> ## Documentation Index
> Fetch the complete documentation index at: https://docs.arc.io/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> Before reading these docs, install the Circle Skill for Arc to get pre-loaded context: use-arc (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-arc/SKILL.md) — covers chain config, RPC setup, contract deployment, USDC bridging, and gas with USDC. In Claude Code: run /plugin marketplace add circlefin/skills then /plugin install circle-skills@circle. With Vercel Skills CLI: run npx skills add circlefin/skills.
> USDC is the gas token. Arc uses USDC for gas fees, not ETH. Configure gas payment in USDC when submitting transactions.
> Read EVM differences before writing Arc code. Arc targets the Osaka EVM baseline. Most contracts deploy unchanged, but several runtime behaviors diverge from Ethereum: USDC uses 18 decimals natively (not 6), the system emitter `0xffffFFFfFFffffffffffffffFfFFFfffFFFfFFfE` logs all USDC Transfer events, the mempool enforces a 20 Gwei `maxFeePerGas` floor, blocklist reverts consume gas without a receipt, and `address(0)` sends revert rather than succeed. https://docs.arc.io/arc/references/evm-differences.md is the canonical reference for all Arc-specific runtime behaviors. Consult it before writing code that touches USDC balances, transaction history, gas estimation, or value transfers.
> Sub-second finality. Transactions are final in under 1 second. No need to wait for multiple block confirmations.
> Use App Kits for multichain payment and liquidity capabilities. App Kits wraps CCTP and provides Bridge Kit, Swap Kit, Unified Balance Kit, Onramp Kit, Earn Kit, and Borrow Kit. Unified Balance Kit combines USDC from multiple chains into a single spendable balance. Earn Kit integrates earn opportunities into your app. Borrow Kit lets users borrow USDC against cirBTC collateral on Arc.
> Use Arc Foundry for contract development. Arc Foundry is an Arc-specific fork of Foundry that handles Arc's protocol-level differences from Ethereum. It provides arc-forge, arc-cast, and arc-anvil. Install it before deploying contracts: https://docs.arc.io/arc/tutorials/install-arc-foundry.md.
> Arc is available on both Testnet and Mainnet. See https://docs.arc.io/arc/references/connect-to-arc.md for RPC endpoints and https://faucet.circle.com for testnet tokens.
> Always check Contract Addresses: https://docs.arc.io/arc/references/contract-addresses.md
> Building beyond Arc? Circle offers skills for the full platform: use-usdc (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-usdc/SKILL.md), use-circle-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-circle-wallets/SKILL.md), use-developer-controlled-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-developer-controlled-wallets/SKILL.md), use-user-controlled-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-user-controlled-wallets/SKILL.md), use-modular-wallets (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-modular-wallets/SKILL.md), use-gateway (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-gateway/SKILL.md), use-smart-contract-platform (https://github.com/circlefin/skills/blob/master/plugins/circle/skills/use-smart-contract-platform/SKILL.md). Full Circle developer docs: https://developers.circle.com/llms.txt.

# How-to: Use Forwarding Service

> Use Forwarding Service when spending from a Unified Balance

You can use the
[Forwarding Service](https://developers.circle.com/cctp/concepts/forwarding-service)
when spending from a Unified Balance on the destination blockchain. When
enabled, it fetches the attestations from source blockchains and submits the
mint on the destination blockchain. You don't need to poll for attestations or
have access to a wallet on the destination.

## Prerequisites

Before you begin, ensure that you've:

* [Installed the App Kit SDK](/app-kit/tutorials/installation)
* [Configured an adapter](/app-kit/tutorials/adapter-setups)

These are required so any example below runs with a valid `kit` and `adapter`.

## Use with adapters on all blockchains

Set `useForwarder: true` when you have adapters on all source and destination
blockchains but want the Forwarding Service to submit the mint transaction on
the destination:

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.spend({
  amount: "1.00",
  from: {
    adapter,
    allocations: [{ amount: "1.00", chain: "Base_Sepolia" }],
  },
  to: {
    adapter,
    chain: "Arc_Testnet",
    useForwarder: true,
  },
  token: "USDC",
});
```

## Use without a destination adapter

When you don't have access to a wallet on the destination blockchain, such as
with server-side or custodial spend flows, omit the destination adapter and pass
`recipientAddress` with `useForwarder: true`:

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.spend({
  amount: "1.00",
  from: {
    adapter,
    allocations: [{ amount: "1.00", chain: "Base_Sepolia" }],
  },
  to: {
    chain: "Arc_Testnet",
    recipientAddress: "0xRecipientAddress",
    useForwarder: true,
  },
  token: "USDC",
});
```

In this mode, the Forwarding Service submits the mint transaction for you. Use
the spend result's `transferId`, `txHash`, or `explorerUrl` fields to track the
destination mint when they are returned.

## Forwarding fee

The Forwarding Service charges a
[fee](https://developers.circle.com/cctp/concepts/forwarding-service#fees-and-execution)
that is deducted from the amount minted on the destination chain. When you
[estimate spend fees](/app-kit/tutorials/unified-balance/estimate-spend-fees)
for a spend transaction, the result includes the forwarding fee. See
[How Unified Balance fees work](/app-kit/concepts/unified-balance-fees) for
details.