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emulate

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by vercel · part of vercel-labs/emulate

Local drop-in API emulator for Vercel, GitHub, Google, Slack, Apple, Microsoft, AWS, Linear, and other developer APIs. Use when the user needs to start emulated services, configure seed data, write tests against local APIs, set up CI without network access, or work with the emulate CLI or programmatic API. Triggers include "start the emulator", "emulate services", "mock API locally", "create emulator config", "test against local API", "npx emulate", or any task requiring local service emulation.

🔥🔥🔥🔥✓ VerifiedFreeQuick setup
#email#automation#slack#payments#database#cloud#git#project-mgmt
🧰 Not standalone. This skill ships with vercel-labs/emulate and only works together with that tool — install the tool first, then add this skill.

This is the playbook your agent receives when the skill activates — you don't need to read it to use the skill, but it's here to audit before installing.

Service Emulation with emulate

Local drop-in replacement services for CI and no-network sandboxes. Fully stateful, production-fidelity API emulation, not mocks.

CLI

# Start all services (zero-config)
npx emulate

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

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

# Use a seed config file
npx emulate --seed config.yaml

# Generate omitted service secrets into a private file
npx emulate start --seed config.yaml --generated-secrets-file .emulate-secrets.json

# Generate a starter config
npx emulate init

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

# List available services
npx emulate list

Options

FlagDefaultDescription
-p, --port4000Base port (auto-increments per service)
-s, --serviceallComma-separated services to enable
--seedauto-detectPath to seed config (YAML or JSON)
--base-urlnoneOverride advertised base URL (supports {service} template)
--portlessoffServe over HTTPS via portless (auto-registers aliases)
--generated-secrets-filenoneGenerate omitted service secrets and write them to a new owner-only JSON file

The port can also be set via EMULATE_PORT or PORT environment variables.

The generated-secrets destination must not exist. emulate removes inherited ACLs, verifies effective owner-only access, and publishes complete JSON before opening listeners or configuring portless. Handled startup failures remove the invocation-owned artifact. A hard termination can leave a complete artifact that must be removed manually after confirming no invocation is using it. Only service-generated values appear in the artifact. Linux requires setfacl and getfacl from the acl package. The flag fails closed when access controls cannot be verified and is not supported on Windows.

The advertised base URL (used in OAuth redirects, webhook URLs, etc.) can be overridden via --base-url, the EMULATE_BASE_URL env var (supports {service} template), or per-service baseUrl in the seed config. When running under portless, the PORTLESS_URL env var is also detected automatically.

Programmatic API

npm install emulate

Each call to createEmulator starts a single service:

import { createEmulator } from 'emulate'

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

github.url   // 'http://localhost:4001'
vercel.url   // 'http://localhost:4002'

await github.close()
await vercel.close()

For GitHub App tests, inspect secret-free minted installation-token metadata at GET /_emulate/installation-tokens.

Options

OptionDefaultDescription
service(required)'vercel', 'github', 'google', 'slack', 'apple', 'microsoft', 'okta', 'aws', 'resend', 'stripe', 'mongoatlas', 'clerk', 'linear', or 'twilio'
port4000Port for the HTTP server
seednoneInline seed data (same shape as YAML config)
baseUrlnoneOverride advertised base URL. Per-service baseUrl in seed config takes highest priority, then this option, then EMULATE_BASE_URL env var (supports {service}), then PORTLESS_URL (supports {service}, automatically set by the portless CLI wrapper), then http://localhost:<port>.

Instance Methods

MethodDescription
urlBase URL of the running server
reset()Wipe the store and replay seed data
close()Shut down the HTTP server, returns a Promise

HTTPS with portless

portless gives emulators trusted HTTPS URLs with auto-generated certs. Use the --portless flag to auto-register each service as a portless alias:

npx emulate start --portless
# github  https://github.emulate.localhost
# google  https://google.emulate.localhost
# ...

