# Cross Chain USDC Transfers (CCTP Integration)

## Overview

:::info
This guide is maintained by [Circle](https://www.circle.com).
:::

Let's set up a cross-chain USDC transfer using [Circle's Cross-Chain Transfer Protocol (CCTP V2)](https://developers.circle.com/stablecoins/cctp-getting-started) and Viem.

In this guide, we'll build a TypeScript script that burns USDC on the **Optimism Sepolia** testnet and mints the equivalent USDC on the **Ethereum Sepolia** testnet – all with just a few steps.

We'll use Viem's wallet client to interact with both chains, and the Circle CCTP Attestation API to retrieve the attestation needed to mint USDC on the destination chain.

By the end, you'll know how to:

* Approve the Circle TokenMessenger contract to spend USDC on the source chain (Optimism Sepolia).
* Burn USDC on the source chain to initiate a cross-chain transfer.
* Fetch an attestation from Circle verifying the burn event.
* Mint USDC on the destination chain (Ethereum Sepolia) using the attestation.
* Verify that the USDC balance moved from the source chain to the destination chain.

## Steps

::::steps
### Set up Viem Clients

First, we need to configure our environment and connect to both blockchains. We'll use **Viem** to create wallet clients for signing transactions and public clients for reading blockchain data.

```ts twoslash [config.ts]
import { createClient, http, publicActions, walletActions } from 'viem';
import { privateKeyToAccount } from 'viem/accounts';
import { sepolia, optimismSepolia } from 'viem/chains';

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

export const client = {
  optimismSepolia: createClient({
    account,
    chain: optimismSepolia,
    transport: http(),
  })
    .extend(publicActions)
    .extend(walletActions),

  sepolia: createClient({
    account,
    chain: sepolia,
    transport: http(),
  })
    .extend(publicActions)
    .extend(walletActions),
}
```

:::note
Ensure your test wallet is funded with Sepolia ETH (for gas) on both networks and some Sepolia USDC on Optimism.
:::

### Define constants

Next, let's define all the constants we'll need: contract addresses, domains, and transfer parameters.

CCTP uses specific contracts on each chain for messaging and token minting/burning, and uses [**domain IDs**](https://developers.circle.com/stablecoins/supported-domains) to identify each blockchain in the protocol. The domain ID for Ethereum (Sepolia) is 0, and for Optimism (Sepolia) it's 2.

We'll also specify the amount of USDC to transfer (in USDC's smallest unit, 6 decimal places) and the max fee for a **Fast** transfer.

```ts [constants.ts]
import { erc20Abi } from 'viem'

export const domain = {
  optimismSepolia: 2,
  mainnet: 0,
}

export const tokenMessengerAbi = [{
  type: 'function', 
  name: 'depositForBurn', 
  stateMutability: 'nonpayable',
  inputs: [
    { name: 'amount', type: 'uint256' }, 
    { name: 'destinationDomain', type: 'uint32' },
    { name: 'mintRecipient', type: 'bytes32' }, 
    { name: 'burnToken', type: 'address' },
    { name: 'destinationCaller', type: 'bytes32' }, 
    { name: 'maxFee', type: 'uint256' },
    { name: 'minFinalityThreshold', type: 'uint32' },
  ],
  outputs: [],
}, {
  type: 'function', 
  name: 'receiveMessage', 
  stateMutability: 'nonpayable',
  inputs: [ 
    { name: 'message', type: 'bytes' }, 
    { name: 'attestation', type: 'bytes' } 
  ],
  outputs: [],
}] as const

export const tokenMessengerAddress = {
  optimismSepolia: '0x8fe6b999dc680ccfdd5bf7eb0974218be2542daa',
  mainnet: '0xe737e5cebeeba77efe34d4aa090756590b1ce275',
}

export const usdcAbi = erc20Abi

export const usdcAddress = {
  optimismSepolia: '0x5fd84259d66cd46123540766be93dfe6d43130d7',
  mainnet: '0x1c7d4b196cb0c7b01d743fbc6116a902379c7238',
}
```

### Approve USDC for transfer

Before we can burn USDC, we must approve the TokenMessenger contract to spend it.\
To prevent a race condition, we call waitForTransactionReceipt to ensure the approval transaction is confirmed before we proceed to the next step.

```ts
import { parseUnits } from 'viem'
import { client } from './config'
import { tokenMessengerAddress, usdcAddress, usdcAbi } from './constants'

async function approveUSDC() {
  // approve 1 USDC
  const hash = await client.optimismSepolia.writeContract({
    abi: usdcAbi,
    address: usdcAddress.optimismSepolia,
    functionName: 'approve',
    args: [tokenMessengerAddress.optimismSepolia, parseUnits('1', 6)],
  });
  await client.optimismSepolia.waitForTransactionReceipt({ hash });
}
```

### Burn USDC to initiate the transfer

Now comes the core of CCTP: **burning** USDC on the source chain to initiate the transfer.

