# `withRelay`

Creates a transport that routes Tempo relay traffic between a default transport and a relay service.

* [View Guide](https://docs.tempo.xyz/guide/payments/sponsor-user-fees)
* [View Specification](https://docs.tempo.xyz/protocol/transactions/spec-tempo-transaction)

`withRelay` forwards every `eth_fillTransaction` request to the relay and preserves the request's `feePayer` value so the relay can decide whether to sponsor the transaction.

* `feePayer: true` asks the relay to sponsor the transaction.
* If `feePayer` is omitted, `undefined`, or `null`, that value is forwarded as-is.
* An explicit `feePayer` address is preserved.

## Usage

:::code-group

```ts twoslash [example.ts]
import { privateKeyToAccount } from 'viem/accounts'
import { createClient, http, withRelay } from 'viem/tempo'

const client = createClient({
  account: privateKeyToAccount('0x...'),
  testnet: true,
  transport: withRelay(
    http(),                             // ← Default Transport
    http('https://relay.example.com'),  // ← Relay Transport // [!code hl]
  ),
})

// Regular transaction
const receipt1 = await client.sendTransactionSync({
  to: '0x742d35Cc6634C0532925a3b844Bc9e7595f0bEbb',
})

// Sponsored transaction // [!code hl]
const receipt2 = await client.sendTransactionSync({ // [!code hl]
  feePayer: true, // [!code hl]
  to: '0x742d35Cc6634C0532925a3b844Bc9e7595f0bEbb', // [!code hl]
})
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/tempo/viem.config.ts:setup]
```

:::

### Example Relay Service

Below is an end-to-end example of a client/server relay setup.

See `server.ts` for the server-side implementation. It uses [`Handler.relay` provided by `accounts/server`](https://docs.tempo.xyz/accounts/server/handler.relay) to handle relay requests.

:::code-group

```ts twoslash [client.ts]
import { privateKeyToAccount } from 'viem/accounts'
import { createClient, http, withRelay } from 'viem/tempo'

const client = createClient({
  account: privateKeyToAccount('0x...'),
  testnet: true,
  transport: withRelay(
    http(),
    http('http://localhost:3000'),
  ),
})

const hash = await client.sendTransactionSync({
  feePayer: true,
  to: '0x0000000000000000000000000000000000000000',
})
```

```ts twoslash [server.ts]
// @noErrors
import { createServer } from 'node:http'
import { privateKeyToAccount } from 'viem/accounts'
import { createClient } from 'viem/tempo'
import { Handler } from 'accounts/server'

const client = createClient({
  // Note: the relay can specify its own fee token.
  feeToken: '0x20c0000000000000000000000000000000000001',
  testnet: true,
})

const handler = Handler.relay({ // [!code hl]
  feePayer: { // [!code hl]
    account: privateKeyToAccount('0x...'), // [!code hl]
  }, // [!code hl]
}) // [!code hl]

const server = createServer(handler.listener)
server.listen(3000)
```

:::

## Return Type

```ts
type ReturnType = Transport<'relay'>
```

## Parameters

### defaultTransport

* **Type:** `Transport`

The default transport to use for regular relay traffic and for broadcasting transactions when `policy` is `'sign-only'`.

### relayTransport

* **Type:** `Transport`

The relay transport to use for `eth_fillTransaction` and sponsored transaction handling.