The Complete Production SOP: How to Deploy and Run Mock Service Worker (MSW) Network-Level API Mocking in Autonomous AI Agents

---
name: "msw-mock-service-worker-api-tests"
description: "Mock REST and GraphQL APIs at the network level for tests, storybook, and development with MSW."
metadata:
  author: "Artem Zakharchenko"
  version: "1.0.0"
  runtime_compatibility:
    - "codex"
    - "claude-code"
    - "cursor"
    - "windsurf"
triggers:
  - "setup msw 2.0 handlers"
  - "mock http request msw"
  - "msw in vitest"
permissions:
  fs_read: true
  fs_write: "workspace"
  shell_exec: false
  net_access: false
---

# Mock Service Worker (MSW) Network-Level API Mocking

Implements Mock Service Worker (MSW 2.0): http and graphql handlers (http.get, http.post), HttpResponse.json, setting up setupServer for Node.js tests, setupWorker for browsers, and intercepting fetch requests seamlessly.

## Execution SOP & Rules
1. Define handlers in src/mocks/handlers.ts using `http.*` and `HttpResponse.json()`.
2. Configure Node server in src/mocks/node.ts via `setupServer(...handlers)`.
3. Hook server lifecycle in vitest.setup.ts: listen, resetHandlers after each test, close.
4. Fail fast on unhandled requests with `server.listen({ onUnhandledRequest: 'error' })`.
5. When simulating errors per test, use `server.use()` overrides, never mutate production endpoints.

import { setupServer } from 'msw/node';
import { handlers } from './handlers';

export const server = setupServer(...handlers);

import { http, HttpResponse } from 'msw';

export const handlers = [
  http.get('https://api.example.com/v1/users/:id', ({ params }) => {
    const { id } = params;
    return HttpResponse.json({
      id: String(id),
      name: 'Agent Validator',
      role: 'engineer'
    });
  }),
  http.post('https://api.example.com/v1/auth/login', async ({ request }) => {
    const payload = (await request.json()) as { username?: string };
    if (!payload.username) {
      return new HttpResponse(null, { status: 400 });
    }
    return HttpResponse.json({ token: 'mock-jwt-token-xyz' }, { status: 200 });
  })
];