We'll call the TokenMessenger's `depositForBurn(...)` function on Optimism Sepolia.\
This will burn the specified USDC from our wallet and emit a message that Circle's infrastructure will pick up.

Our function burnUSDC will handle this and return the transaction result, which we'll need for the next step (to retrieve the attestation).

```ts
import { erc20Abi, padHex, parseUnits } from 'viem'
import { client } from './config'
import { domain, tokenMessengerAddress, usdcAddress, usdcAbi } from './constants'

const destinationAddress = '0x...'

async function burnUSDC() {
  return await client.optimismSepolia.writeContract({
    abi: tokenMessengerAbi,
    address: tokenMessengerAddress.optimismSepolia,
    functionName: 'depositForBurn',
    args: [
      parseUnits('1', 6), 
      domain.sepolia, 
      padHex(destinationAddress, { dir: 'left', size: 32 }),
      usdcAddress.optimismSepolia, 
      '0x0000000000000000000000000000000000000000000000000000000000000000', 
      parseUnits('0.0005', 6), 
      1000,
    ],
  })
}
```

### Retrieve the attestation from Circle

After burning USDC on the source chain, Circle's CCTP service needs to provide an **attestation** – basically a signed confirmation that the burn event happened and is valid.

The attestation will later be used to authorize minting on the destination chain. Circle provides a public API endpoint to fetch this attestation by the source domain and transaction hash.

```ts
import { Hex } from 'viem'
import { domain } from './constants'

async function retrieveAttestation(burnTx: Hex) {
  const url = `https://iris-api-sandbox.circle.com/v2/messages/${domain.optimismSepolia}?transactionHash=${burnTx}`;

  return new Promise((resolve, reject) => {
    const interval = setInterval(async () => {
      try {
        const response = await fetch(url)
        if (!response.ok) return
        
        const data = await response.json()
        if (!data?.messages?.[0]) return
        if (data.messages[0].status !== 'complete') return

        clearInterval(interval)
        resolve(data.messages[0])
      } catch (error: any) {
        clearInterval(interval)
        reject(error)
      }
    }, 5000);
  });
}
```

### Mint USDC on the destination chain

With the attestation, we call the receiveMessage function on the MessageTransmitter contract on Ethereum Sepolia. This function verifies the attestation and mints the USDC to our destination address.

```ts
import { client } from './config'
import { tokenMessengerAddress, tokenMessengerAbi } from './constants'

async function mintUSDC(attestation: { attestation: Hex, message: Hex }) {
  return await client.sepolia.writeContract({
    to: tokenMessengerAddress.sepolia,
    abi: tokenMessengerAbi,
    functionName: 'receiveMessage',
    args: [attestation.message, attestation.attestation],
  });
}
```

### Execute the transfer

Finally, we execute the steps in sequence and verify the final balances on both chains.

```ts
import { client } from './config'

// 1. Approve USDC on source chain
await approveUSDC();

// 2. Burn USDC on source (initiates transfer)
const burnTx = await burnUSDC();

// 3. Retrieve attestation for the burn transaction
const attestation = await retrieveAttestation(burnTx);

// 4. Mint USDC on destination chain using the attestation
await mintUSDC(attestation);

const sourceBalance = await client.optimismSepolia.readContract({
  address: usdcAddress.optimismSepolia,
  abi: usdcAbi,
  functionName: 'balanceOf',
  args: [account.address],
});
const destBalance = await client.sepolia.readContract({
  address: usdcAddress.sepolia,
  abi: usdcAbi,
  functionName: 'balanceOf',
  args: [account.address],
});

console.log(`USDC on Optimism Sepolia: ${formatUnits(sourceBalance, 6)}`);
console.log(`USDC on Ethereum Sepolia: ${formatUnits(destBalance, 6)}`);
```
::::