# Use an ERC-20 paymaster with a smart account

> Enable users to pay gas fees with an ERC-20 token using a smart account and paymaster.

This tutorial walks you through using an ERC-20 paymaster with [MetaMask Smart Accounts](/smart-accounts-kit/concepts/smart-accounts), enabling users to pay gas fees in USDC.
This tutorial uses Pimlico's paymaster, but you can use any paymaster of your choice.

## About paymasters

A paymaster is an important component of the [account abstraction (ERC-4337)](/smart-accounts-kit/concepts/smart-accounts) standard, responsible for abstracting gas fees for end users.
There are different types of paymasters, such as gasless paymasters and ERC-20 paymasters.
While a gasless paymaster covers the transaction on behalf of the user, an ERC-20 paymaster allows users to pay gas fees using a supported ERC-20 token.
This removes the need for users to hold native tokens, allowing them to perform onchain actions using only stablecoins.

## Prerequisites

- Install [Node.js](https://nodejs.org/en/blog/release/v18.18.0) v18 or later.
- Install [Yarn](https://yarnpkg.com/),
  [npm](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm), or another package manager.
- [Create a Pimlico API key](https://docs.pimlico.io/guides/create-api-key#create-api-key).

## Steps

### 1. Install the Smart Accounts Kit

Install the [Smart Accounts Kit](https://www.npmjs.com/package/@metamask/smart-accounts-kit) in your project:

```bash npm2yarn
npm install @metamask/smart-accounts-kit
```

### 2. Set up a Public Client

Set up a Public Client using Viem's [`createPublicClient`](https://viem.sh/docs/clients/public) function.
You will configure a <GlossaryTerm term="MetaMask smart account">smart account</GlossaryTerm> and Bundler Client with the Public Client, which you can use to query the <GlossaryTerm term="Signer">signer</GlossaryTerm>'s account state and interact with the blockchain network.

```typescript

const publicClient = createPublicClient({
  chain,
  transport: http(),
})
```

### 3. Set up a Paymaster Client

Set up a <GlossaryTerm term="Paymaster">Paymaster Client</GlossaryTerm>
using Viem's [`createPaymasterClient`](https://viem.sh/account-abstraction/clients/paymaster) function. This client interacts with the paymaster service, enabling users to pay gas fees in USDC.

Replace `<YOUR-API-KEY>` with your Pimlico API key:

```typescript

const paymasterClient = createPaymasterClient({
  transport: http('https://api.pimlico.io/v2/11155111/rpc?apikey=<YOUR-API-KEY>'),
})
```

### 4. Set up a Bundler Client

Set up a Bundler Client using Viem's [`createBundlerClient`](https://viem.sh/account-abstraction/clients/bundler) function.
You can use the <GlossaryTerm term="Bundler">bundler</GlossaryTerm> service to estimate gas for user operations and submit transactions to the network.

To use the ERC-20 paymaster, configure the `paymasterContext` with the ERC-20 token you wish to use to pay for gas fees.
For this tutorial, specify the Sepolia USDC token address.

```typescript

// USDC address on Ethereum Sepolia.
const token = '0x1c7D4B196Cb0C7B01d743Fbc6116a902379C7238'

const bundlerClient = createBundlerClient({
  client: publicClient,
  transport: http('https://your-bundler-rpc.com'),
  paymasterContext: {
    token,
  },
})
```

### 5. Create and fund a smart account

Create a [Hybrid smart account](/smart-accounts-kit/guides/smart-accounts/create-smart-account/#hybrid-smart-account).
A Hybrid smart account is a flexible smart account implementation that supports both an <GlossaryTerm term="Externally owned account (EOA)">EOA</GlossaryTerm> owner and any number of <GlossaryTerm term="Passkey">passkey</GlossaryTerm> (WebAuthn) signers.

```typescript

const account = privateKeyToAccount('0x...')

const smartAccount = await toMetaMaskSmartAccount({
  client: publicClient,
  implementation: Implementation.Hybrid,
  deployParams: [account.address, [], [], []],
  deploySalt: '0x',
  signer: { account },
})
```

Fund the smart account with some Sepolia USDC to pay gas fees.

:::note
You can use [Circle's faucet](https://faucet.circle.com/) to get Sepolia USDC.
:::

### 6. Send a user operation

The ERC-20 paymaster works by transferring the token from the smart account, and reimbursing itself for paying the gas fees on the user's behalf.

To send a <GlossaryTerm term="User operation">user operation</GlossaryTerm> with the ERC-20 paymaster, use the [`sendUserOperation`](https://viem.sh/account-abstraction/actions/bundler/sendUserOperation) method from the Bundler Client.
You must include a call approving the ERC-20 token to be used by the paymaster.
To modify the token allowance for the paymaster, perform a write operation on the USDC contract.
For this tutorial, set an allowance of 10 USDC tokens.

:::note
In a production dapp, you should first check the existing token allowance and only approve the amount required by the paymaster.
:::

Batch the approve call with other onchain actions you want to perform using the ERC-20 paymaster.
Pass the `paymasterClient` from [Step 3](#3-set-up-a-paymaster-client) to the `paymaster` property.

```typescript

// Appropriate fee per gas must be determined for the bundler being used.
const maxFeePerGas = 1n
const maxPriorityFeePerGas = 1n

const pimlicoPaymasterAddress = '0x777777777777AeC03fd955926DbF81597e66834C'

// 10 USDC in wei format. Since USDC has 6 decimals, the wei value is 10 * 10^6.
const approvalAmount = 10000000n

const userOperationHash = await bundlerClient.sendUserOperation({
  account: smartAccount,
  calls: [
    // USDC approve call
    {
      // USDC token address
      to: token,
      abi: parseAbi(['function approve(address,uint)']),
      functionName: 'approve',
      args: [pimlicoPaymasterAddress, approvalAmount],
    },
    // Batch the approve call with other onchain actions.
    {
      to: '0x1234567890123456789012345678901234567890',
      value: parseEther('1'),
    },
  ],
  maxFeePerGas,
  maxPriorityFeePerGas,
  paymaster: paymasterClient,
})
```

## Next steps

- Learn more about [smart account implementations](/smart-accounts-kit/guides/smart-accounts/create-smart-account).
- To sponsor gas for end users, see how to [send a gasless transaction](/smart-accounts-kit/guides/smart-accounts/send-gasless-transaction).