Arc Testnet chain definition declares 6 native decimals (should be 18) and points at a dead explorer

[JspIIV]

Two problems in the hand-rolled Arc Testnet chain definition in lib/circle/gateway-sdk.ts:

export const arcTestnet = {
  id: 5042002,
  name: 'Arc Testnet',
  nativeCurrency: { name: 'USD Coin', symbol: 'USDC', decimals: 6 },
  rpcUrls: { default: { http: [arcRpcUrl] } },
  blockExplorers: {
    default: { name: 'Explorer', url: 'https://explorer.arc.testnet.circle.com' },
  },
  testnet: true,
} as const satisfies Chain;

1. nativeCurrency.decimals should be 18, not 6

Arc's native USDC balance is 18-decimal. The 6-decimal representation is the ERC-20 interface at 0x3600...0000, which is a separate view of the same balance.

Arc's own docs are explicit about this, in Two interfaces, one balance:

Interface	Decimals	Purpose
Native	18	Gas accounting, native sends, and msg.value
ERC-20	6	application-level transfers, approvals, and allowances

Everything else agrees on 18:

• viem's built-in arcTestnet (and arc) both declare decimals: 18
• circlefin/arc-defi-lend-borrow, lib/wagmi.ts: decimals: 18
• circlefin/arc-prediction-markets, lib/chain.ts: decimals: 18
• circlefin/arc-p2p-payments, components/web3-provider.tsx: decimals: 18

I checked on testnet as well: eth_getBalance and the precompile's balanceOf differ by exactly 1e12 across every funded account I sampled, which is only consistent with 18 native / 6 ERC-20.

Impact today is limited, but the value is still wrong. Inside this repo arcTestnet is only used for read clients (earn-ops.ts) and amounts are formatted with an explicit USDC_DECIMALS, so nothing currently renders incorrectly. But the object is exported, and nativeCurrency.decimals is what viem and wagmi use for native-balance formatting, and what gets forwarded to wallet_addEthereumChain. Anyone reusing this chain object in a browser wallet flow would add Arc to MetaMask with 6 decimals and see native balances 10^12 too large.

Since viem now ships arcTestnet, the simplest fix may be to import it and override only the RPC URL, rather than maintaining a local copy.

2. The block explorer URL no longer resolves

https://explorer.arc.testnet.circle.com does not resolve for me. Repeated requests fail at DNS, and I could not find an A record for the host.

The current explorer is https://testnet.arcscan.app, which is what Arc's docs and viem's own arcTestnet definition both point at.

---

Happy to send a PR for both if useful. It is a two-line change.