Agent X Alpha

CORS Preflight Troubleshooter & Config Generator

Server API

Simulate OPTIONS preflights, audit Access-Control headers, compare browser vs edge execution, and generate copy-paste server CORS fixes.

Cross-Origin Request Parameters

RFC 6454 / W3C CORS Spec
Credentials Mode

Cookies / Auth headers

Presets:

Instant Server Fix Code Generator

Copy-paste production CORS rules pre-populated with your target parameters

// Next.js Route Handler (app/api/route.ts)
import { NextResponse } from 'next/server'

const CORS_HEADERS = {
  'Access-Control-Allow-Origin': 'https://myapp.com',
  'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
  'Access-Control-Allow-Headers': 'Content-Type, Authorization',
  'Access-Control-Max-Age': '86400',
}

// 1. Handle Preflight OPTIONS
export async function OPTIONS() {
  return new NextResponse(null, { status: 204, headers: CORS_HEADERS })
}

// 2. Handle actual requests
export async function GET() {
  return NextResponse.json({ message: 'CORS OK' }, { headers: CORS_HEADERS })
}

Preflight OPTIONS Simulation

Dispatches synthetic OPTIONS preflight requests carrying Origin and Access-Control headers to diagnose CORS errors before shipping frontend code.

Dual-Vantage Diagnostic

Compares direct local browser fetch behavior against Cloudflare Edge telemetry to isolate client browser shielding from server misconfigurations.

Instant Server Fix Generator

Generates copy-paste production CORS middleware configurations for Next.js, Express, FastAPI, Nginx, and Cloudflare Workers.

Frequently Asked Questions

Why do CORS errors only happen in web browsers and not in Postman or cURL?
Cross-Origin Resource Sharing (CORS) is a browser security mechanism enforced strictly by client web browsers (Chrome, Firefox, Safari) to prevent malicious websites from reading sensitive data from another domain. Command-line tools like cURL and Postman do not execute arbitrary untrusted third-party JavaScript, so they do not enforce CORS restrictions.
What triggers an automatic HTTP OPTIONS Preflight request?
A preflight request is automatically triggered whenever an HTTP request uses methods other than GET, POST, or HEAD, or sends non-simple headers (such as "Authorization", "Content-Type: application/json", or custom "X-*" headers), or specifies credentials. The browser first sends an OPTIONS request to ask the origin server for permission.
Why does Access-Control-Allow-Origin: * fail when sending cookies or tokens?
The W3C CORS specification strictly forbids the wildcard "*" origin whenever "credentials: include" (cookies, HTTP basic auth, or TLS client certificates) is set. In credentialed mode, the server must explicitly echo back the caller's exact origin (e.g. "Access-Control-Allow-Origin: https://myapp.com") alongside "Access-Control-Allow-Credentials: true".
How do I fix "No 'Access-Control-Allow-Origin' header is present on the requested resource"?
The target server must be configured to handle HTTP OPTIONS requests and reply with the "Access-Control-Allow-Origin" header matching your frontend origin. Use the built-in server fix generator below to copy ready-to-use configuration blocks for Nginx, Next.js, Express, FastAPI, or Cloudflare Workers.
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