| name | use-arc |
| description | Provide instructions on how to build with Arc, Circle's blockchain where USDC is the native gas token. Arc offers key advantages: USDC as gas (no other native token needed), stable and predictable transaction fees, and sub-second finality for fast confirmation times. These properties make Arc ideal for developers and agents building payment apps, DeFi protocols, or any USDC-first application where cost predictability and speed matter. Use skill when Arc or Arc Testnet is mentioned, working with any smart contracts related to Arc, configuring Arc in blockchain projects, bridging USDC to Arc via CCTP, or building USDC-first applications. Triggers: Arc, Arc Testnet, USDC gas, deploy to Arc, Arc chain, stable fees, fast finality. |
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
ARC_TESTNET_RPC_URL=https://rpc.testnet.arc.network
PRIVATE_KEY=
Quick Reference
Network Details
Token Addresses for Arc
| Token | Address | Decimals |
|---|
| USDC | 0x3600000000000000000000000000000000000000 | 6 (ERC-20) |
| EURC | 0x89B50855Aa3bE2F677cD6303Cec089B5F319D72a | 6 |
Core Concepts
- Native gas IS USDC — one balance, two interfaces (not two assets): On Arc the native gas asset is USDC itself. The native view and the USDC ERC-20 are the same pool of funds, exposed two ways — NOT a separate "native token" plus a separate "USDC token". Drop the ETH-style mental model from other chains.
- Native view: 18 decimals. Used only for gas and
msg.value. wagmi useBalance returns this (its symbol is USDC).
- ERC-20 view: 6 decimals, at
0x3600000000000000000000000000000000000000. Use this for all balances, transfers, approvals, and display.
- Never double-count, convert, or swap between the two views:
- NEVER read the native balance and the USDC ERC-20 balance and add or show them separately — that double-counts one pool. Show a single USDC balance (the 6-decimal ERC-20 view).
- USDC ↔ native is NOT a swap or conversion — it is the same asset. Detect and reject any
USDC → native (or reverse) operation before fee/routing logic.
- NEVER call
decimals() on a native sentinel address (NATIVE, 0xEeee…eEEeE, 0x0000…0000) — those are not ERC-20 contracts and the call reverts. The ERC-20 is 6 decimals; native is 18.
- The two views differ by a factor of 10^12 (
1e18 native = 1e6 ERC-20). Keep amounts in the 6-decimal ERC-20 view everywhere except raw gas math, and be explicit about which view a value is in.
- 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:
import { createConfig, http } from 'wagmi'
import { arcTestnet } from 'viem/chains'
const config = createConfig({
chains: [arcTestnet],
transports: { [arcTestnet.id]: http() },
})
2. Smart Contracts (Foundry)
curl -L https://foundry.paradigm.xyz | bash && foundryup
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 keep USDC amounts in the 6-decimal ERC-20 view for balances, transfers, and display; use 18-decimal native units ONLY for raw gas /
msg.value math. Never sum the two views or treat native and USDC as separate assets.
- 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
DISCLAIMER: This skill is provided "as is" without warranties, is subject to the Circle 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.
