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

[Jenny1610]

Hi Viem team 👋,
I'm encountering a confusing issue using viem@2.21.31 and wagmi@2.x 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: viem@2.21.31

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

[christianalberto]

Hello @Jenny1610, this is a known behavior when interacting with MetaMask Mobile via WalletConnect (especially on non-Ethereum mainnet chains like BSC Testnet).

---

Root Cause

The Invalid chainId error on WalletConnect mobile sessions usually stems from one of two scenarios:

1. WalletConnect Session Mismatch: MetaMask Mobile maintains an internal active chain ID for the WalletConnect session. Even if the wallet UI displays BSC Testnet, the underlying WalletConnect provider session object might still be bound to a different chain ID (or default 1) if the session wasn't explicitly switched using WalletConnect's provider namespace.
2. writeContract Client Resolution: When writeContract is called from Wagmi, it resolves the underlying connector client. If writeContract receives an explicit chainId parameter but the WalletConnect provider session handshake reports a mismatch, Viem throws an InvalidChainIdError pre-flight check before broadcasting the payload to the RPC.

---

Recommended Solutions

1. Explicitly Await switchChain via Connector

Before triggering writeContract, explicitly ensure the WalletConnect connector session completes the chain switch event:

import { useSwitchChain, useWriteContract } from 'wagmi';

const { switchChainAsync } = useSwitchChain();
const { writeContractAsync } = useWriteContract();

async function handleTransaction() {
  // 1. Force explicit session-level chain switch on WalletConnect
  await switchChainAsync({ chainId: 97 });

  // 2. Small async delay (500ms) to allow Mobile MetaMask RPC provider to update session state
  await new Promise((resolve) => setTimeout(resolve, 500));

  // 3. Omit explicit chainId in writeContract to let it infer from the switched connector
  await writeContractAsync({
    abi: [...],
    address: tokenAddress,
    functionName: 'transfer',
    args: [receiver, amount],
  });
}

2. Specify RPC URLs in wagmiConfig
   MetaMask Mobile requires explicit RPC URL mappings for testnets during WalletConnect session initiation:

import { http, createConfig } from 'wagmi';
import { bscTestnet } from 'wagmi/chains';

export const config = createConfig({
  chains: [bscTestnet],
  transports: {
    [bscTestnet.id]: http('[https://data-seed-prebsc-1-s1.binance.org:8545/](https://data-seed-prebsc-1-s1.binance.org:8545/)'),
  },
});

3. Bypass Session Check using getWalletClient directly
   If writeContract continues to validate against stale session metadata, invoke writeContract directly on the walletClient instance obtained via getWalletClient:

import { getWalletClient } from '@wagmi/core';

const walletClient = await getWalletClient(wagmiConfig, { chainId: 97 });

const hash = await walletClient.writeContract({
  abi: [...],
  address: tokenAddress,
  functionName: 'transfer',
  args: [receiver, amount],
  chain: bscTestnet, // Explicitly bind Viem chain object
});

If this resolves the chain ID synchronization issue on mobile, please consider marking this response as the accepted answer! 🟢

By the way, if you work with EVM wallet integrations, HD key derivations, or balance tracking across multiple chains, check out my open-source tool Wallet Hunter — if you find it helpful, dropping a ⭐️ star on the repo is greatly appreciated!