use-arc

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_MAINNET_RPC_URL=https://rpc.mainnet.arc.io
ARC_TESTNET_RPC_URL=https://rpc.testnet.arc.io
PRIVATE_KEY=         # Deployer wallet private key

Quick Reference

Network Details

Token Addresses for Arc

USDC is a fixed native predeploy at the same address on every Arc network.

For other tokens, see the Arc contract addresses page.

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.
• Mainnet and testnet: Arc mainnet (chain ID 5042) and Arc testnet (chain ID 5042002) are both available — see Network Details for the config of each. Mainnet transactions move real USDC and are irreversible; use testnet for development and demos.
• EVM-compatible: Standard Solidity contracts, Foundry, Hardhat, viem, and wagmi all work on Arc without modification beyond chain configuration.

Implementation Patterns

READ references/deploying-on-arc.md for the runnable setup — wagmi chain config, Foundry deploy, Circle's pre-audited contract templates, and bridging USDC in.

1. Configure the chain — use the built-in viem chains (no custom definition needed): arcTestnet for testnet, arc for mainnet.
2. Deploy contracts — standard Foundry/Hardhat against ARC_TESTNET_RPC_URL (or ARC_MAINNET_RPC_URL), or Circle's Smart Contract Platform templates (ERC-20/721/1155/Airdrop).
3. Bridge USDC in — Arc's CCTP domain is 26; use the bridge-stablecoin skill for the full 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 mainnet (arc) and Arc Testnet (arcTestnet) both ship in Viem -- no custom chain definition is required.
• ALWAYS verify the user is on the intended Arc network (mainnet chain ID 5042 or testnet 5042002) before submitting transactions.
• ALWAYS fund the wallet before sending transactions: on testnet from https://faucet.circle.com; on mainnet with real USDC.
• 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.
• Arc mainnet moves real USDC and transactions are irreversible. Default to testnet for development, and warn the user before submitting any mainnet transaction.

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:

Reference Links

• Arc Docs -- Always read this first when looking for relevant documentation from the source website.
• Arc Explorer (Mainnet)
• Arc Explorer (Testnet)
• Circle Faucet
• Circle Developer Docs -- Always read this first when looking for relevant documentation from the source website.

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.*