MockFast - API Mock & Response Interceptor

Intercept, mock and modify HTTP API responses instantly. No proxy, no code changes. Built for frontend developers.

MockFast lets you intercept and mock HTTP API responses directly in Chrome — no proxy setup, no backend changes, no extra software. ⚡ WHAT IT DOES Click Activate, add a URL pattern, and MockFast intercepts every matching request. Replace the real response with your own JSON, change the status code, simulate a timeout, or redirect the request to your local dev server — all in seconds. 🔧 KEY FEATURES Mock API responses Return custom JSON, HTML or plain text for any API request. Set any HTTP status code (200, 404, 500, etc.) and response headers. Perfect for building UI against APIs that don't exist yet. Simulate network errors & delays Trigger network failures (timeout, connection refused, DNS error) or add artificial latency to test how your frontend handles slow or broken APIs. Redirect requests to localhost Route production API calls to your local dev server. Test your code on the real production domain without touching the live backend. Live request log See every request in real time. Mocked requests are highlighted so you always know what's intercepted and what's passing through. Profile system Create multiple rule profiles (dev, staging, demo) and switch between them instantly. Export and import profiles to share with your team. Template library 9 built-in response templates: empty success, paginated list, login success, 401, 404, 500 and more. Start mocking in one click. 📦 FREE TIER - Up to 5 mock rules - 1 profile - All core features 🔒 PRO (£5.99/year) - Unlimited rules and profiles - Team export/import - Advanced regex URL matching - Response header editing ⚠️ DEBUGGER NOTE MockFast uses the Chrome Debugger API to intercept responses at the network level — the same approach used by ModResponse and other professional developer tools. Chrome will show a "MockFast started debugging this browser" banner while active. This is a Chrome security feature and cannot be hidden. Click "Stop" in MockFast to remove it. 💻 PERFECT FOR - Frontend developers building against unstable or unbuilt APIs - QA engineers testing edge cases and error handling - Developers demoing products with controlled mock data - Engineers testing local changes on production domains