This requires the portless proxy to be running (portless proxy start). If portless is not installed, emulate will prompt to install it.

The --portless flag overwrites any existing portless aliases matching *.emulate. Aliases are removed automatically when emulate shuts down.

For a single service behind portless:

portless github.emulate emulate start --service github

For a custom base URL without portless (any reverse proxy):

npx emulate start --base-url "https://{service}.myproxy.test"
# or
EMULATE_BASE_URL="https://{service}.myproxy.test" npx emulate start

The PORTLESS_URL env var is automatically set by the portless CLI wrapper when running a command through it (e.g. portless github.emulate emulate start), typically to a value like https://{service}.emulate.localhost. It supports {service} interpolation, just like --base-url and EMULATE_BASE_URL. When no explicit baseUrl is provided, it is used as a fallback.

Per-service overrides in the seed config (these take highest priority over all other base URL sources):

github:
  baseUrl: https://github.emulate.localhost
google:
  baseUrl: https://google.emulate.localhost

Pointing Your App at the Emulator

Set environment variables to override real service URLs:

VERCEL_EMULATOR_URL=http://localhost:4000
GITHUB_EMULATOR_URL=http://localhost:4001
GOOGLE_EMULATOR_URL=http://localhost:4002
SLACK_EMULATOR_URL=http://localhost:4003
APPLE_EMULATOR_URL=http://localhost:4004
MICROSOFT_EMULATOR_URL=http://localhost:4005
AWS_EMULATOR_URL=http://localhost:4007
LINEAR_EMULATOR_URL=http://localhost:4012

Then use these in your app to construct API and OAuth URLs. See each service's skill for SDK-specific override instructions.

Framework Integration (Embedded Mode)

The @emulators/adapter-next package embeds emulators directly into a Next.js app on the same origin. See the next skill (skills/next/SKILL.md) for full setup, Auth.js configuration, persistence, and font tracing details.

The @emulators/adapter-nuxt package embeds emulators directly into a Nuxt app on the same origin. See the nuxt skill (skills/nuxt/SKILL.md) for the server route, Nuxt config, OAuth configuration, and persistence setup.

Persistence

By default, all emulator state is in-memory. For persistence across process restarts and serverless cold starts, use a PersistenceAdapter.

Built-in file persistence

import { filePersistence } from '@emulators/core'

// CLI or local dev: persists to a JSON file
const adapter = filePersistence('.emulate/state.json')

Custom adapters

import type { PersistenceAdapter } from '@emulators/core'

const kvAdapter: PersistenceAdapter = {
  async load() { return await kv.get('emulate-state') },
  async save(data) { await kv.set('emulate-state', data) },
}

State is loaded on cold start and saved after every mutating request (POST, PUT, PATCH, DELETE). Saves are serialized to prevent race conditions. Generated GitHub App identities require initialize to atomically create the initial value or return the value another instance created first.

Architecture

packages/
  emulate/           # CLI entry point + programmatic API
  @emulators/
    core/            # HTTP server, Store, plugin interface, middleware
    adapter-next/    # Next.js App Router integration
    adapter-nuxt/    # Nuxt server route integration
    vercel/          # Vercel API service plugin
    github/          # GitHub API service plugin
    google/          # Google OAuth 2.0 / OIDC plugin
    slack/           # Slack Web API, OAuth, incoming webhooks plugin
    linear/          # Linear GraphQL API, OAuth, webhooks plugin
    twilio/          # Twilio Messaging, Verify, Voice, webhooks plugin
    apple/           # Sign in with Apple / OIDC plugin
    microsoft/       # Microsoft Entra ID OAuth 2.0 / OIDC plugin
    aws/             # AWS S3, SQS, IAM, STS plugin

The core provides a generic Store with typed Collection<T> instances supporting CRUD, indexing, filtering, and pagination. Each service plugin registers routes with the shared internal app and uses the store for state.