# Pay Fees in a Stablecoin

:::info
This guide is intended to be low-level. If you are looking for a high-level abstraction, check out
[Viem's Pay Fees in a Stablecoin guide](https://viem.sh/tempo/guides/pay-fees).
:::

## Overview

Tempo transaction envelopes can select a TIP-20 token with the `feeToken` field. The protocol's
Fee AMM converts the chosen USD-denominated token into the validator's preferred fee token.

Ox encodes the token address into the transaction envelope. It does not read token metadata,
balances, pause state, or Fee AMM liquidity from the network.

[See the Tempo fee specification](https://docs.tempo.xyz/protocol/fees)

## Recipes

### Set the Fee Token

Pass a token address to `feeToken` when calling
[`TxEnvelopeTempo.from`](/tempo/reference/TxEnvelopeTempo/from).

```ts twoslash
import { TxEnvelopeTempo } from 'ox/tempo'

const alphaUsd = '0x20c0000000000000000000000000000000000001'
const envelope = TxEnvelopeTempo.from({
  calls: [{ to: '0xcafebabecafebabecafebabecafebabecafebabe' }],
  chainId: 4217,
  feeToken: alphaUsd, // [!code hl]
  gas: 21_000n,
  maxFeePerGas: 2_000_000_000n,
  nonce: 0n,
  nonceKey: 1n,
})
```

### Sign and Serialize the Selection

For a transaction without sponsorship, the sender commits to `feeToken` through
[`TxEnvelopeTempo.getSignPayload`](/tempo/reference/TxEnvelopeTempo/getSignPayload).

```ts twoslash
import { Secp256k1 } from 'ox'
import { TxEnvelopeTempo } from 'ox/tempo'

const privateKey = Secp256k1.randomPrivateKey()
const envelope = TxEnvelopeTempo.from({
  calls: [{ to: '0xcafebabecafebabecafebabecafebabecafebabe' }],
  chainId: 4217,
  feeToken: '0x20c0000000000000000000000000000000000001',
  gas: 21_000n,
  maxFeePerGas: 2_000_000_000n,
  nonce: 0n,
  nonceKey: 1n,
})

// [!code focus:start]
const signature = Secp256k1.sign({
  payload: TxEnvelopeTempo.getSignPayload(envelope),
  privateKey,
})
const serialized = TxEnvelopeTempo.serialize(envelope, { signature })
// [!code focus:end]
```

### Inspect the Encoded Fee Token

Deserialize the transaction to inspect its encoded fee token selection.

```ts twoslash
import { TxEnvelopeTempo } from 'ox/tempo'

const envelope = TxEnvelopeTempo.from({
  calls: [{ to: '0xcafebabecafebabecafebabecafebabecafebabe' }],
  chainId: 4217,
  feeToken: '0x20c0000000000000000000000000000000000001', // [!code hl]
})
const serialized = TxEnvelopeTempo.serialize(envelope)

const decoded = TxEnvelopeTempo.deserialize(serialized)
const feeToken = decoded.feeToken // [!code hl]
```

## Best Practices

### Check Eligibility Onchain

An address-shaped value can still be an invalid fee token. Before signing, use a Tempo client to
confirm that the token is an active USD-denominated TIP-20 and that the Fee AMM can settle it.

### Check the Fee Payer's Balance

Without sponsorship, the sender pays. With sponsorship, the fee payer pays. Check the relevant
account's balance in the selected token before submission.

### Treat Sponsored Fee Selection Differently

Fee sponsorship intentionally lets the fee payer select and commit to the fee token. Follow the
[Sponsor User Fees](/tempo/guides/transaction-envelopes/sponsor-user-fees) signing flow instead
of reusing the single-signer flow above.

## See More

<Cards>
  <Card icon="lucide:hand-coins" title="Sponsor User Fees" description="Add a fee payer signature to a Tempo envelope." to="/tempo/guides/transaction-envelopes/sponsor-user-fees" />

  <Card icon="lucide:send" title="Transaction Envelopes" description="Construct, sign, serialize, and submit Tempo envelopes." to="/tempo/guides/transaction-envelopes" />

  <Card icon="lucide:book-open" title="Viem: Pay Fees in a Stablecoin" description="Select a fee token and submit transactions with a Viem client." to="https://viem.sh/tempo/guides/pay-fees" />
</Cards>