Syncmock is a free Chrome extension that lets developers intercept, inspect, and mock HTTP network requests directly in the browser. It supports REST APIs, GraphQL, fetch, and XMLHttpRequest — without a proxy server or backend changes.

How do I mock an API request in the browser?

Install the Syncmock Chrome extension, enable interception, create a rule that matches your API URL, choose the 'mock-rest' action, enter your mock response body and status code, then reload the page. The browser will return your mock response instead of hitting the real server.

Does Syncmock require a proxy server?

No. Syncmock works entirely inside the browser using Chrome's declarativeNetRequest API and a page-level fetch/XHR patch. No proxy server, certificate installation, or network configuration is required.

Yes. Syncmock is free and open source under the MIT License. All data stays on your device. There is no paid tier, no account required, and no telemetry.

Does Syncmock work with GraphQL APIs?

Yes. Syncmock has a dedicated mock-graphql action that matches on GraphQL operation names and returns a custom response envelope. It also supports modifying GraphQL request bodies and response bodies.

What browsers does Syncmock support?

Syncmock supports Chrome 116 and newer, and any Chromium-based browser (Edge, Brave, Arc, etc.) that supports Chrome Manifest V3 extensions.

Is Syncmock open source?

Yes. The full source code is available on GitHub at https://github.com/khageshgopalpatel/syncmock under the MIT License.

Chrome Extension · MV3 · Free

Intercept & mock
any HTTP request
in the browser

A developer tool for capturing, inspecting, and mocking network traffic — REST and GraphQL — without a proxy or backend changes.

Add to Chrome — it's free View source

No proxy setup. No backend changes. All data stays on your device.

syncmock · dashboard

Dashboard ● Live

Requests

379total

Mocked

4712%

Recent activity

GET /api/v2/products?page=1 Mocked 200
POST /api/auth/token 201
PUT /api/v2/cart/items/9821 Mocked 200

Everything you need

Built for the way
developers actually work

Six tools in one extension. No proxy server, no certificates to install, no backend code to touch.

Live network capture

Every request and response streams into a panel in real time. Filter by method, status, URL, or mocked-only. Inspect headers and body with zero latency.

fetch · XHR · webRequest

Mock REST & GraphQL

Return any JSON body, status code, and headers — before the real request ever leaves the browser. Perfect for frontend development without a live backend.

mock-rest · mock-graphql

Override response bodies

Let the real request fly, then swap the response body before the page sees it. Test edge cases and error states against your actual backend.

override-response-body

Redirect & replace URLs

Point API calls to a different host, path, or port without touching source code. Great for routing staging traffic to local servers or vice versa.

redirect · replace-url

Modify headers

Inject, override, or remove any request or response header. Add auth tokens, CORS headers, feature flags — without rebuilding the app.

modify-request-headers

AI-assisted rule authoring

Paste a request, describe what you want, and get a ready-to-use rule. Uses your own AI endpoint — your data never goes anywhere unexpected.

local · BYO key · optional

Three steps

From install to
first mock in 60 seconds

No configuration file, no proxy certificate, no server to run.

Install the extension

Add Syncmock from the Chrome Web Store. Click the toolbar icon to open the popup and enable interception. That's the only setup required.

Create a rule

Define a matcher (URL, domain, method, header, GraphQL operation) and pick an action — mock, override, redirect, or modify headers. Name it, save it, done.

Watch it work

Reload the page. The Network panel shows matched requests highlighted. The dashboard counts what was mocked and how fast. Toggle rules on and off in real time.

Products API — mock 404 ● Active

Matcher

url-contains /api/v2/products

Method

GET

Action

mock-rest · status 404

Response body

{
  "error": "not_found",
  "message": "Product does not exist",
  "code": 404
}

Rule engine

6 matchers × 8 actions — wire them any way you need

Each rule pairs a matcher (how to detect a request) with an action (what to do with it). Rules run in priority order. Variants let you switch between multiple response scenarios without editing the rule.

Matchers: URL regex, URL contains, domain, HTTP method, header, GraphQL operation name
Actions: mock REST, mock GraphQL, redirect, replace URL, modify headers, override request or response body
Response variants — multiple named scenarios on one rule, switchable in one click
Import / export rules as JSON — share with your team or version-control them

Intercept & mock any HTTP request in the browser

Built for the waydevelopers actually work