Emulate

You help developers set up and use emulate — a local drop-in replacement for production APIs (GitHub, Vercel, Google, Slack, Apple, Microsoft, AWS). All state is in-memory, fully stateful, and production-fidelity. Not mocks.

When to use this skill

• Setting up emulate in a new or existing project
• Writing integration tests that hit emulated APIs
• Configuring seed data for emulated services
• Adding emulate to CI pipelines
• Switching from mocked API calls to emulated services
• Debugging emulate configuration or connectivity issues

Available services

CLI usage

# Zero-config — all services on port 4000+
npx emulate

# Specific services
npx emulate --service github,vercel

# Custom port
npx emulate --port 3000

# With seed config
npx emulate --seed emulate.config.yaml

# Generate a starter config
npx emulate init
npx emulate init --service github

# List available services
npx emulate list

Port auto-increments per service: github on 4000, vercel on 4001, google on 4002, etc.

Environment variables EMULATE_PORT or PORT override the default base port.

Programmatic API (for tests)

import { createEmulator } from "emulate";

const github = await createEmulator({
  service: "github",
  port: 14000,
  seed: {
    github: {
      users: [{ login: "test-user" }],
      repos: [{ owner: "test-user", name: "my-repo", auto_init: true }],
    },
  },
});

// github.url → "http://localhost:14000"
// github.reset() → wipe store, re-seed
// github.close() → shutdown server

Vitest integration pattern

import { describe, it, expect, beforeAll, afterAll, afterEach } from "vitest";
import { createEmulator, type Emulator } from "emulate";

let github: Awaited<ReturnType<typeof createEmulator>>;

beforeAll(async () => {
  github = await createEmulator({
    service: "github",
    port: 14000,
    seed: {
      tokens: {
        "test-token": { login: "testuser", scopes: ["repo", "user"] },
      },
      github: {
        users: [{ login: "testuser", name: "Test User", email: "[email protected]" }],
        repos: [{ owner: "testuser", name: "my-repo" }],
      },
    },
  });
});

afterEach(() => {
  github.reset(); // Clean state between tests
});

afterAll(async () => {
  await github.close();
});

it("fetches the authenticated user", async () => {
  const res = await fetch(`${github.url}/user`, {
    headers: { Authorization: "Bearer test-token" },
  });
  expect(res.status).toBe(200);
  const body = await res.json();
  expect(body.login).toBe("testuser");
});

Multiple services

const [github, vercel, google] = await Promise.all([
  createEmulator({ service: "github", port: 14000 }),
  createEmulator({ service: "vercel", port: 14001 }),
  createEmulator({ service: "google", port: 14002 }),
]);

// Point your app at emulated URLs
process.env.GITHUB_API_URL = github.url;
process.env.VERCEL_API_URL = vercel.url;
process.env.GOOGLE_API_URL = google.url;

Seed config format

The config file (emulate.config.yaml) has two top-level sections:

1. Tokens (shared across services)

tokens:
  gho_test_token_admin:
    login: admin
    scopes: [repo, user, admin:org, admin:repo_hook]
  gho_test_token_user1:
    login: octocat
    scopes: [repo, user]

Tokens map to users. Pass as Authorization: Bearer <token> or Authorization: token <token>.

Default token (when no tokens configured): ghotesttoken_admin with login admin.

2. Service-specific seed data

Each service key contains arrays of entities to pre-populate:

github:
  users:
    - login: octocat
      name: The Octocat
      email: [email protected]
  orgs:
    - login: my-org
      name: My Organization
  repos:
    - owner: octocat
      name: hello-world
      description: My first repository
      language: JavaScript
      topics: [hello, world]
      auto_init: true
  oauth_apps:
    - client_id: my_client_id
      client_secret: my_client_secret
      name: My App
      redirect_uris:
        - http://localhost:3000/callback
  apps:
    - app_id: 12345
      slug: my-github-app
      name: My GitHub App
      private_key: |
        -----BEGIN RSA PRIVATE KEY-----
        ...
        -----END RSA PRIVATE KEY-----
      permissions:
        contents: read
        issues: write
      installations:
        - installation_id: 100
          account: octocat
          repository_selection: all

vercel:
  users:
    - username: developer
      email: [email protected]
  teams:
    - slug: my-team
      name: My Team
  projects:
    - name: my-app
      team: my-team
      framework: nextjs
  integrations:
    - client_id: oac_abc123
      client_secret: secret_abc123
      name: My Vercel App
      redirect_uris:
        - http://localhost:3000/callback

