What is a CORS preflight request?

A preflight request is an HTTP OPTIONS request sent automatically by the browser before the actual request. It checks whether the server permits the requested origin, HTTP method, and custom headers. If the server responds with matching Access-Control-Allow-* headers and an HTTP 200 or 204 status, the browser sends the real request.

Why can't I use Access-Control-Allow-Origin: * with credentials?

The W3C/WHATWG CORS specification strictly forbids combining Access-Control-Allow-Origin: * with Access-Control-Allow-Credentials: true. If a server responds with both, the browser will block the response to prevent malicious sites from reading authenticated user sessions. The server must dynamically reflect the incoming Origin header instead.

What does Access-Control-Max-Age do?

Access-Control-Max-Age tells the browser how many seconds it can cache the preflight OPTIONS response. Setting it to 86400 (24 hours) reduces network overhead by eliminating redundant preflight requests for subsequent API calls.

How do I handle CORS in Next.js App Router?

In Next.js App Router, export an explicit OPTIONS route handler from your route.ts file that returns a NextResponse with status 204 and the required CORS headers, alongside your GET, POST, or PUT handlers.

Does this tool transmit my API URLs or server configs?

No. All configuration generation and preflight simulations run 100% locally in your web browser using client-side JavaScript. Zero server requests or origin details are collected.

Why does my cURL request work but the browser request fails?

cURL and Postman do not enforce the browser's Same-Origin Policy (SOP). CORS is strictly a browser-enforced security mechanism. A request that succeeds in cURL can still fail in a browser if the server does not return the proper CORS headers.

CORS Header & Preflight Response Generator Online

Target Server / Framework

Allowed Origin(s) (e.g., * or https://app.com)

Allowed HTTP Methods

Allowed Headers (Comma separated)

Preflight Cache Duration (Max-Age in Seconds)

Allow Credentials (Cookies, Authorization Bearer)

🚨 CORS Specification Violation:
Browsers strictly block requests if Access-Control-Allow-Credentials: true is combined with a wildcard Access-Control-Allow-Origin: *. Your server MUST dynamically reflect the incoming Origin header instead!

Generated Server Configuration:

Generating...

Frequently Asked Questions

Is CORS Header & Preflight Generator free to use?

Yes, CORS Header & Preflight Generator is completely free with no signup or registration required. All processing happens directly in your browser.

Is my data safe?

Absolutely. Your data never leaves your device. Everything runs locally in your browser — no uploads, no servers, no tracking.

Do I need to install anything?

No installation needed. CORS Header & Preflight Generator works entirely in your web browser on both desktop and mobile devices.

How do I use

Simply enter or paste your input in the tool above, and the result will be generated instantly. No configuration required.

Frequently Asked Questions

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