your tool inputs are processed in your browser and are not uploaded or sent to analytics.

Your request

What the server responded with

Advanced response options
Browser
your page
OPTIONS
Server
preflight check
response
Browser
preflight passed
GET
Server
actual request
response
Blocked
response hidden from JS
ℹ
Cross-origin request detected
Your page (https://myapp.com) is calling a different origin (https://api.example.com), so the browser enforces CORS.
ℹ
Preflight required (OPTIONS request)
GET with custom headers (Authorization) isn't a "simple request", so the browser sends an OPTIONS preflight first.
✓
Preflight method check passes
GET is in Access-Control-Allow-Methods.
✓
Preflight header check passes
All custom headers (Authorization) are allowed.
✕
Response is missing Access-Control-Allow-Origin
The request completes on the wire, but the browser blocks JavaScript from reading the response.
What happened
Blocked — your JavaScript will not be able to read this response.
Why
The request completes on the wire, but the browser blocks JavaScript from reading the response.
Try
Set Access-Control-Allow-Origin to "https://myapp.com" (or "*" if you don't need credentials) on the server's response.

Common misconception: CORS doesn't stop the request from reaching the server (except a failed preflight) — it stops the browser from handing the response back to your JavaScript. The server usually still processed it. This tool models the browser's CORS algorithm from the headers you enter; it never makes a real network request.

Understand CORS Doctor

What this tool helps you understand

Describe your request and response headers and CORS Doctor walks through the actual browser CORS algorithm step by step, showing exactly where it fails — missing Access-Control-Allow-Origin, a credentials mismatch, a preflight that never got the right method — instead of just repeating the cryptic browser error.

When to use it

Use CORS Doctor when engineering, backend, and networking work raises questions about cors error debugger, cors checker, and preflight request and you need a concrete view before making the next move.

How to read the result

Read each finding as a diagnostic signal, then confirm it against the contract, system, file, or production context you actually control.

Assumptions and limits

The result follows the request and response details you enter. It does not send a network request, inspect the live server, change headers, or bypass the browser’s CORS enforcement.

Toolooo is part of Khizooology, created byKhizar Imtiaz (khizooo).