resend

---
name: resend
description: Emulated Resend email API for local development and testing. Use when the user needs to send emails locally, test transactional email flows, implement magic link or verification code auth, inspect sent emails, manage domains/contacts/API keys, or work with the Resend API without sending real emails. Triggers include "Resend API", "emulate Resend", "send email locally", "test email", "magic link", "verification email", "email inbox", "RESEND_BASE_URL", or any task requiring a local email API.
allowed-tools: Bash(npx emulate:*), Bash(curl:*)
---

# Resend Email API Emulator

Fully stateful Resend API emulation. Emails, domains, API keys, audiences, and contacts persist in memory. Sent emails are captured and viewable through the inbox UI or the REST API.

No real emails are sent. Accepted calls to `POST /emails` store the message locally so you can inspect it programmatically or in the browser. Validated sends also record attempts, including recipient rejections.

## Start

Local listeners bind to `127.0.0.1` by default. For access from containers or other machines, pass `--host 0.0.0.0` to the CLI or `hostname: '0.0.0.0'` to `createEmulator`. Configure `--base-url` or `baseUrl` for advertised URLs reachable by those clients.

```bash
# Resend only
npx emulate --service resend

# Default port (when run alone)
# http://localhost:4000
```

Or programmatically:

```typescript
import { createEmulator } from 'emulate'

const resend = await createEmulator({ service: 'resend', port: 4000 })
// resend.url === 'http://localhost:4000'
```

## Auth

Pass tokens as `Authorization: Bearer <token>`. Any `re_` prefixed token is accepted.

```bash
curl http://localhost:4000/emails \
  -H "Authorization: Bearer re_test_key"
```

When no token is provided, requests fall back to the default user.

## Pointing Your App at the Emulator

### Environment Variable (Resend SDK)

The official Resend Node.js SDK reads `RESEND_BASE_URL` at module load time. Set it to the emulator URL and the SDK works without any code changes:

```bash
RESEND_BASE_URL=http://localhost:4000
```

```typescript
import { Resend } from 'resend'

// No baseUrl argument needed; the SDK reads RESEND_BASE_URL automatically.
const resend = new Resend('re_test_key')

await resend.emails.send({
  from: '[email protected]',
  to: '[email protected]',
  subject: 'Hello',
  html: '<p>It works!</p>',
})
```

### Embedded in Next.js (adapter-next)

When using `@emulators/adapter-next`, the emulator runs inside your Next.js app at `/emulate/resend`. Set `RESEND_BASE_URL` via `next.config.ts`:

```typescript
// next.config.ts
import { withEmulate } from '@emulators/adapter-next'

export default withEmulate({
  env: {
    RESEND_BASE_URL: `http://localhost:${process.env.PORT ?? '3000'}/emulate/resend`,
  },
})
```

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

export const { GET, POST, PUT, PATCH, DELETE } = createEmulateHandler({
  services: {
    resend: {
      emulator: resend,
      seed: {
        domains: [{ name: 'example.com' }],
      },
    },
  },
})
```

### Direct fetch

If you cannot use the SDK or env var, call the emulator directly:

```typescript
await fetch('http://localhost:4000/emails', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Authorization': 'Bearer re_test_key',
  },
  body: JSON.stringify({
    from: '[email protected]',
    to: '[email protected]',
    subject: 'Hello',
    html: '<p>It works!</p>',
  }),
})
```

## Seed Config

```yaml
resend:
  domains:
    - name: example.com
      region: us-east-1
  contacts:
    - email: [email protected]
      first_name: Test
      last_name: User
      audience: Default
```

## Retrieving Sent Emails

This is the key differentiator of the emulator: every email sent via `POST /emails` is stored and queryable.

### Inbox UI

Browse sent emails in the browser:

```
http://localhost:4000/inbox
```

### REST API

```bash
# List all sent emails
curl http://localhost:4000/emails \
  -H "Authorization: Bearer re_test_key"

