About Syncmock

What is Syncmock?

Syncmock is a free, open-source Chrome extension that lets developers intercept, inspect, and mock HTTP and GraphQL network traffic directly in the browser — with no proxy server, no certificate installation, and no changes to backend code.

The project was built out of a frustration with the friction involved in testing frontend applications against edge cases, error states, and slow or unavailable backends. Syncmock removes that friction: you define a rule, reload the page, and your mock is live.

The mission

Make network interception a first-class, zero-setup developer experience. Every frontend developer should be able to mock any API response in under 60 seconds, without reading documentation, configuring a proxy, or asking a backend engineer to add an endpoint.

Principle: All data stays on your device. Syncmock has no telemetry, no accounts, no cloud sync, and no servers. It is infrastructure you own.

What Syncmock does

Live network capture — streams every fetch and XHR request into a panel in real time.
Mock REST APIs — return any JSON body, status code, and headers before the request leaves the browser.
Mock GraphQL — match on operation names and return custom response envelopes.
Override response bodies — let the real request fly, then swap the response the page receives.
Override request bodies — modify outgoing payloads before they hit the server.
Redirect and replace URLs — reroute requests to a different host, path, or port.
Modify headers — inject, override, or remove any request or response header.
AI-assisted rule authoring — describe what you want; the extension generates the rule using your own AI endpoint.

How it works technically

Syncmock is a Manifest V3 Chrome extension with two interception lanes:

chrome.declarativeNetRequest — handles redirects and header modifications natively at the network layer, with no service-worker wakeup penalty.
Page-bridge MAIN-world content script — patches window.fetch and XMLHttpRequest on each page so body-touching actions (mocking, overriding, body inspection) work without a proxy.

Rules are stored in chrome.storage.local on your device and never leave it. The extension requires no account and phones home to no server.

Open source

The full source code is available under the MIT License at github.com/khageshgopalpatel/syncmock. Contributions, bug reports, and feature requests are welcome via GitHub Issues.

Contact

For bug reports and feature requests, please use GitHub Issues. For security vulnerabilities, see our Security policy.