[Arc Testnet quirk] wallet_switchEthereumChain returns success without actually switching

[osr21]

Symptom

On Arc Testnet, calling wallet_switchEthereumChain sometimes returns a resolved promise (no error thrown) while the wallet stays on the previous chain. Standard code that trusts the promise resolution will silently send transactions to the wrong network.

Affected versions

• MetaMask (all recent versions tested against Arc Testnet chain ID 5042002)
• Likely affects all wallets that pass through Arc's custom RPC

Reproduction

await window.ethereum.request({
  method: 'wallet_switchEthereumChain',
  params: [{ chainId: '0x4CEF52' }], // Arc Testnet
});
// ⚠️  Promise resolved — but current chain may NOT be Arc yet
const chainId = await window.ethereum.request({ method: 'eth_chainId' });
console.log(chainId); // May still be the old chain!

Workaround

Always verify the active chain ID after the call. If the chain didn't switch, fall back to wallet_addEthereumChain (which adds and switches atomically):

await window.ethereum.request({ method: 'wallet_switchEthereumChain', params: [{ chainId }] });
const actual = parseInt(await window.ethereum.request({ method: 'eth_chainId' }), 16);
if (actual !== expectedChainId) {
  // Fall through to wallet_addEthereumChain
  await window.ethereum.request({ method: 'wallet_addEthereumChain', params: [chainConfig] });
}

References

• Tracked in arc-node#89
• Fixed in this repo: src/lib/bridge.ts → switchToChain()

[osr21]

Connection to issue #6

This is the root cause of #6. The wallet_switchEthereumChain false-resolve creates the race window where MetaMask's RPC router hasn't switched yet.

In wagmi v3 apps, the mitigation is:

1. After switchChainAsync resolves, add a ~400ms delay before the first transaction call.
2. Pass chain explicitly to writeContractAsync — wagmi v3 will validate via connector.getChainId() before dispatching and throw ChainMismatchError if still mismatched, giving you a clean error rather than a silent wrong-chain submission.

For non-wagmi apps (raw ethers.js/viem), the getSignerOnChain() retry loop in src/lib/bridge.ts is the right approach — it uses the RPC transport directly (provider.getNetwork()) rather than relying on the wallet's cached state.

This behaviour appears to be a MetaMask-specific issue; Coinbase Wallet handles wallet_switchEthereumChain more synchronously and hasn't exhibited the same race.