> ## 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.
# App Kit SDK: Unified Balance
> Create a chain-agnostic USDC balance and spend it instantly on any blockchain with the App Kit SDK's Unified Balance capability.
The [App Kit SDK](/app-kit) includes the Unified Balance capability that
combines USDC from multiple blockchains into a single, instantly spendable
balance. It is built on top of
[Circle Gateway](https://developers.circle.com/gateway) and handles the Gateway
workflow for deposits and spends across EVM and non-EVM blockchains.
## How it works
Unified Balance works by depositing funds held across multiple blockchains into
a single, chain-agnostic Unified Balance. Those funds are then available to
spend instantly on any blockchain.
The process is illustrated below:
```mermaid theme={null}
flowchart LR
A[Chain A wallet] -->|"deposits funds"| UB[(Unified Balance)]
B[Chain B wallet] -->|"deposits funds"| UB
C[Chain C wallet] -->|"deposits funds"| UB
UB -->|"spends funds"| D[Chain D wallet]
```
### What to know about wallet models
* Some wallet models cannot sign their own Unified Balance spends, including
Circle Wallets SCAs and Privy server wallets. Use the
[delegate workflow](/app-kit/quickstarts/unified-balance-delegate-deposit-and-spend):
the wallet remains the depositor, and an authorized EOA signs each spend.
* Circle Wallets are chain-specific. For Unified Balance flows that draw from
multiple source blockchains, use the wallet address for each source
blockchain.
* For Unified Balance spends, create one source per wallet and chain as needed.
The Circle Wallets adapter is stateless, so you can reuse the same adapter
configuration and pass the wallet address in each operation context.
* Circle Wallets smart contract account (SCA) deposits require
`allowanceStrategy: "approve"`. USDC permit signatures use `ecrecover`, which
does not accept the SCA's ERC-1271 signature, so the SDK uses an onchain
`approve`.
Unified Balance is built on Circle Gateway. For production considerations
related to deposits, spends, and fund removal, see the
[Gateway implementation checklist](https://developers.circle.com/gateway/references/technical-guide#implementation-considerations).
## Quick look
This code snippet creates a Unified Balance by depositing funds from two
blockchains to spend on a third:
```typescript TypeScript theme={null}
// Deposit 1.00 USDC into the Unified Balance from Base
const depositBase = await kit.unifiedBalance.deposit({
from: { adapter: viemAdapter, chain: "Base_Sepolia" },
amount: "1.00",
token: "USDC",
});
// Deposit 1.00 USDC into the Unified Balance from Arbitrum
const depositArb = await kit.unifiedBalance.deposit({
from: { adapter: viemAdapter, chain: "Arbitrum_Sepolia" },
amount: "1.00",
token: "USDC",
});
// Spend 1.50 USDC from the Unified Balance on Arc
const spendResult = await kit.unifiedBalance.spend({
amount: "1.50",
from: { adapter: viemAdapter },
to: {
adapter: viemAdapter,
chain: "Arc_Testnet",
recipientAddress: "0xRecipientAddress",
},
});
```
For a complete end-to-end flow, follow the quickstart for your scenario:
* [Deposit and spend a Unified Balance](/app-kit/quickstarts/unified-balance-deposit-and-spend)
* [Use a delegate to deposit and spend a Unified Balance](/app-kit/quickstarts/unified-balance-delegate-deposit-and-spend)
## Installation
[Install the App Kit SDK](/app-kit/tutorials/installation) to use Unified
Balance. If you only need Unified Balance and don't want to install the full App
Kit SDK, follow the steps below to install the standalone Unified Balance Kit.
```bash npm theme={null}
npm install @circle-fin/unified-balance-kit
```
```bash yarn theme={null}
yarn add @circle-fin/unified-balance-kit
```
Use the [adapter setup guide](/app-kit/tutorials/adapter-setups) to choose the
right wallet model for your app, including browser wallets and Circle Wallets.
```bash npm theme={null}
npm install @circle-fin/adapter-viem-v2 viem
```
```bash yarn theme={null}
yarn add @circle-fin/adapter-viem-v2 viem
```
```bash npm theme={null}
npm install @circle-fin/adapter-ethers-v6 ethers
```
```bash yarn theme={null}
yarn add @circle-fin/adapter-ethers-v6 ethers
```
```bash npm theme={null}
npm install @circle-fin/adapter-solana-kit @solana/kit @solana/web3.js
```
```bash yarn theme={null}
yarn add @circle-fin/adapter-solana-kit @solana/kit @solana/web3.js
```
```bash npm theme={null}
npm install @circle-fin/adapter-circle-wallets
```
```bash yarn theme={null}
yarn add @circle-fin/adapter-circle-wallets
```