google:
  users:
    - email: [email protected]
      name: Test User
  oauth_clients:
    - client_id: google_client_id
      client_secret: google_client_secret
      name: My Google App
      redirect_uris:
        - http://localhost:3000/callback
  messages:
    - id: msg_welcome
      user_email: [email protected]
      from: [email protected]
      to: [email protected]
      subject: Welcome
      body_text: Hello world
      label_ids: [INBOX, UNREAD]
  calendars:
    - id: primary
      user_email: [email protected]
      summary: [email protected]
      primary: true
  calendar_events:
    - id: evt_1
      user_email: [email protected]
      calendar_id: primary
      summary: Meeting
      start_date_time: "2025-01-10T09:00:00.000Z"
      end_date_time: "2025-01-10T09:30:00.000Z"
  drive_items:
    - id: drv_folder
      user_email: [email protected]
      name: Documents
      mime_type: application/vnd.google-apps.folder
      parent_ids: [root]

slack:
  team:
    name: My Workspace
    domain: my-workspace
  users:
    - name: bot
      real_name: Bot User
      email: [email protected]
      is_admin: false
  channels:
    - name: general
      topic: General discussion
  oauth_apps:
    - client_id: slack_client_id
      client_secret: slack_client_secret
      name: My Slack App
      redirect_uris:
        - http://localhost:3000/callback
  incoming_webhooks:
    - channel: general
      label: Notifications

apple:
  users:
    - email: [email protected]
      name: Apple User
  oauth_clients:
    - client_id: com.example.app
      team_id: TEAMID123
      name: My iOS App
      redirect_uris:
        - http://localhost:3000/callback

microsoft:
  users:
    - email: [email protected]
      name: MS User
      tenant_id: "9188040d-6c67-4c5b-b112-36a304b66dad"
  oauth_clients:
    - client_id: ms_client_id
      client_secret: ms_client_secret
      name: My Azure App
      redirect_uris:
        - http://localhost:3000/callback

aws:
  region: us-east-1
  s3:
    buckets:
      - name: my-bucket
        region: us-east-1
  sqs:
    queues:
      - name: my-queue
        fifo: false
  iam:
    users:
      - user_name: deploy-user
        create_access_key: true

Authentication patterns

Token auth (GitHub, Vercel, Slack)

curl -H "Authorization: Bearer gho_test_token_admin" http://localhost:4000/user

OAuth flows (GitHub, Google, Vercel, Apple, Microsoft)

The emulator implements full OAuth 2.0 authorization code flow:

1. Redirect to emulator's authorize endpoint
2. Emulator shows user-picker consent screen
3. Redirect back with authorization code
4. Exchange code for access token
5. Use token to call API

GitHub Apps (JWT)

Sign a JWT with { iss: "<app_id>" } using the app's private key (RS256). Create installation access tokens via:

curl -X POST -H "Authorization: Bearer <jwt>" \
  http://localhost:4001/app/installations/100/access_tokens

CI setup

# GitHub Actions example
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: pnpm/action-setup@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 22
          cache: pnpm
      - run: pnpm install --frozen-lockfile
      - run: pnpm test
        env:
          GITHUB_API_URL: http://localhost:14000
          VERCEL_API_URL: http://localhost:14001

No special CI setup needed — emulate runs in-process via createEmulator() in your test files.

For CLI-based CI (starting emulator as a sidecar):

- name: Start emulator
  run: npx emulate --service github --port 14000 --seed emulate.config.yaml &
- name: Wait for emulator
  run: curl --retry 5 --retry-delay 1 --retry-connrefused http://localhost:14000/meta
- name: Run tests
  run: pnpm test

Common patterns

Replace environment variables for your app

// In test setup, point your app's SDK clients at the emulator
process.env.GITHUB_URL = github.url;        // Instead of https://api.github.com
process.env.VERCEL_API = vercel.url;         // Instead of https://api.vercel.com
process.env.GOOGLE_AUTH_URL = google.url;    // Instead of https://accounts.google.com

Test webhook delivery

// Seed a GitHub app with webhook config
const github = await createEmulator({
  service: "github",
  port: 14000,
  seed: {
    github: {
      apps: [{
        app_id: 1,
        slug: "test-app",
        name: "Test App",
        webhook_url: "http://localhost:3000/webhooks/github",
        webhook_secret: "test-secret",
        installations: [{ installation_id: 100, account: "testuser", repository_selection: "all" }],
      }],
    },
  },
});

Generate starter config

npx emulate init                    # All services
npx emulate init --service github   # Just GitHub

This creates emulate.config.yaml with commented examples.

Debugging

• Set DEBUG=1 or EMULATE_DEBUG=1 to enable debug logging
• The emulator prints a startup banner showing URLs, enabled services, and loaded config
• Rate limiting is built-in (5000 req/hour per token) with X-RateLimit-* headers
• All state is in-memory — restart or reset() to get a clean slate
• Config auto-detection checks: emulate.config.yaml, emulate.config.yml, emulate.config.json