> ## 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 Kits
> Build payment and liquidity workflows across blockchains with App Kits
App Kits is a suite of SDKs for composing multichain payment and liquidity flows
without orchestrating separate, low-level protocol integrations for each
blockchain or use case.
The App Kit SDK is the all-in-one core package. Install it to access every
capability (Send, Bridge, Swap, Unified Balance, Onramp, Earn, and Borrow)
behind one type-safe interface. Or, you can install Bridge Kit, Swap Kit,
Unified Balance Kit, Onramp Kit, Earn Kit, or Borrow Kit separately.
## Quick install
To get started quickly, install the App Kit SDK and the Viem adapter:
```bash npm theme={null}
npm install @circle-fin/app-kit @circle-fin/adapter-viem-v2 viem
```
```bash yarn theme={null}
yarn add @circle-fin/app-kit @circle-fin/adapter-viem-v2 viem
```
Need a different adapter or individual kits? See the full
[installation](/app-kit/tutorials/installation) guide.
## Core capabilities
Combine and use any of the App Kit SDK's built-in core capabilities in your app.
Bridge, Swap, Unified Balance, Onramp, Earn, and Borrow are also available as
individual kits.
Transfer USDC and EURC across blockchains.
Exchange one token for another on the same blockchain or across blockchains.
Transfer tokens between wallets on the same blockchain.
Create a chain-abstracted balance and spend it instantly.
Embed a widget so users can buy stablecoins on Arc with fiat.
Deposit USDC or EURC to earn returns from supported lending protocols.
Borrow USDC against cirBTC collateral on Arc.
## Key benefits
* **Simple setup**: Get up and running with minimal configuration and a few
lines of code.
* **Application monetization**: Collect a custom fee from end users without
writing new code.
* **Flexible configurations**: Specify custom RPC endpoints and wallet clients.
* **Broad compatibility**: Works with Viem, Ethers, Solana, and Circle Wallets,
integrating smoothly with existing developer workflows.
* **Protocol abstraction**: Build against a single interface over underlying
protocols such as [Gateway](https://developers.circle.com/gateway) and
[CCTP](https://developers.circle.com/cctp).
* **Composable workflows**: Combine multiple capabilities in one product flow
without stitching together separate protocol integrations.
## Quick look
The following examples show how each capability can be integrated with a single
method call.
```typescript TypeScript theme={null}
// Transfer 1.00 USDC from Ethereum to Arc
const result = await kit.bridge({
from: { adapter: viemAdapter, chain: "Ethereum_Sepolia" },
to: { adapter: viemAdapter, chain: "Arc_Testnet" },
amount: "1.00",
});
```
Ready to start bridging? Follow the
[Bridge tokens across blockchains](/app-kit/quickstarts/bridge-tokens-across-blockchains)
quickstart.
```typescript TypeScript theme={null}
// Swap 1.00 USDC for EURC on Arc Testnet
const result = await kit.swap({
from: { adapter: viemAdapter, chain: "Arc_Testnet" },
tokenIn: "USDC",
tokenOut: "EURC",
amountIn: "1.00",
config: {
kitKey: process.env.KIT_KEY as string, // Your kit key from the Circle Console
},
});
```
Ready to start swapping? Follow the
[Swap tokens on a blockchain](/app-kit/quickstarts/swap-tokens-same-chain)
quickstart.
```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({
from: { adapter: viemAdapter },
amountIn: "1.50",
to: {
adapter: viemAdapter,
chain: "Arc_Testnet",
recipientAddress: "0xRecipientAddress",
},
});
```
Ready to start using a Unified Balance? Follow the quickstarts:
* [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)
```typescript TypeScript theme={null}
// Send 1.00 USDC from one wallet to another on Arc Testnet
const result = await kit.send({
from: { adapter: viemAdapter, chain: "Arc_Testnet" },
to: "RECIPIENT_ADDRESS",
amount: "1.00",
token: "USDC",
});
```
Ready to start sending tokens? Follow the
[Send tokens across wallets](/app-kit/quickstarts/send-tokens-same-chain)
quickstart.
```typescript TypeScript theme={null}
// Mint a session on your server, then mount the Onramp widget inline
const session = await kit.onramp.fetchSession({
url: "/api/onramp/sessions",
body: { userId: "user-123", destinationAddress: "USER_WALLET_ADDRESS" },
});
const widget = kit.onramp.mountIframe({
session,
container: document.getElementById("onramp-root")!,
});
```
Ready to start onramping? Follow the
[Embed the Onramp widget](/app-kit/quickstarts/onramp-embed-widget) quickstart.
```typescript TypeScript theme={null}
// Deposit 100.00 USDC into an Earn vault on Arc
const result = await kit.earn.deposit({
from: { adapter: viemAdapter, chain: "Arc_Testnet" },
vaultAddress: "0xVaultAddress",
amount: "100.00",
});
```
Ready to add earning to your app? Follow the
[Deposit into an Earn vault](/app-kit/quickstarts/earn-deposit) quickstart.
```typescript TypeScript theme={null}
// Borrow 1,000 USDC against cirBTC on Arc Testnet
const result = await kit.borrow.borrow({
from: { adapter: viemAdapter, chain: "Arc_Testnet" },
marketId:
"0x1111111111111111111111111111111111111111111111111111111111111111",
borrowAmount: "1000.0",
});
```
Ready to start borrowing? Follow the
[Originate a loan](/app-kit/quickstarts/originate-a-loan) quickstart.