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# Fees

Pay transaction fees with a supported USD-denominated TIP-20 token. Tempo has no native gas token. The sender can pay, or a separate fee payer can sponsor the transaction.

:::info[Interactive walkthrough]
Try [Sponsor fees](https://tempo.xyz/developers/docs/guide/payments/sponsor-user-fees#fee-sponsorship-demo) in Accounts: send a test payment with a fee payer, then follow the wallet implementation.
:::

Use the [Network example setup](https://tempo.xyz/developers/docs/network/examples) for these Moderato examples. The sender needs test AlphaUSD and a recipient address.

## Choose a fee token

Set a client default with `feeToken`, or pass it on an individual transaction. The token must have enough Fee AMM liquidity when conversion to the validator's preferred token is needed.

To save an account preference onchain:

:::code-group
```ts [Viem]
await client.fee.setUserTokenSync({ token: alphaUsd })
```

```bash [CLI]
cast send 0xfeec000000000000000000000000000000000000 \
  'setUserToken(address)' "$TOKEN" \
  --rpc-url "$RPC_URL" --tempo.fee-token "$TOKEN" --interactive
```

```ts [Wagmi]
  await Actions.fee.setUserTokenSync(config, { token: alphaUsd })
```

```python [Python]
from pytempo.contracts import FeeManager
from network_config import ALPHA_USD, make_transaction, submit

call = FeeManager.set_user_token(token=ALPHA_USD)
submit(make_transaction((call,)))
```
:::

An explicit transaction fee token overrides the saved preference. [Choose a fee token](https://tempo.xyz/developers/docs/guide/payments/pay-fees-in-any-stablecoin) includes a working demo and per-transaction examples.

## Sponsor fees

The sender signs the transaction and supplies any funds it transfers. The fee payer separately authorizes payment of its transaction fees.

Configure the public testnet relay in the [Viem setup](https://tempo.xyz/developers/docs/network/examples#sponsored-transactions) or [Wagmi wallet setup](https://tempo.xyz/developers/docs/network/examples#wagmi-setup), then request sponsorship. The CLI passes the relay URL directly. Foundry 1.8.5 estimates gas before attaching the sponsor signature, so this example adds a 20% gas margin:

:::code-group
```ts [Viem]
await sponsored.token.transferSync({
  token: alphaUsd,
  to: recipient,
  amount: 1_000_000n,
  feePayer: true,
})
```

```bash [CLI]
GAS=$(cast estimate "$TOKEN" 'transfer(address,uint256)' "$TEMPO_RECIPIENT" 1000000 \
  --from "$SENDER" --rpc-url "$RPC_URL" --tempo.fee-token "$TOKEN")
cast erc20 transfer "$TOKEN" "$TEMPO_RECIPIENT" 1000000 \
  --rpc-url "$RPC_URL" --sponsor-url https://sponsor.moderato.tempo.xyz \
  --gas-limit "$(((GAS * 120 + 99) / 100))" --interactive
```

```ts [Wagmi]
  await Actions.token.transferSync(config, {
    token: alphaUsd,
    to: recipient,
    amount: 1_000_000n,
    feePayer: true,
  })
```
:::

The public testnet endpoint needs no API key. For authenticated sandbox and production sponsorship, use the [Fee Payer API](https://tempo.xyz/developers/docs/api/fee-payer). The [Python SDK](https://tempo.xyz/developers/docs/sdk/python#fee-sponsorship-with-python) supports co-signing with a separate fee payer account. A raw RPC request must already contain the required signatures.

Use the [sponsorship demo](https://tempo.xyz/developers/docs/guide/payments/sponsor-user-fees) to try the flow, or the [relay handler](https://tempo.xyz/developers/docs/server/relay-handler) to enforce your own sponsorship policy.

## Estimate costs

Estimate the actual call with its sender and fee configuration. A transfer to a new recipient can require more gas than a transfer to an existing token holder, so avoid hard-coding one gas limit for every payment.

Estimate a one-AlphaUSD transfer. The HTTP tab uses `SENDER`, `TOKEN`, and `CALL_DATA` from the [RPC setup](https://tempo.xyz/developers/docs/network/examples#http-requests).

:::code-group
```ts [Viem]
const gas = await client.token.transfer.estimateGas({
  account: client.account,
  chain: client.chain,
  token: alphaUsd,
  to: recipient,
  amount: 1_000_000n,
})
```

```bash [CLI]
GAS=$(cast estimate "$TOKEN" 'transfer(address,uint256)' \
  "$TEMPO_RECIPIENT" 1000000 --from "$SENDER" \
  --rpc-url "$RPC_URL" --tempo.fee-token "$TOKEN")
```

```ts [Wagmi]
  const transfer = ViemActions.token.transfer.call({
    token: alphaUsd,
    to: recipient,
    amount: 1_000_000n,
  })
  const gas = await estimateGas(config, { ...transfer, account: accountAddress })
```

```python [Python]
from network_config import ALPHA_USD, TIP20, account, recipient, w3

call = TIP20(ALPHA_USD).transfer(to=recipient, amount=1_000_000)
request = {"type": "0x76", "from": account.address, "feeToken": ALPHA_USD,
           "calls": [{"to": ALPHA_USD, "data": "0x" + call.data.hex()}]}
gas = int(w3.manager.request_blocking("eth_estimateGas", [request]), 16)
```

```bash [API (RPC)]
curl "$RPC_URL" -H 'Content-Type: application/json' --data @- <<JSON
{"jsonrpc":"2.0","id":1,"method":"eth_estimateGas","params":[{
  "type":"0x76","from":"$SENDER","feeToken":"$TOKEN",
  "calls":[{"to":"$TOKEN","data":"$CALL_DATA"}]
}]}
JSON
```
:::

Gas prices are in attodollars (10⁻¹⁸ USD) per gas. Convert the estimate to USD, rounding up to six decimal places:

:::code-group
```ts [Viem]
const gasPrice = await client.getGasPrice()
const microdollars = (gas * gasPrice + 10n ** 12n - 1n) / 10n ** 12n
console.log('Estimated fee (USD):', formatUnits(microdollars, 6))
```

```bash [CLI]
GAS_PRICE=$(cast gas-price --rpc-url "$RPC_URL")
MICRODOLLARS=$(((GAS * GAS_PRICE + 999999999999) / 1000000000000))
cast format-units "$MICRODOLLARS" 6
```

```ts [Wagmi]
  const gasPrice = await getGasPrice(config)
  const microdollars = (gas * gasPrice + 10n ** 12n - 1n) / 10n ** 12n
  console.log('Estimated fee (USD):', formatUnits(microdollars, 6))
```

```python [Python]
gas_price = w3.eth.gas_price
microdollars = (gas * gas_price + 10**12 - 1) // 10**12
print(f"Estimated fee (USD): {microdollars / 10**6:.6f}")
```
:::

This is an estimate at the current suggested gas price. Gas usage, the base fee, and network state can change before inclusion. Read the receipt for actual gas usage and the fee-token transfer for the amount charged.

The [fee specification](https://tempo.xyz/developers/docs/protocol/fees/spec-fee#fee-units) defines fee units, maximum-fee reservation, and refunds. [Fee AMM liquidity](https://tempo.xyz/developers/docs/guide/stablecoin-dex/managing-fee-liquidity) supports conversion between fee tokens; it is separate from swapping application funds on the Exchange.