CORS Preflight Simulator & Debugger

Simulate browser CORS preflight requests, audit Access-Control-Allow headers, diagnose blocked fetch calls, and export Next.js/Express/Nginx fixes.

Developer

Test Scenarios:

1. Browser Fetch RequestPreflight Required

Origin (Window / Page Origin)

Target API Endpoint

HTTP Method

Content-Type

Custom Request Headers

Credentials Mode: include (Cookies / Auth)

2. Server Response HeadersOPTIONS / Actual

Access-Control-Allow-Origin

•

Access-Control-Allow-Methods

Access-Control-Allow-Headers

Allow-Credentials

Max-Age (seconds)

CORS Preflight & Request Passed Successfully

The browser permits JavaScript access to the response body.

200 OK

W3C Fetch Specification ChecklistContent-Type 'application/json' requires preflight.

Origin Allowed

Origin 'http://localhost:3000' successfully matched by 'http://localhost:3000'.

Credentials Mode

Explicit origin specified and Allow-Credentials is true.

HTTP Method Permitted

Method 'POST' is approved in preflight response.

Headers Permitted

All requested custom headers approved by server.

Ready Server Fix

// app/api/example/route.ts
import { NextResponse } from 'next/server';

export async function OPTIONS(request: Request) {
  return new NextResponse(null, {
    status: 204,
    headers: {
      'Access-Control-Allow-Origin': 'http://localhost:3000',
      'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
      'Access-Control-Allow-Headers': 'Content-Type, Authorization',
      'Access-Control-Allow-Credentials': 'true',
      'Access-Control-Max-Age': '86400',
    },
  });
}

export async function POST(request: Request) {
  return NextResponse.json(
    { success: true, message: 'Data fetched successfully' },
    {
      headers: {
        'Access-Control-Allow-Origin': 'http://localhost:3000',
        'Access-Control-Allow-Credentials': 'true',
      },
    }
  );
}

Step-by-Step Guide

How to Use CORS Preflight Simulator & Debugger

CORS Preflight Simulator & Debugger is a fast, free, browser-based online utility designed to help developers, designers, and tech professionals streamline their workflows. Built with a local-first architecture, it processes your input instantly inside your browser without uploading your sensitive data to external servers.

Input or Paste Your Data

Enter, paste, or upload your source data into the editor or input field provided above for CORS Preflight Simulator & Debugger.

Customize Options & Settings

Select your desired configuration options, formatting preferences, or parameters. The tool updates and validates in real-time.

Copy or Download the Result

Once the output is generated, use the one-click copy button to save the result to your clipboard or download it directly.

Performance & Privacy

Key Features of CORS Preflight Simulator & Debugger

100% Client-Side Privacy

All data transformations occur locally in your browser. No server roundtrips, logging, or third-party tracking.

Instant Real-Time Execution

Eliminate wait times and server queues. Calculations, conversions, and rendering happen instantaneously.

Mobile-First & Accessible

Engineered with responsive design, keyboard shortcuts, high-contrast themes, and full WCAG compliance.

Zero Setup & Completely Free

No account registration, no credit cards, and no usage caps. Access the full suite of developer tools anytime.

Real-World Applications

Common Use Cases & Workflows

Software Engineering & Debugging

Inspect payloads, validate syntax errors, and quickly reformat messy code or logs during active development.

API & Backend Integration

Prepare clean data structures, verify response payloads, and generate standard configurations effortlessly.

Rapid Prototyping

Speed up build iterations by generating assets and formatting inputs without opening heavyweight IDEs or CLI tools.

Got Questions?

Frequently Asked Questions

Everything you need to know about using CORS Preflight Simulator & Debugger on ToolMate.

Is my data private and secure when using CORS Preflight Simulator & Debugger?

Yes, 100%. ToolMate processes your data entirely within your web browser using client-side JavaScript. None of your input code, text, or data structures are sent to remote servers or stored in any database. Your privacy is guaranteed.

How does CORS Preflight Simulator & Debugger work?

ToolMate's CORS Preflight Simulator & Debugger leverages modern browser APIs and optimized client-side processing algorithms. It parses your input in real time, applies necessary validations or transformations, and outputs the result with zero latency and zero server dependencies.

Can I use CORS Preflight Simulator & Debugger offline or on mobile devices?

Yes. ToolMate is responsive and functions as a Progressive Web App (PWA). Once loaded in your browser, you can access and run the tool seamlessly on desktop, tablet, or smartphone even with intermittent or zero internet connectivity.

Is CORS Preflight Simulator & Debugger completely free to use?

Yes, CORS Preflight Simulator & Debugger is completely free with no usage limits, hidden subscriptions, or account requirements. You can format, convert, or generate output as many times as you need.

Do I need to install any browser extensions or software?

No software installation or plugins are required. CORS Preflight Simulator & Debugger runs out of the box in any modern browser including Google Chrome, Mozilla Firefox, Safari, Microsoft Edge, and Brave.

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