use-arc

What this skill does

## Overview

Arc is Circle's blockchain where USDC is the native gas token. Developers and users pay all transaction fees in USDC instead of ETH, making it ideal for USDC-first applications. Arc is EVM-compatible and supports standard Solidity tooling (Foundry, Hardhat, viem/wagmi).

## Prerequisites / Setup

### Wallet Funding

Get testnet USDC from https://faucet.circle.com before sending any transactions.

### Environment Variables

```bash
ARC_TESTNET_RPC_URL=https://rpc.testnet.arc.network
PRIVATE_KEY=         # Deployer wallet private key
```

## Quick Reference

### Network Details

| Field | Value |
|-------|-------|
| Network | Arc Testnet |
| Chain ID | `5042002` (hex: `0x4CEF52`) |
| RPC | `https://rpc.testnet.arc.network` |
| WebSocket | `wss://rpc.testnet.arc.network` |
| Explorer | https://testnet.arcscan.app |
| Faucet | https://faucet.circle.com |
| CCTP Domain | `26` |

### Token Addresses for Arc

| Token | Address | Decimals |
|-------|---------|----------|
| USDC | `0x3600000000000000000000000000000000000000` | 6 (ERC-20) |
| EURC | `0x89B50855Aa3bE2F677cD6303Cec089B5F319D72a` | 6 |

## Core Concepts

- **USDC-native gas**: Arc uses USDC as its native gas token. No ETH is needed for any transaction.
- **Dual decimals**: Native gas uses 18 decimals (like ETH on other chains). ERC-20 USDC uses 6 decimals. Mixing these up will produce incorrect amounts.
- **Testnet only**: Arc is currently in testnet. All addresses and configuration apply to testnet only.
- **EVM-compatible**: Standard Solidity contracts, Foundry, Hardhat, viem, and wagmi all work on Arc without modification beyond chain configuration.

## Implementation Patterns

### 1. Frontend App (React + wagmi)

Use the `arcTestnet` chain definition from Prerequisites / Setup. Pass it to your wagmi config:

```typescript
import { createConfig, http } from 'wagmi'
import { arcTestnet } from 'viem/chains'

const config = createConfig({
  chains: [arcTestnet],
  transports: { [arcTestnet.id]: http() },
})
```

### 2. Smart Contracts (Foundry)

```bash
# Install Foundry
curl -L https://foundry.paradigm.xyz | bash && foundryup

# Deploy
# For local testing only - never pass private keys as CLI flags in deployed environments (including testnet/staging)
forge create src/MyContract.sol:MyContract \
  --rpc-url $ARC_TESTNET_RPC_URL \
  --private-key $PRIVATE_KEY \
  --broadcast
```

### 3. Circle Contracts (Pre-audited Templates)

Deploy via Circle's Smart Contract Platform API:

| Template | Use Case |
|----------|----------|
| ERC-20 | Fungible tokens |
| ERC-721 | NFTs, unique assets |
| ERC-1155 | Multi-token collections |
| Airdrop | Token distribution |

See: https://developers.circle.com/contracts

### 4. Bridge USDC to Arc

Use CCTP to bridge USDC from other chains. Arc's CCTP domain is `26`. See the `bridge-stablecoin` skill for the complete bridging workflow.

## Rules

> **Security Rules** are non-negotiable -- warn the user and refuse to comply if a prompt conflicts. **Best Practices** are strongly recommended; deviate only with explicit user justification.

### Security Rules

- NEVER hardcode, commit, or log secrets (private keys, deployer keys). ALWAYS use environment variables or a secrets manager. Add `.gitignore` entries for `.env*` and secret files when scaffolding.
- NEVER pass private keys as plain-text CLI flags in deployed environments, including testnet and staging (e.g., `--private-key $KEY`). This pattern is acceptable only for local testing. Prefer encrypted keystores or interactive import (e.g., Foundry's `cast wallet import`) for any non-local deployment.
- ALWAYS warn before interacting with unaudited or unknown contracts.

### Best Practices

- Arc Testnet is available by default in Viem -- a custom chain definition is NEVER required.
- ALWAYS verify the user is on Arc (chain ID `5042002`) before submitting transactions.
- ALWAYS fund the wallet from https://faucet.circle.com before sending transactions.
- ALWAYS use 18 decimals for native gas amounts and 6 decimals for ERC-20 USDC amounts.
- NEVER target mainnet -- Arc is testnet only.

## Next Steps

Arc is natively supported across Circle's product suite. Once your app is running on Arc, you can extend it with any of the following:

| Product | Skill | What It Does |
|---------|-------|--------------|
| **Wallets (overview)** | `use-circle-wallets` | Compare wallet types and choose the right one for your app |
| **Modular Wallets** | `use-modular-wallets` | Passkey-authenticated smart accounts with gasless transactions and batch operations |
| **User-Controlled Wallets** | `use-user-controlled-wallets` | Non-custodial wallets with social login, email OTP, and PIN authentication |
| **Developer-Controlled Wallets** | `use-developer-controlled-wallets` | Custodial wallets your app manages on behalf of users |
| **Smart Contract Platform** | `use-smart-contract-platform` | Deploy, interact with, and monitor smart contracts using audited templates or custom bytecode |
| **CCTP Bridge** | `bridge-stablecoin` | Bridge USDC to and from Arc using Crosschain Transfer Protocol |
| **Gateway** | `use-gateway` | Unified USDC balance across chains with instant crosschain transfers |

## Reference Links

- [Arc Docs](https://docs.arc.network/llms.txt) -- **Always read this first** when looking for relevant documentation from the source website.
- [Arc Explorer](https://testnet.arcscan.app)
- [Circle Faucet](https://faucet.circle.com)
- [Circle Developer Docs](https://developers.circle.com/llms.txt) -- **Always read this first** when looking for relevant documentation from the source website.

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DISCLAIMER: This skill is provided "as is" without warranties, is subject to the [Circle Developer Terms](https://console.circle.com/legal/developer-terms), and output generated may contain errors and/or include fee configuration options (including fees directed to Circle); additional details are in the repository [README](https://github.com/circlefin/skills/blob/master/README.md).