# Get a single email by ID
curl http://localhost:4000/emails/<id> \
  -H "Authorization: Bearer re_test_key"
```

### Extracting Data from Emails (tests, agents)

Useful for completing magic link, verification code, or password reset flows programmatically:

```bash
# Get the latest email ID
EMAIL_ID=$(curl -s http://localhost:4000/emails \
  -H "Authorization: Bearer re_test_key" | jq -r '.data[0].id')

# Extract a 6-digit code from the HTML body
CODE=$(curl -s http://localhost:4000/emails/$EMAIL_ID \
  -H "Authorization: Bearer re_test_key" | jq -r '.html' | grep -oE '[0-9]{6}')

echo "Verification code: $CODE"
```

## API Endpoints

### Emails

```bash
# Send an email
curl -X POST http://localhost:4000/emails \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"from": "[email protected]", "to": "[email protected]", "subject": "Hello", "html": "<p>Hi</p>"}'

# Send batch (up to 100)
curl -X POST http://localhost:4000/emails/batch \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '[{"from": "[email protected]", "to": "[email protected]", "subject": "One", "html": "<p>1</p>"}]'

# List all emails
curl http://localhost:4000/emails \
  -H "Authorization: Bearer $TOKEN"

# Get email by ID
curl http://localhost:4000/emails/<id> \
  -H "Authorization: Bearer $TOKEN"

# Cancel a scheduled email
curl -X POST http://localhost:4000/emails/<id>/cancel \
  -H "Authorization: Bearer $TOKEN"
```

Supported fields: `from`, `to`, `subject`, `html`, `text`, `cc`, `bcc`, `reply_to`, `headers`, `tags`, `scheduled_at`.

### Idempotent Sends

`POST /emails` and `POST /emails/batch` accept the case-insensitive `Idempotency-Key` header. Keys must be 1 to 256 characters and remain active for 24 hours. A retry with the same key and validated payload returns the original response, email IDs, and status without creating emails or dispatching duplicate `email.sent` and `email.delivered` webhooks. Reusing a key with a different payload or endpoint returns `409 invalid_idempotent_request`; invalid key lengths return `400 invalid_idempotency_key`. Omitting the header preserves normal behavior.

```bash
curl -X POST http://localhost:4000/emails \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: signup-email-123" \
  -d '{"from": "[email protected]", "to": "[email protected]", "subject": "Hello"}'
```

### Recipient restrictions and send attempts

Set `resend.allowlist` in seed config to test recipient rejection:

```yaml
resend:
  allowlist:
    - [email protected]
    - [email protected]
```

Omitting `allowlist` accepts every recipient; `allowlist: []` rejects every recipient. Every address in `to`, `cc`, and `bcc` must be listed. Matching ignores surrounding whitespace and letter case; entries are individual addresses, with no wildcard or domain matching. This is a local testing control, independent of Resend's domain verification and account restrictions.

A rejected send returns `403 validation_error` naming the rejected addresses and configured allowlist. No email ID, email record, idempotency record, or webhook is created. A batch is rejected as a whole if any recipient is disallowed. Retrying a rejected request, even with the same idempotency key, creates a new attempt. Replaying an accepted idempotent request returns its original response even if the allowlist has since changed.

Validated sends append one record per email to the `resend.send_attempts` store collection, including emails in a rejected batch. Each record contains `from`, `to`, `cc`, `bcc`, `subject`, `idempotency_key` (or `null`), and the store's `id`, `created_at`, and `updated_at`. Message bodies are not copied into attempts. Malformed requests, invalid or conflicting idempotency keys, and idempotency replays do not append attempts.

Read attempts with `getResendStore(store).sendAttempts.all()` from `@emulators/resend`, or inspect the persisted state. Shared adapter snapshots use the key `resend:resend.send_attempts`. Attempts and recipient restrictions survive saved-state restoration, including rejected sends. There is no additional Resend API endpoint for attempts. A rejected send has an attempt without an email; a send that never happened has neither.

### Domains

```bash
# Create domain
curl -X POST http://localhost:4000/domains \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name": "example.com", "region": "us-east-1"}'

