vercel-labs/emulate

Skill by ara.so — Daily 2026 Skills collection.

emulate provides fully stateful, production-fidelity local HTTP servers that replace Vercel, GitHub, and Google APIs. Designed for CI pipelines and no-network sandboxes — not mocks, real in-memory state with proper pagination, OAuth, webhooks, and cascading deletes.

Installation

# CLI (no install needed)
npx emulate

# Or install as a dev dependency
npm install --save-dev emulate

CLI Usage

# Start all services with defaults
npx emulate

# Start specific services
npx emulate --service vercel,github

# Custom base port (auto-increments per service)
npx emulate --port 3000

# Start with seed data
npx emulate --seed emulate.config.yaml

# Generate a starter config
npx emulate init

# Generate config for a specific service
npx emulate init --service github

# List available services
npx emulate list

Default ports:

• Vercel → http://localhost:4000
• GitHub → http://localhost:4001
• Google → http://localhost:4002

Port can also be set via EMULATE_PORT or PORT environment variables.

Programmatic API

import { createEmulator, type Emulator } from 'emulate'

// Start a single service
const github = await createEmulator({ service: 'github', port: 4001 })
const vercel = await createEmulator({ service: 'vercel', port: 4002 })

console.log(github.url)  // 'http://localhost:4001'
console.log(vercel.url)  // 'http://localhost:4002'

// Reset state (replays seed data)
github.reset()

// Shutdown
await github.close()
await vercel.close()

Options

Instance Methods

Vitest / Jest Setup

// vitest.setup.ts
import { createEmulator, type Emulator } from 'emulate'

let github: Emulator
let vercel: Emulator

beforeAll(async () => {
  ;[github, vercel] = await Promise.all([
    createEmulator({ service: 'github', port: 4001 }),
    createEmulator({ service: 'vercel', port: 4002 }),
  ])
  process.env.GITHUB_URL = github.url
  process.env.VERCEL_URL = vercel.url
})

afterEach(() => {
  github.reset()
  vercel.reset()
})

afterAll(() => Promise.all([github.close(), vercel.close()]))

// vitest.config.ts
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    setupFiles: ['./vitest.setup.ts'],
    environment: 'node',
  },
})

Seed Configuration

Create emulate.config.yaml in your project root (auto-detected):