[Question] Invalid chainId on mobile with correct chain connected (MetaMask, BSC Testnet)

[Jenny1610]

Hi Viem team 👋,
I'm encountering a confusing issue using [email protected] and [email protected] when calling writeContract on mobile devices (MetaMask via WalletConnect).

✅ Context:
I'm working on a dApp that supports multiple chains (BSC, Polygon, Base, etc.).
The user connects via MetaMask mobile (WalletConnect).
Chain bscTestnet (chainId = 97) is already added and active in MetaMask.
I'm using getWalletClient(...) from wagmi/actions to confirm chain is correct.
The full flow works fine on desktop MetaMask, but fails on mobile.

📋 Code Summary:

const client = await getWalletClient(wagmiConfig, { chainId });
toastInfo(wallet client id: ${client?.chain?.id}); // Logs: 97

await writeContract({
abi: [...],
address: tokenAddress,
functionName: 'transfer',
args: [receiver, amount],
chainId: 97, // Also tried without this param, still fails
});

🔥 Error on mobile:
Details: Invalid chainId
Version: [email protected]

Even though:
chainId is 97
wagmiConfig.chains includes bscTestnet
getWalletClient() returns chainId 97
MetaMask is visibly on BSC Testnet

🧪 Tried:
Ensuring chainId is number
Switching chain via switchChainAsync(...)
Confirming bscTestnet is part of both Viem + Wagmi config
Logging chainId at every step (it's always 97)
Forcing chainId into writeContract(...)
Still get the same Invalid chainId error only on mobile MetaMask (WalletConnect).

❓Questions:

1. Is this a known limitation with MetaMask mobile / WalletConnect and Viem?
2. Are there extra steps to ensure WalletClient is ready for writeContract()?
3. Is this a timing issue between chain switching and transaction sending?

P/S: i tried the newest viem version (2.31.7) but it didn't work and has the same issue