# Verify OAuth access tokens in your TanStack React Start application with Clerk

When building a resource server that needs to accept and verify OAuth access tokens issued by Clerk, it's crucial to verify these tokens on your backend to ensure the request is coming from an authenticated client.

Clerk provides a built-in [auth()](https://clerk.com/docs/tanstack-react-start/reference/tanstack-react-start/auth.md) function that supports token validation via the `acceptsToken` parameter. This lets you specify which type(s) of token your API route should accept in your TanStack React Start application.

By default, `acceptsToken` is set to `session_token`, which means OAuth tokens will **not** be accepted unless explicitly configured. You can pass either a **single token type** or an **array of token types** to `acceptsToken`. To learn more about the supported token types, see the [auth() parameters documentation](https://clerk.com/docs/tanstack-react-start/reference/tanstack-react-start/auth.md#parameters).

## Example 1: Accepting a single token type

In the following example, the `acceptsToken` parameter is set to only accept `oauth_token`s.

- If the token is invalid or missing, `auth()` will return `null` for `subject` and other properties, and the request will be rejected with a `401` response.
- If the token is valid, it returns the authenticated user's subject and their associated scopes for use in the application logic.

```tsx
export async function clerkAuth({ request }: { request: Request }) {
  const { subject, scopes } = await auth({
    acceptsToken: 'oauth_token',
  })

  // If auth() returns null, the token is invalid
  if (!subject) {
    throw new Error('OAuth access token is invalid')
  }

  return { subject, scopes }
}
```

## Example 2: Accepting any token type

In the following example, the `acceptsToken` parameter is set to accept any token type.

- If the token is a `session_token`, it logs that the request is from a user session.
- Otherwise, it logs that the request uses a machine token and specifies its type.

```tsx
import { createServerFn } from '@tanstack/react-start'
import { auth } from '@clerk/tanstack-react-start/server'

const authStateFn = createServerFn().handler(async () => {
  const { tokenType } = await auth({ acceptsToken: 'any' })

  if (tokenType === 'session_token') {
    console.log('This is a session token from a user')
  } else {
    console.log(`This is a ${tokenType} token`)
  }

  return {}
})
```

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