# List domains
curl http://localhost:4000/domains \
  -H "Authorization: Bearer $TOKEN"

# Get domain
curl http://localhost:4000/domains/<id> \
  -H "Authorization: Bearer $TOKEN"

# Verify domain (instantly marks all records as verified)
curl -X POST http://localhost:4000/domains/<id>/verify \
  -H "Authorization: Bearer $TOKEN"

# Delete domain
curl -X DELETE http://localhost:4000/domains/<id> \
  -H "Authorization: Bearer $TOKEN"
```

### API Keys

```bash
# Create API key
curl -X POST http://localhost:4000/api-keys \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name": "Production"}'

# List API keys
curl http://localhost:4000/api-keys \
  -H "Authorization: Bearer $TOKEN"

# Delete API key
curl -X DELETE http://localhost:4000/api-keys/<id> \
  -H "Authorization: Bearer $TOKEN"
```

### Audiences

```bash
# Create audience
curl -X POST http://localhost:4000/audiences \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"name": "Newsletter"}'

# List audiences
curl http://localhost:4000/audiences \
  -H "Authorization: Bearer $TOKEN"

# Delete audience
curl -X DELETE http://localhost:4000/audiences/<id> \
  -H "Authorization: Bearer $TOKEN"
```

### Contacts

```bash
# Create contact in an audience
curl -X POST http://localhost:4000/audiences/<audience_id>/contacts \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"email": "[email protected]", "first_name": "Jane", "last_name": "Doe"}'

# List contacts in an audience
curl http://localhost:4000/audiences/<audience_id>/contacts \
  -H "Authorization: Bearer $TOKEN"

# Delete contact
curl -X DELETE http://localhost:4000/audiences/<audience_id>/contacts/<id> \
  -H "Authorization: Bearer $TOKEN"
```

## Webhooks

The emulator dispatches webhook events when state changes:

- `email.sent` and `email.delivered` on `POST /emails`
- `domain.created` and `domain.deleted` on domain operations
- `contact.created` and `contact.deleted` on contact operations

## Common Patterns

### Magic Link / Verification Code Flow

```bash
TOKEN="re_test_key"
BASE="http://localhost:4000"

# 1. Send verification email
curl -X POST $BASE/emails \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"from": "[email protected]", "to": "[email protected]", "subject": "Your code", "html": "<p>Code: <strong>482910</strong></p>"}'

# 2. Retrieve the email
EMAIL_ID=$(curl -s $BASE/emails -H "Authorization: Bearer $TOKEN" | jq -r '.data[0].id')

# 3. Read the HTML body
curl -s $BASE/emails/$EMAIL_ID -H "Authorization: Bearer $TOKEN" | jq -r '.html'
```

### Send and Verify in a Test

```typescript
import { createEmulator } from 'emulate'
import { Resend } from 'resend'

const emu = await createEmulator({ service: 'resend', port: 4000 })

process.env.RESEND_BASE_URL = emu.url
const resend = new Resend('re_test_key')

// Send
await resend.emails.send({
  from: '[email protected]',
  to: '[email protected]',
  subject: 'Verify',
  html: '<p>Code: <strong>123456</strong></p>',
})

// Retrieve
const res = await fetch(`${emu.url}/emails`, {
  headers: { Authorization: 'Bearer re_test_key' },
})
const { data: emails } = await res.json()
console.log(emails[0].html) // contains "123456"
```

## Custom emulators alongside built-ins

Use `npx emulate init --custom inventory` to scaffold a third-party API emulator and test. Register it in `emulate.config.ts` with `defineConfig` from `emulate`, alongside built-in entries such as `{ emulator: "resend" }`. Run `npx emulate start --watch` to reload imports and inspect custom state at the printed `/_emulate` URL. Successful reloads reset the run to seed. Existing flat seed configs still work; `--config` selects an explicit file. For authoring and testing third-party API emulators, see https://emulate.dev/docs/custom-emulators.