# Token

Handles two grants, selected by the `grant_type` body field: `authorization_code` (with mandatory PKCE) and `refresh_token` (with rotation and reuse detection). Accepts both `application/json` and `application/x-www-form-urlencoded`. Prefer form encoding, since that's what most OAuth libraries send by default.

Access tokens are JWTs signed with `ES256`, valid for 900 seconds. Refresh tokens are opaque strings, valid for 60 days from whenever they were last issued, and rotate on every use.

## Client authentication

How the client proves its identity depends on the `token_endpoint_auth_method` it [registered](/guides/oauth-register#param-token-endpoint-auth-method) with:

- **Public clients (`none`)** don't send a secret. PKCE (`code_verifier`) is the only proof. This is the default and covers native, CLI, and other clients that can't keep a secret.
- **Confidential clients** additionally present their `client_secret`, on top of PKCE:
  - `client_secret_basic`: send `client_id` and `client_secret` in the HTTP `Authorization: Basic` header (each URL-encoded, joined with `:`, base64-encoded). With this method `client_id` can be omitted from the body.
  - `client_secret_post`: send `client_secret` as a body parameter alongside `client_id`.

A client must use exactly one mechanism. Sending both a Basic header and a body `client_secret` fails with `invalid_request`.

## PKCE

Before starting the [authorization request](/guides/oauth-authorize), generate:

- `code_verifier`: a high-entropy random string, 43–128 characters, from the
  unreserved character set (`A-Z`, `a-z`, `0-9`, `-`, `.`, `_`, `~`). Keep it
  client-side only.
- `code_challenge`: the base64url-encoded SHA-256 hash of `code_verifier`,
  sent during authorization.

```javascript
import { createHash, randomBytes } from 'node:crypto';

function base64url(input) {
  return Buffer.from(input).toString('base64url');
}

const codeVerifier = base64url(randomBytes(64));
const codeChallenge = base64url(
  createHash('sha256').update(codeVerifier).digest(),
);
```

## Authorization code grant

### Body Parameters

- `grant_type` (string, required) — Must be `"authorization_code"`.

- `client_id` (string) — Required, except with `client_secret_basic`, where the `Authorization` header already carries it.

- `client_secret` (string) — Required for a confidential client using `client_secret_post`. Omit for public clients and for `client_secret_basic` (send the secret in the `Authorization` header instead). See [Client authentication](#client-authentication).

- `code` (string, required) — The code from the `/oauth/authorize` callback. Single-use: redeeming it twice fails with `invalid_grant`. Expires 10 minutes after it was issued.

- `redirect_uri` (string, required) — Must exactly match the `redirect_uri` used in the authorization request.

- `code_verifier` (string, required) — The original value that [code\_challenge](/guides/oauth-authorize#param-code-challenge) was derived from.

:::code-group
```bash Public client
curl -X POST 'https://api.resend.com/oauth/token' \
     -H 'Content-Type: application/x-www-form-urlencoded' \
     -d 'grant_type=authorization_code&client_id=550e8400-e29b-41d4-a716-446655440000&code=AUTHORIZATION_CODE&redirect_uri=http%3A%2F%2F127.0.0.1%3A49152%2Foauth%2Fcallback&code_verifier=CODE_VERIFIER_VALUE'
```

```bash Client Secret Basic
curl -X POST 'https://api.resend.com/oauth/token' \
     -H 'Content-Type: application/x-www-form-urlencoded' \
     -u '550e8400-e29b-41d4-a716-446655440000:CLIENT_SECRET' \
     -d 'grant_type=authorization_code&code=AUTHORIZATION_CODE&redirect_uri=http%3A%2F%2F127.0.0.1%3A49152%2Foauth%2Fcallback&code_verifier=CODE_VERIFIER_VALUE'
```

```bash Client Secret Post
curl -X POST 'https://api.resend.com/oauth/token' \
     -H 'Content-Type: application/x-www-form-urlencoded' \
     -d 'grant_type=authorization_code&client_id=550e8400-e29b-41d4-a716-446655440000&client_secret=CLIENT_SECRET&code=AUTHORIZATION_CODE&redirect_uri=http%3A%2F%2F127.0.0.1%3A49152%2Foauth%2Fcallback&code_verifier=CODE_VERIFIER_VALUE'
```
:::

:::code-group
```json Response
{
  "access_token": "eyJhbGciOiJFUzI1NiIsImtpZCI6Im9hdXRoX2tleSIsInR5cCI6ImF0K2p3dCJ9...",
  "token_type": "Bearer",
  "expires_in": 900,
  "refresh_token": "JcL7aYfE7S9h3L4qv0o2e1w8m6n5b3x9RkP2tD4uV6Q",
  "scope": "emails:send"
}
```
:::

### Body Parameters

- `grant_type` (string, required) — Must be `"refresh_token"`.

- `client_id` (string) — Required, except with `client_secret_basic`, where the `Authorization` header already carries it.

- `client_secret` (string) — Required for a confidential client using `client_secret_post`. Omit for public clients and for `client_secret_basic`. See [Client authentication](#client-authentication).

- `refresh_token` (string, required)

- `scope` (string) — Optionally narrow the scope of the new access token. Must be a subset of what the grant already has. You can't use this to escalate scope.

:::code-group
```bash cURL
curl -X POST 'https://api.resend.com/oauth/token' \
     -H 'Content-Type: application/x-www-form-urlencoded' \
     -d 'grant_type=refresh_token&client_id=550e8400-e29b-41d4-a716-446655440000&refresh_token=JcL7aYfE7S9h3L4qv0o2e1w8m6n5b3x9RkP2tD4uV6Q'
```
:::

:::code-group
```json Response
{
  "access_token": "eyJhbGciOiJFUzI1NiIsImtpZCI6Im9hdXRoX2tleSIsInR5cCI6ImF0K2p3dCJ9...",
  "token_type": "Bearer",
  "expires_in": 900,
  "refresh_token": "N3wRefreshTokenValue...",
  "scope": "emails:send"
}
```
:::

## Errors

| Status | `error`               | When                                                                                                                                                                                           |
| ------ | --------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `400`  | `invalid_request`     | A required field is missing or malformed, or the client sent credentials via more than one mechanism (both a Basic header and a body `client_secret`).                                         |
| `400`  | `invalid_scope`       | Refresh requests a scope outside what the grant already has.                                                                                                                                   |
| `400`  | `invalid_grant`       | The code/refresh token is invalid, expired, already used, or reused after rotation (which also revokes the grant). Also returned when PKCE verification fails or `redirect_uri` doesn't match. |
| `401`  | `invalid_client`      | Unknown or disabled `client_id`, or a confidential client failed authentication (missing or wrong `client_secret`, or `client_id` in the Basic header doesn't match the request).              |
| `400`  | `unauthorized_client` | The client isn't registered for the grant type it's using.                                                                                                                                     |

## Related pages

- [Account Management](./account-management-index.md)
- [API Keys](./api-keys-2-index.md)
- [API Keys](./api-keys-index.md)
- [API Reference](./api-reference-index.md)
- [AudiencesDEPRECATED](./audiencesdeprecated-index.md)
- [Authorized Apps](./authorized-apps-index.md)
- [Automations](./automations-index.md)
- [Broadcasts](./broadcasts-index.md)
- [Build with AI](./build-with-ai-index.md)
- [Changelog](../changelog.md)

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