Next.js Integration

The @emulators/adapter-next package embeds emulators directly into a Next.js App Router app, running them on the same origin. This is particularly useful for Vercel preview deployments where OAuth callback URLs change with every deployment.

Install

npm install @emulators/adapter-next @emulators/github @emulators/google

Only install the emulators you need. Each @emulators/* package is published independently, keeping serverless bundles small.

Route Handler

Create a catch-all route that serves emulator traffic:

// app/emulate/[...path]/route.ts
import { createEmulateHandler } from '@emulators/adapter-next'
import * as github from '@emulators/github'
import * as google from '@emulators/google'

export const { GET, POST, PUT, PATCH, DELETE, OPTIONS } = createEmulateHandler({
  services: {
    github: {
      emulator: github,
      seed: {
        users: [{ login: 'octocat', name: 'The Octocat' }],
        repos: [{ owner: 'octocat', name: 'hello-world', auto_init: true }],
      },
    },
    google: {
      emulator: google,
      seed: {
        users: [{ email: 'test@example.com', name: 'Test User' }],
      },
    },
  },
})

This creates the following routes:

• /emulate/github/** serves the GitHub emulator
• /emulate/google/** serves the Google emulator

GitHub installation-token metadata is available server-side at /emulate/github/_emulate/installation-tokens.

Auth.js / NextAuth Configuration

Point your provider at the emulator paths on the same origin:

import GitHub from 'next-auth/providers/github'

const baseUrl = process.env.VERCEL_URL
  ? `https://${process.env.VERCEL_URL}`
  : 'http://localhost:3000'

GitHub({
  clientId: 'any-value',
  clientSecret: 'any-value',
  authorization: { url: `${baseUrl}/emulate/github/login/oauth/authorize` },
  token: { url: `${baseUrl}/emulate/github/login/oauth/access_token` },
  userinfo: { url: `${baseUrl}/emulate/github/user` },
})

No oauth_apps need to be seeded. When none are configured, the emulator skips client_id, client_secret, and redirect_uri validation.

Font Tracing for Serverless

Emulator UI pages use bundled fonts. Wrap your Next.js config to include them in the serverless trace:

// next.config.mjs
import { withEmulate } from '@emulators/adapter-next'

export default withEmulate({
  // your normal Next.js config
})

If you mount the catch-all at a custom path, pass the matching prefix:

export default withEmulate(nextConfig, { routePrefix: '/api/emulate' })

Persistence

By default, emulator state is in-memory and resets on every cold start. To persist state across restarts, pass a persistence adapter.

Custom Adapter (Vercel KV, Redis, etc.)

import { createEmulateHandler } from '@emulators/adapter-next'
import * as github from '@emulators/github'

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

export const { GET, POST, PUT, PATCH, DELETE, OPTIONS } = createEmulateHandler({
  services: { github: { emulator: github } },
  persistence: kvAdapter,
})

File Persistence (Local Dev)

For local development, @emulators/core ships a file-based adapter:

import { filePersistence } from '@emulators/core'

// persists to a JSON file
persistence: filePersistence('.emulate/state.json'),

GitHub App seeds may omit private_key. Retain the handler and call server-only generatedSecrets(); explicit keys are excluded. Keep persisted snapshots private and implement initialize atomically.

How Persistence Works

• Cold start: The adapter loads state from the persistence adapter. If found, it restores the full Store and token map (skipping seed). If not found, it seeds from config and saves the initial state.
• After mutating requests (POST, PUT, PATCH, DELETE): State is saved. Saves are serialized via an internal queue to prevent race conditions.
• No persistence configured: Falls back to pure in-memory. Seed data re-initializes on every cold start.

How It Works

1. Incoming request: /emulate/github/login/oauth/authorize?client_id=...
2. Parse: service = github, rest = /login/oauth/authorize
3. Strip prefix: A new Request is created with the stripped path and forwarded to the GitHub service app
4. Rewrite response: HTML action and href attributes, CSS url() font references, and Location headers get the service prefix prepended
5. Persist: After mutating requests, state is saved via the persistence adapter

Limitations

• Requires the Node.js runtime (not Edge) since emulators use crypto.randomBytes
• Concurrent mutations use last-write-wins semantics. Generated identities require initialize to select the initial snapshot atomically across cold starts.

Config Reference

createEmulateHandler(config)

Field	Type	Description
services	Record<string, EmulatorEntry>	Map of service name to emulator config
persistence?	PersistenceAdapter	Optional persistence adapter for state across cold starts

Each EmulatorEntry:

Field	Type	Description
emulator	EmulatorModule	The emulator package (e.g. import * as github from '@emulators/github')
seed?	Record<string, unknown>	Seed data matching the service's config schema

withEmulate(nextConfig, options?)

Wraps a Next.js config to include emulator font files in the serverless output trace. Call it around your exported config in next.config.mjs or next.config.ts.

Option	Type	Default	Description
routePrefix	string	"/emulate"	The path prefix where the catch-all route is mounted

PersistenceAdapter

interface PersistenceAdapter {
  load(): Promise<string | null>
  save(data: string): Promise<void>
  initialize?(data: string): Promise<string>
}

initialize must atomically create the initial value or return the value another instance created first. Implement it with compare-and-set semantics such as Redis SET NX. The built-in filePersistence(path) from @emulators/core provides this behavior for local development.

Custom emulators

The adapter accepts definitions created with defineEmulator from emulate in services[name].emulator. Reuse the definition from CLI/tests; custom inspector is opt-in. Root-relative redirect locations stay under the service mount, while custom HTML bodies pass through unchanged. Await the returned handler's close() in test teardown. Install emulate as a runtime dependency for deployed definitions. See https://emulate.dev/docs/custom-emulators for seeds, snapshots, persistence, and lifecycle rules.