What Mockingbird does
Mockingbird is a focused network-debugging and HTTP mocking Chrome extension for developers and QA engineers. Arm a single tab and it shows you that tab's live traffic — method, URL, status, size and timing — with inspectable headers, query params and pretty-printed JSON bodies. From there you can hold a request, rewrite it, or stand in a mock response, without touching the backend.
What you can do
- Capture live traffic for one tab and inspect headers, query params and JSON bodies.
- Mock any endpoint — match by URL (glob, regex or substring) and return a synthetic status, headers, body and latency. Cross-origin mocks work automatically with CORS handled for you.
- Passthrough & rewrite — let a request hit the network, then override status, merge headers, or rewrite JSON fields with JSONPath.
- Breakpoints — hold a request at the request and/or response stage, edit it live, then continue or abort. Add delay to simulate slow servers.
- Replay & compose — re-send any captured request or build one from scratch, diff two responses, and export the session as HAR or rule sets as JSON.
Private by design
Mockingbird has no server and collects nothing. Captured traffic stays in memory; rules and settings are stored locally, and the extension makes no network requests of its own. It uses the Chrome DevTools Protocol (the debugger permission), so Chrome shows a "debugging this browser" banner while a tab is armed — one tab at a time, by design.
Who it's for
Frontend and QA engineers who need to reproduce edge cases, simulate failures, or build against an endpoint that doesn't exist yet — without spinning up a mock server. It pairs well with API tests from XHRScribe and contract monitoring from Telltale. Free on the Chrome Web Store; works in any Chromium browser.
FAQ
Do I need a backend or account? No — Mockingbird runs entirely in the browser with no server and no sign-up.
Can it mock cross-origin requests? Yes — CORS is handled for you automatically.
Why does Chrome show a debugging banner? Mockingbird uses the Chrome DevTools Protocol to intercept traffic, so Chrome displays its standard banner while a tab is armed.
Is it free? Yes, free on the Chrome Web Store.