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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.
Result
Blocked — your JavaScript will not be able to read this response.
Why it matters
The request completes on the wire, but the browser blocks JavaScript from reading the response.
Action
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.

What this tool does and why

Before you start

What

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

Why

Browser CORS failures depend on the exact request, response, credentials, and preflight rules, so a precise walkthrough is more useful than a generic error message.

Your Result and Action update in the tool as you change its inputs.

Model note

This browser-side model explains the inputs you choose. Check an important decision against the real system, brief, or production constraints.

Next

Use the result to choose a connected next move.

HTTP Cache LabConfigure Cache-Control, ETag, and Last-Modified headers and simulate how browsers and CDNs actually cache the response.Fan-Out Latency SimulatorModel fan-out tail latency, critical paths, optional work, and deadline headroom.
How this tool stays honest

How it works

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.

Assumptions

The tool runs in your browser and models the information you provide. It does not know facts outside that model.

Limitations

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.

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

When a browser blocks a cross-origin API request.

Try a scenario

Model a credentialed request with a wildcard allow-origin response. Review preflight and response requirements without treating the walkthrough as a live server test.

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).