emulate-seed

Emulate Seed Configs

Generate and manage seed configs for emulate (Apache-2.0) — Vercel Labs' stateful API emulation tool. Each category has individual rule files in rules/ loaded on-demand.

Not mocks. Emulate provides full state machines with cascading deletes, cursor pagination, webhook delivery, and HMAC signature verification. Create a PR via the API and it appears in GET /repos/:owner/:repo/pulls. Delete a repo and its issues, PRs, and webhooks cascade-delete.

Quick Reference

Category	Rules	Impact	When to Use
Seed Config	1	HIGH	Setting up emulate.config.yaml for test environments
Service Selection	1	MEDIUM	Choosing GitHub/Vercel/Google for your tests
Webhook Setup	1	MEDIUM	Testing webhook delivery with HMAC verification
Parallel CI	1	HIGH	Running tests in parallel without port collisions
Auth Tokens	1	MEDIUM	Seeding tokens mapped to emulated users

Total: 5 rules across 5 categories

Quick Start

# Install
npm install --save-dev emulate

# Start all services
npx emulate

# Start specific services with seed data
npx emulate --service github,slack --seed ./emulate.config.yaml

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

Services

Service	Default Port	Coverage
GitHub	:4001	Repos, PRs, issues, comments, reviews, Actions, webhooks, orgs, teams
Vercel	:4000	Projects, deployments, domains, env vars, teams
Google OAuth	:4002	OAuth 2.0 authorize, token exchange, userinfo
Slack	:4003	Chat, conversations, users, reactions, OAuth v2 with consent UI
Apple Auth	:4004	Sign in with Apple — OIDC discovery, JWKS (RS256), auth flow, token exchange
Microsoft Entra	:4005	OAuth 2.0/OIDC v2.0, authorization code + PKCE, refresh token rotation
AWS	:4006	S3 buckets, SQS queues, IAM users/roles, STS identity
MongoDB Atlas	:4007	Admin API v2 + Data API v1 — query, update, aggregation pipeline

See references/api-coverage.md for full endpoint lists.

Seed Config Structure

A seed config pre-populates the emulator with tokens, users, repos, and projects so tests start from a known state.

# emulate.config.yaml
tokens:
  dev_token:
    login: yonatangross
    scopes: [repo, workflow, admin:org]
  ci_token:
    login: ci-bot
    scopes: [repo]

github:
  users:
    - login: yonatangross
      name: Yonatan Gross
    - login: ci-bot
      name: CI Bot
  repos:
    - owner: yonatangross
      name: my-project
      private: false
      default_branch: main
      topics: [typescript, testing]

vercel:
  users:
    - username: yonatangross
      email: [email protected]
  projects:
    - name: my-docs
      framework: next

See rules/seed-config.md for full schema and best practices.

Programmatic SDK

import { createEmulator } from 'emulate'

const github = await createEmulator({ service: 'github', port: 4001 })
// github.url -> 'http://localhost:4001'

// State is real — create a PR and it appears in the list
const res = await fetch(`${github.url}/repos/org/repo/pulls`, {
  method: 'POST',
  headers: { Authorization: 'Bearer dev_token' },
  body: JSON.stringify({ title: 'Test PR', head: 'feature', base: 'main' })
})

const prs = await fetch(`${github.url}/repos/org/repo/pulls`)
// -> includes the PR we just created

// Cleanup
github.reset()       // Synchronous state wipe
await github.close() // Shut down server

See references/sdk-patterns.md for advanced patterns (multi-service, lifecycle hooks).

Webhook Delivery

Emulate delivers real webhooks with HMAC-SHA256 signatures when state changes:

import crypto from 'crypto'

function verifyWebhook(payload: string, signature: string, secret: string): boolean {
  const expected = 'sha256=' + crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex')
  return crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expected))
}

See rules/webhook-setup.md for webhook receiver patterns.

CI Integration

# .github/workflows/test.yml
jobs:
  test:
    steps:
      - uses: actions/checkout@v4
      - name: Start emulate
        run: npx emulate --service github --seed .emulate/ci.yaml &
      - name: Wait for emulate
        run: sleep 2
      - name: Run tests
        run: npm test
        env:
          GITHUB_API_BASE: http://localhost:4001
          VERCEL_API_BASE: http://localhost:4000

Parallel Test Execution

Each test worker gets its own port to avoid race conditions:

// vitest.config.ts
const workerPort = 4001 + parseInt(process.env.VITEST_WORKER_ID || '0')

See rules/parallel-ci.md for full parallel isolation patterns.

Decision Matrix

Tool	When to Use	Stateful?	Platforms
emulate (FIRST CHOICE)	GitHub/Vercel/Google API testing	YES	GitHub, Vercel, Google
Pact	Contract verification between services	No	Any
MSW	In-browser/Node HTTP mocking	No	Any
Nock	Node.js HTTP intercept	No	Any
WireMock	HTTP stub server	Partial	Any

Use emulate when:

• Testing code that calls GitHub, Vercel, or Google APIs
• You need state persistence across multiple API calls in a test
• You want webhook delivery with real HMAC signatures
• You need cascading side-effects (delete repo -> PRs cascade-delete)

Use MSW/Nock when:

• Mocking arbitrary HTTP APIs not covered by emulate
• You need in-browser interception (MSW)
• Tests only need single request/response pairs

Related Skills

• testing-integration — Integration test patterns (emulate as first choice for API tests)
• testing-e2e — End-to-end test patterns with emulated backends
• testing-unit — Unit test patterns (use emulate for API-dependent units)
• security-patterns — Auth token patterns (emulate token seeding)

CLI Reference

See references/cli-reference.md for all CLI flags and commands.

SDK Patterns

See references/sdk-patterns.md for programmatic createEmulator() usage.