bug: wallet_switchEthereumChain fails silently for Arc Testnet in MetaMask — wallet_addEthereumChain must be used unconditionally

[osr21]

Summary

In MetaMask, calling wallet_switchEthereumChain for Arc Testnet (Chain ID 0x4CEF52 / 5042002) fails silently or throws an unhandled error — even when the network was previously added. The only reliable method is wallet_addEthereumChain, which both adds the network if missing and switches to it if already present.

Steps to reproduce

1. Add Arc Testnet to MetaMask manually or via a previous wallet_addEthereumChain call
2. Switch to a different network (e.g. Ethereum Mainnet)
3. Call:

   await window.ethereum.request({
     method: "wallet_switchEthereumChain",
     params: [{ chainId: "0x4CEF52" }],
   });

4. MetaMask either throws 4902 (chain not found) or silently does nothing — the switch does not occur

Workaround

Always use wallet_addEthereumChain regardless of whether the user has already added Arc Testnet:

await window.ethereum.request({
  method: "wallet_addEthereumChain",
  params: [{
    chainId: "0x4CEF52",
    chainName: "Arc Testnet",
    nativeCurrency: { name: "ETH", symbol: "ETH", decimals: 18 },
    rpcUrls: ["https://rpc.testnet.arc.network"],
    blockExplorerUrls: ["https://testnet.arcscan.app"],
  }],
});

This works reliably as both an add and a switch operation.

Environment

• MetaMask browser extension (latest)
• Arc Testnet Chain ID: 5042002 (0x4CEF52)
• Observed in Chrome and Brave browsers, May 2025

Impact

All Arc Testnet DApp onboarding flows that use wallet_switchEthereumChain will silently fail to switch the user to the correct network. This is a significant UX issue for new users.

Suggested fix / docs update

Document that wallet_addEthereumChain is the correct and only reliable method for switching users to Arc Testnet, and remove any references to wallet_switchEthereumChain from Arc developer guides.

[osr21]

Confirmed + implementation note for DApp onboarding flows

The wallet_addEthereumChain workaround works reliably. One additional note: the call
silently succeeds even if the user is already on Arc Testnet, so it is safe to call
unconditionally at the start of any DApp session.

Pattern for DApp wallet connection:

async function ensureArcTestnet() {
  // Works as both "add if missing" and "switch if already added"
  // Safe to call even when the user is already on Arc Testnet
  await window.ethereum.request({
    method: "wallet_addEthereumChain",
    params: [{
      chainId: "0x4CEF52",             // 5042002
      chainName: "Arc Testnet",
      nativeCurrency: { name: "ETH", symbol: "ETH", decimals: 18 },
      rpcUrls: ["https://rpc.testnet.arc.network"],
      blockExplorerUrls: ["https://testnet.arcscan.app"],
    }],
  });
}

// In your wallet connect handler:
async function connectWallet() {
  await window.ethereum.request({ method: "eth_requestAccounts" });
  await ensureArcTestnet();
  // At this point the wallet is on Arc Testnet — safe to proceed
}

Why the native currency symbol is "ETH": Arc uses USDC as its gas token, but MetaMask
requires nativeCurrency to be formatted with decimals: 18. MetaMask will show gas
costs as "ETH" in the UI — this is a wallet display limitation and does not affect transaction
execution.