API Interceptor - Mock & Modify HTTP Responses

Mock API responses & intercept HTTP requests. Override JSON, headers, status codes. Free API mocker & debugger for developers.

Mock API responses, intercept HTTP requests, and override JSON — all without touching your backend. API Interceptor is a free Chrome developer tool for frontend developers who need to test, debug, and simulate API behavior in real-time. Whether you need to mock an API, fake a backend, simulate error responses, or override a live API with test data, this extension handles it with a simple rule-based system. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ WHAT DEVELOPERS USE IT FOR ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ • API Mocking — Mock API responses during frontend development before the backend exists • HTTP Interception — Intercept and modify live API responses in real-time • Error Simulation — Fake 500, 404, 401, or any status code to test error handling • JSON Override — Replace or patch specific JSON fields without modifying the server • CORS Testing — Add or remove CORS headers to test cross-origin scenarios • Demo Mode — Lock in consistent mock data for client demos and presentations • Legacy API Testing — Intercept third-party APIs you cannot modify ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ KEY FEATURES ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Response Body Modification • Replace the entire JSON response body with custom mock data • Modify specific fields using JSON path notation (e.g. data.user.name) • Use regex find & replace for text transformations • Write custom JavaScript functions to transform responses programmatically Header Interception & Override • Add, modify, or remove any response header • Fix CORS issues by injecting Access-Control-Allow-Origin • Simulate different server configurations without backend access Status Code Override • Change any HTTP status code (200, 400, 401, 403, 404, 500, 503) • Turn errors into successes or successes into errors for edge-case testing • Simulate flaky endpoints Flexible URL Matching • Wildcard, regex, exact, and "contains" URL patterns • Filter by HTTP method (GET, POST, PUT, DELETE, PATCH) Rule Management • Organize rules into named groups • Enable/disable rules or entire groups instantly • Import & export rules as JSON for team sharing • Built-in rule templates to get started fast ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ HOW IT WORKS ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 1. Click the extension icon and attach the debugger to your tab 2. Go to Options and create a rule with a URL pattern 3. Choose what to mock: body, headers, status code, or all three 4. Refresh the page — matching API responses are intercepted and modified automatically 5. Detach the debugger when done; the real API behavior returns instantly ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ PRIVACY & SECURITY ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ • 100% local — all rules and data stay on your device • No analytics, no tracking, no external servers • Debugger only activates on tabs you explicitly attach it to