> ## 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.

# DeFi integration

> Port AMMs, liquidity pools, and DeFi protocols to Arc using USDC as the canonical pool token and native gas asset.

For DeFi protocols on Arc, use the ERC-20 USDC interface at
`0x3600000000000000000000000000000000000000` for all pool accounting, token
addresses, and reserve math. Arc's native USDC has a
[built-in ERC-20 interface](/arc/concepts/stablecoin-native-model) at that
address, so no WETH-style adapter contract is needed.

## Key differences for DeFi protocols

Three Arc design decisions affect every DeFi porting decision:

| Concern | Arc behavior | Impact on DeFi protocols |
| :- | :- | :- |
| **One balance, two decimal views** | Native USDC uses 18 decimals (`msg.value`, `address.balance`); the ERC-20 interface uses 6 decimals (`balanceOf`, pool math). Both views draw from the same underlying balance. | Use 6-decimal ERC-20 values exclusively in pool invariant math. Mixing 18-decimal native values with 6-decimal ERC-20 values breaks constant-product calculations by a factor of 10<sup>12</sup>. Do not pair the native interface against the ERC-20 interface as two separate pool tokens; they are the same asset. |
| **No protocol adapter needed** | Native USDC has a built-in ERC-20 interface at `0x3600000000000000000000000000000000000000`. No WETH-style adapter is required. See [EVM differences](/arc/references/evm-differences) for the full list of protocol divergences. | Use the ERC-20 address directly as the pool pair token. |
| **Allowances do not gate native transfers** | ERC-20 `approve` and `allowance` control only the `transferFrom` path. Native transfers (`msg.value`, direct sends) bypass the allowance check entirely. | Pool sweep logic that relies on allowance limits does not protect the native balance. |

## What you need

Each integration guide in this section requires the following:

| Requirement | Details |
| :- | :- |
| RPC access | `https://rpc.testnet.arc.io` |
| Chain ID | `5042002` (testnet) |
| USDC ERC-20 contract | [`0x3600000000000000000000000000000000000000`](https://explorer.testnet.arc.io/address/0x3600000000000000000000000000000000000000) |
| Existing EVM contract codebase | A Solidity codebase to port to Arc |

## Integration guides

<CardGroup cols={2}>
  <Card title="AMM and pools" icon="chart-mixed" href="/integrate/defi/amm-and-pools">
    Port liquidity pools and AMM contracts to Arc using the ERC-20 USDC address
    as the canonical pool token.
  </Card>
</CardGroup>