Phantom Mock

Mock REST API responses and override request/response headers directly from your browser.

Phantom Mock is a Chrome DevTools extension for frontend developers who need to reproduce different API responses, header values, locales, feature flags, and authenticated states on a real page — without spinning up a mock server, hand-editing the Application tab, or shipping debug code. Open DevTools, click the Phantom Mock tab, and you get: ▸ Mock REST API responses Match a URL by exact value, substring, or regex (optionally narrowed to GET / POST / PUT / etc.). Reply with a custom status code, delay, content type, and body. Patches both fetch() and XMLHttpRequest, so it works regardless of which HTTP client your app uses. ▸ Override request and response headers Add, set, append, or remove arbitrary headers on the wire — backed by Chrome's native declarativeNetRequest, so the headers show up in the Network tab exactly like a real server set them. Useful for X-Tenant-ID, Authorization, CORS, Cache-Control, and feature-flag headers. ▸ Switch localStorage values with one click (Storage tab) Define a profile per key (for example a "preferred locale" profile pointing at your app's locale key, with values en_GB / de_DE / fr_FR) and flip between them with a chip selector. Optional prefix/suffix wraps each value before it lands in storage — handy for JSON-quoted strings like "en_GB". Optional auto-reload of the inspected page after each switch. ▸ Switch cookies with one click (Cookies tab) Same chip-selector UX for cookies, including httpOnly auth cookies that document.cookie can't touch. Define a profile per cookie name with an optional path scope. Common case: flip an app_locale cookie between en / de / fr without ever touching the Application panel. ▸ Live request log + DNR debug The Hit Log streams every mocked request as it happens. The Debug tab surfaces the live state of chrome.declarativeNetRequest: which dynamic rules are registered, the last sync error if any, a Test-against-URL form using testMatchOutcome, and a live tail of every header-rule match. ▸ Organize and share Group rules, toggle whole groups on or off, or kill everything with the master switch in the popup. Export rules, storage profiles, and cookie profiles as a single JSON bundle for your team — with per-item conflict resolution on import. Why developers like it: • Reproduce bugs that need a specific tenant / locale / auth state without rebuilding the backend • Test loading, error, and slow-response paths on real pages • QA feature-flag flipping without a coordinator on every cycle • Demo product features in a specific state during stakeholder meetings Privacy: Phantom Mock works fully offline. No accounts, no telemetry, no servers, no third-party SDKs. Every rule, profile, and preference lives in your browser's chrome.storage.local. We never make network requests of our own and we never log or persist cookie values. The cookies permission is used only when you click a chip in the Cookies tab; cookies are never read in the background.