> ## 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 fast deposits

> Deposit USDC into a Unified Balance from a source blockchain onto a fast finality destination without waiting for source chain finality

Standard deposits wait for source chain finality, which can take 15 minutes or
longer. Fast deposits use
[CCTP's Fast Transfer](https://developers.circle.com/cctp/concepts/finality-and-block-confirmations#fast-transfer-attestation-times)
to credit the destination in seconds while the source settles in the background.
They are opt-in: pass `config: { transferSpeed: "FAST" }` with `to`. Omitting it
fails validation rather than falling back to a slower deposit.

## 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`.

## Supported blockchains

The following blockchains support a fast transfer as the source or destination.

<Info>
  This list differs from Unified Balance's
  [supported blockchains](/app-kit/references/supported-blockchains): some sources
  are not full deposit blockchains, and only a subset of Unified Balance
  blockchains are fast deposit destinations. For chain identifiers used in code,
  see
  [Blockchain identifiers](/app-kit/references/supported-blockchains#blockchain-identifiers).
</Info>

| Role | Blockchains |
| - | - |
| Source | Arbitrum, Codex, Ethereum, Ink, Linea, OP Mainnet, Plume, Unichain, World Chain |
| Destination | Arc, Avalanche, Polygon PoS |

## Optional: set an allowance in advance

A fast deposit uses the `TokenMessengerWithFees` contract to spend the deposit
amount and forwarder fee from your wallet. If the remaining USDC allowance is
too low, the kit submits an approval transaction and waits for it to be mined
before it submits the burn transaction.

To remove that wait from the deposit path, set an allowance before you estimate
and execute the deposit. The allowance applies only to the same signing wallet
and source blockchain. Each fast deposit consumes part of it, so the
optimization lasts only while the remaining allowance covers the deposit amount
plus the forwarder fee.

All supported EVM source blockchains share the same address in an environment:

| Environment | `TokenMessengerWithFees` address |
| - | - |
| Mainnet | `0x71f54F818671cD0D7ea140Da213e5C8b5C92a408` |
| Testnet | `0x8745D906D67C346E5eb1aEEED38Eb87F34DF0C0A` |

Approval isn't part of the Unified Balance API. This example uses a
user-controlled EVM adapter on Ethereum Sepolia:

```typescript TypeScript theme={null}
import { EthereumSepolia } from "@circle-fin/app-kit/chains";

const sourceChain = EthereumSepolia;
const tokenMessengerWithFees: string =
  "0x8745D906D67C346E5eb1aEEED38Eb87F34DF0C0A";
const maxUint256: bigint = 2n ** 256n - 1n;

function isEvmTransactionHash(value: string): value is `0x${string}` {
  return /^0x[0-9a-fA-F]{64}$/.test(value);
}

const approval = await adapter.prepareAction(
  "usdc.approve",
  { amount: maxUint256, delegate: tokenMessengerWithFees },
  { chain: sourceChain },
);

const approvalTransactionHash: string = await approval.execute();

if (!isEvmTransactionHash(approvalTransactionHash)) {
  throw new Error("The approval returned an invalid EVM transaction hash.");
}

type ApprovalReceipt = Awaited<ReturnType<typeof adapter.waitForTransaction>>;

const approvalReceipt: ApprovalReceipt = await adapter.waitForTransaction(
  approvalTransactionHash,
  { timeout: 60_000 },
  sourceChain,
);

if (approvalReceipt.status !== "success") {
  throw new Error("The USDC approval transaction reverted.");
}
```

`maxUint256` grants the largest possible `uint256` USDC allowance. Later fast
deposits from the same wallet and source blockchain can skip the approval wait.
The allowance stays active until you change or revoke it. Use a bounded amount
if you want to limit how much USDC the contract can spend.

<Note>
  Pre-approving moves the approval cost earlier; it doesn't remove the cost. If
  you use a developer-controlled adapter, also include its wallet `address` in the
  `prepareAction` context.
</Note>

## Estimate fees before depositing

Fast deposits carry two fees:

* **Gas fee**: the cost to submit the source transaction. Paid in the source
  blockchain's native token, such as ETH.
* **Forwarder fee**: charged in USDC from your source chain wallet on top of the
  deposit amount. Your wallet needs deposit + fee available; the full deposit
  amount arrives on the destination chain.

