Mock Service Worker

Spec sheet

What it does

MSW is an API mocking library for browser and Node.js that intercepts outgoing requests so they can be observed and answered with mocked responses.Read 30 Sep 2026 · mswjs.io

Purpose

Mock Service Worker (MSW) is an API mocking library for browser and Node.js that intercepts outgoing requests so developers can observe and respond with mocked responses.Read 30 Sep 2026 · mswjs.io

Support

The quick start directs users to a debugging runbook, MSW Discord, and GitHub Discussions for help.Read 30 Sep 2026 · mswjs.io

Additional integrations

The MSW organization lists extensions for Playwright, Storybook, Cloudflare workerd, and SvelteKit.Read 30 Sep 2026 · github.com

Browser integration

In browsers, MSW uses the Service Worker API to intercept requests at the network level.Read 30 Sep 2026 · mswjs.io

Browser limitation

Because browser interception relies on the Service Worker API, browser-specific limitations apply; the docs also state that Firefox does not notify the worker about page XMLHttpRequest requests.Read 30 Sep 2026 · mswjs.io

Client compatibility

The documentation names native fetch and third-party clients including Axios, React Query, and Apollo as supported request clients.Read 30 Sep 2026 · mswjs.io

Learning resources

The documentation says its official Egghead courses require a paid subscription and help support the project.Read 30 Sep 2026 · mswjs.io

Limitations

MSW's GraphQL documentation says it supports queries and mutations, while subscriptions are described as coming in the future.Read 30 Sep 2026 · mswjs.io

Network APIs

MSW supports mocking REST, GraphQL, Server-Sent Events, and WebSocket APIs.Read 30 Sep 2026 · mswjs.io

Network standards

MSW uses the Fetch API to handle intercepted requests and construct mocked responses, including requests made with other clients.Read 30 Sep 2026 · mswjs.io

Node integration

In Node.js, MSW intercepts raw socket connections before they reach the network code written in C.Read 30 Sep 2026 · mswjs.io

Node.js integration

In Node.js, MSW provides setupServer for enabling API mocking in an application process or test runner.Read 30 Sep 2026 · mswjs.io

Node.js limitation

MSW cannot intercept requests made directly through net.connect() or net.createConnection(), which some libraries use.Read 30 Sep 2026 · mswjs.io

Open source

The project describes MSW as open-source and free to try.Read 30 Sep 2026 · mswjs.io

Paid learning

The documentation says its official Egghead courses require a paid subscription.Read 30 Sep 2026 · mswjs.io

Protocols

MSW supports mocking HTTP, GraphQL, Server-Sent Events, and WebSocket APIs.Read 30 Sep 2026 · mswjs.io

Request clients

MSW works with native fetch and third-party clients such as Axios, React Query, and Apollo.Read 30 Sep 2026 · mswjs.io

Reusable mocks

The same mock handlers can be reused for development, integration and end-to-end testing, Storybook, and live demos.Read 30 Sep 2026 · mswjs.io

Shared mocks

The same client-agnostic mocks can be reused across frameworks, tools, and environments, including development, integration and end-to-end testing, Storybook, and live demos.Read 30 Sep 2026 · mswjs.io

Tool integrations

The site provides integration guides for Cloudflare, React Native, Storybook, Vite, and Vitest, as well as browser and Node.js.Read 30 Sep 2026 · mswjs.io

Where Specifiction places it

• API Mocking Tools#21 of 267.1