Mocker

Mock any API response from simple YAML files — on your own or with your AI coding agent. No proxy, no code changes.

Mocker serves fake API responses to the page you are developing, from simple YAML files on your own disk. No account, no proxy to run, no dev-server flag, no branch of throwaway code in your app. A scenario is a file. Turn it on from the toolbar, reload the page, and every request it covers answers with the status, body and delay you wrote. WHAT YOU CAN DO • Mock any endpoint: method, URL, status code, response body and an optional delay. • Group mocks into scenarios ("Empty list", "Expired session", "Save error") and flip them one switch at a time, or mock by mock. • Match URLs without repeating hosts: a bare pathname matches any host, fragments match partially, and * covers ids in the middle. • Reuse one mock across environments with {{variables}} resolved per environment (local, staging, …), validated live as you type. • Simulate what is hard to reproduce: a 500 on save, a 401 mid-session, a listing that answers after three seconds, an empty result set. • See the tab's real traffic in the popup and turn any captured request into a new mock with one click. • Fire saved WebSocket pushes on demand, wrapped in the Centrifugo envelope for a channel or raw for any WebSocket app. • Switch a whole domain off when you want the real backend back — the toolbar icon greys out so you always know. MADE TO SHARE WITH YOUR AI AGENT Because a mock is just a file, an AI coding agent can write it for you: "mock the items endpoint as empty", "make the save fail with a 500", and the scenario appears in the extension without a reload or an import. The loop closes without a browser. Mocker writes the requests it sees to a log next to your scenarios — method, URL, request body, response status and body, and, when a mock matched, which scenario served it. Your agent reads that log to see what the app really called, writes the YAML, and checks that its mock matched. No screenshots to paste, no copying payloads by hand. Mocker ships a Claude Code skill that teaches an agent the whole workflow (file format, URL matching, verification), so it gets it right on the first try. FILES, NOT A DATABASE Everything mocker knows lives in a folder you pick: one project file plus one YAML file per scenario. "New project" initializes that folder anywhere — an empty directory is enough. Put it next to the app you are working on, or in a scratch directory, or anywhere you like: mocker works the same, and anyone you hand the folder to gets exactly the same mocks. Saving from the extension writes the YAML; editing the YAML in your editor shows up in the extension. Nothing is hidden in browser storage. MOCKED REQUESTS ARE NEVER A MYSTERY Every served mock logs to the page console with the original URL intact, the status coloured green or red, and the scenario that answered in parentheses. Amber counters in the popup show how many calls each scenario has served. HOW TO START 1. Open Settings and click "New project" to initialize a folder, or "Import project" to pick one that already has scenarios. 2. Create a scenario, add a mock (method, URL, status, body) — or ask your agent to write it. 3. Turn the scenario on in the popup and reload your app. WHAT IT DOES NOT DO Mocker does not send anything anywhere. It has no account, no server and no telemetry: the requests it captures are written only to the folder you picked, on your own disk, and are wiped at the start of every browser session. Open source: https://github.com/arturosdg/mocker