> ## 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: Select source blockchains for a Unified Balance spend

> Control how USDC is drawn from your Unified Balance across blockchains when you spend, using automatic routing or explicit amounts

A spend pulls USDC from the balances you have deposited on supported
blockchains. Choose one of these approaches to control which blockchains supply
the USDC:

* **Automatic routing**: Specify a total spend amount and let the App Kit SDK
  choose which blockchains to draw from based on your confirmed balances.
* **Explicit amounts**: Set how much USDC comes from each source blockchain
  yourself.

## 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`.

## The App Kit SDK allocates for you

The App Kit SDK chooses how to fund the spend from your confirmed balances. The
kit prefers the destination blockchain first, then pulls from your other
blockchains from highest balance to lowest. Ethereum mainnet is the exception:
it is always last, including when it is the spend destination.

Pass `amount`, `from` with only an `adapter` (no `allocations`), and `to`.

This example spends 2.00 USDC with automatic routing:

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.spend({
  amount: "2.00",
  from: { adapter },
  to: {
    adapter,
    chain: "Arc_Testnet",
    recipientAddress: "0xRecipientAddress",
  },
});
```

## Explicit per-chain allocations

List each blockchain and how much USDC to draw in `from.allocations`. Each entry
contains an `amount` and `chain`. When you use `allocations`, their amounts must
add up to the top-level `amount`.

This example spends 2.00 USDC (1.00 USDC from Arc Testnet and 1.00 USDC from
Base Sepolia) delivered to the recipient on Arc Testnet:

```typescript TypeScript theme={null}
const result = await kit.unifiedBalance.spend({
  amount: "2.00",
  from: {
    adapter,
    allocations: [
      { amount: "1.00", chain: "Arc_Testnet" },
      { amount: "1.00", chain: "Base_Sepolia" },
    ],
  },
  to: {
    adapter,
    chain: "Arc_Testnet",
    recipientAddress: "0xRecipientAddress",
  },
});
```

You can pass multiple adapters in `from` when you need separate sources (for
example EVM and Solana). Each source uses the same shape: `adapter` and optional
`allocations`. The top-level `amount` must still match the sum of all allocation
amounts you provide.

## Validation rules

* **Amount**: The top-level `amount` is always required. It is the total USDC
  for the spend.
* **Sums**: If you pass `allocations`, the kit checks that they sum to `amount`.
  If they do not match, you get a clear error with both values.
* **Consistency**: Either every `from` entry specifies `allocations`, or none
  do. Mixing sources with allocations and sources without is not supported. If
  you need a computed split first, use `estimateSpend`, then pass the returned
  `allocations` into `spend`.
* **Retry**: Retrying only the mint step after a failure uses a separate
  `retrySpend` flow and parameters, not a partial `spend` call.