Use `estimateDeposit` to preview fees before committing. The returned `quote`
locks in the forwarder fee, preventing fee changes from affecting your deposit.
For broader fee estimation guidance, see
[Estimate spend fees](/app-kit/tutorials/unified-balance/estimate-spend-fees).

```typescript TypeScript theme={null}
const estimate = await kit.unifiedBalance.estimateDeposit({
  from: { adapter, chain: "Ethereum_Sepolia" },
  amount: "10",
  token: "USDC",
  to: { chain: "Arc_Testnet" },
  config: { transferSpeed: "FAST" },
});

console.log(estimate.fees);
// [
//   { type: 'gasFee',    token: 'ETH',  amount: '0.0012' },
//   { type: 'forwarder', token: 'USDC', amount: '0.50'   },
// ]
```

<Note>
  The gas fee estimate may differ from the actual fee due to network conditions at
  execution time. Review the estimate before proceeding.
</Note>

## Execute the fast deposit

Pass the `estimate` object into `deposit()` to reuse the locked fee quote. If
you omit the quote, the kit fetches a fresh one. If a supplied quote expired,
`deposit()` throws an error. Call `estimateDeposit()` again and pass the new
estimate without changing its quoted fields.

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.deposit({
  ...estimate, // spreads amount, to, config, and quote
  from: { adapter, chain: "Ethereum_Sepolia" },
});
```

<Note>
  `allowanceStrategy` cannot be used with the `to` parameter. Fast deposits manage
  USDC approval internally. A fast finality source blockchain (for example
  Avalanche or Polygon PoS) is rejected rather than silently falling back to a
  standard deposit. Use a [supported source](#supported-blockchains) instead.
</Note>

## Deposit for another account

`depositFor` is permissionless. Any wallet can fund another account's Unified
Balance. Estimate first, then pass the same `to` and `config` fields as
`deposit`:

```typescript TypeScript theme={null}
const estimate = await kit.unifiedBalance.estimateDeposit({
  from: { adapter, chain: "Ethereum_Sepolia" },
  amount: "1.00",
  token: "USDC",
  to: { chain: "Arc_Testnet" },
  config: { transferSpeed: "FAST" },
});

const result = await kit.unifiedBalance.depositFor({
  ...estimate, // spreads amount, to, config, and quote
  from: { adapter, chain: "Ethereum_Sepolia" },
  depositAccount: "0xDepositAccountAddress",
});
```

## Handle the deposit result

Circle's relayer runs in the background. `deposit()` returns a `progress` field
alongside the transaction details:

| Status | Meaning |
| - | - |
| `DONE` | The relayer confirmed the destination deposit. `txHash` is the relay transaction hash. |
| `PENDING` | The kit timed out (\~60 s). `txHash` is the submitted source transaction, whose receipt might still be pending. |
| `FAILED` | The source burn completed, but the relayer returned a terminal destination failure. |

```typescript TypeScript theme={null}
switch (result.progress?.status) {
  case "DONE":
    console.log("Deposited. Relay tx:", result.txHash);
    break;
  case "PENDING":
    console.log("Transfer in progress. Source tx:", result.txHash);
    break;
  case "FAILED":
    console.error(
      "Relay failed. Contact support with source tx:",
      result.txHash,
    );
    break;
}
```

<Note>
  `PENDING` doesn't confirm that the source transaction succeeded. Check the
  source `txHash` before you act. Don't submit another deposit while its receipt
  is pending or after the receipt succeeds. For receipt, polling, and recovery
  guidance, see
  [Troubleshoot Unified Balance deposits](/app-kit/references/unified-balance-error-recovery).
</Note>

Example result shape:

```json JSON theme={null}
{
  "amount": "1.00",
  "token": "USDC",
  "depositedTo": "0xDepositAccountAddress",
  "depositedBy": "0xSignerAddress",
  "chain": "Arc_Testnet",
  "txHash": "0x…",
  "explorerUrl": "https://explorer.testnet.arc.io/tx/0x…",
  "sourceChain": "Ethereum_Sepolia",
  "destinationChain": "Arc_Testnet",
  "fees": [
    { "type": "gasFee", "token": "ETH", "amount": "0.000689…" },
    { "type": "forwarder", "token": "USDC", "amount": "0.023517" }
  ],
  "progress": { "status": "DONE" }
}
```