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How to Fix MetaMask Chain ID Errors

DodaTech Updated 2026-06-24 4 min read

In this tutorial, you'll learn about How to Fix MetaMask Chain ID Errors. We cover key concepts, practical examples, and best practices.

The Problem

Your dApp shows MetaMask: Invalid chain ID or The endpoint returned a different chain ID than the one configured. The network dropdown shows the wrong network, and transactions fail because they are sent to the wrong chain. Chain ID mismatches can cause lost funds.

Quick Fix

Fix 1: MetaMask on the Wrong Network

WRONG — deploying to Sepolia but MetaMask is on Ethereum mainnet:

# (transaction hangs or shows "network does not match")

RIGHT — programmatically switch the network:

// ethers.js v6:
async function ensureCorrectNetwork() {
    if (!window.ethereum) return;
    const provider = new ethers.BrowserProvider(window.ethereum);
    const network = await provider.getNetwork();
    const targetChainId = 11155111n;  // Sepolia

    if (network.chainId !== targetChainId) {
        try {
            await window.ethereum.request({
                method: "wallet_switchEthereumChain",
                params: [{ chainId: "0xaa36a7" }],  // Sepolia chain ID in hex
            });
        } catch (error) {
            if (error.code === 4902) {
                await window.ethereum.request({
                    method: "wallet_addEthereumChain",
                    params: [{
                        chainId: "0xaa36a7",
                        chainName: "Sepolia Testnet",
                        rpcUrls: ["https://sepolia.infura.io/v3/YOUR_KEY"],
                        nativeCurrency: { name: "ETH", symbol: "ETH", decimals: 18 },
                    }],
                });
            }
        }
    }
}

Fix 2: Wrong Chain ID in Hex Format

WRONG — decimal instead of hex:

await window.ethereum.request({
    method: "wallet_switchEthereumChain",
    params: [{ chainId: 11155111 }],  // WRONG — must be hex string
});
// MetaMask: Invalid chain ID (number)

RIGHT — use hex string format:

// Convert to hex:
const chainIdHex = "0x" + (11155111).toString(16);
console.log(chainIdHex);
// "0xaa36a7"

// Use directly:
params: [{ chainId: "0xaa36a7" }]

Fix 3: RPC Returns Wrong Chain ID

// MetaMask shows: "The endpoint returned a different chain ID"
// (the configured RPC URL returns a different chain than expected)

RIGHT — verify RPC endpoint chain ID:

curl -X POST https://sepolia.infura.io/v3/YOUR_KEY \
    -H "Content-Type: application/json" \
    -d '{"jsonrpc":"2.0","id":1,"method":"eth_chainId","params":[]}'
# {"jsonrpc":"2.0","id":1,"result":"0xaa36a7"}
# (should match 11155111 decimal)

Fix 4: Custom Network Configuration

WRONG — guessing the chain ID:

// User manually adds a network but enters wrong chain ID:
// Chain ID: 1 (Ethereum mainnet) — but RPC URL points to Sepolia
// (transactions fail: "nonce too low" or sender doesn't have enough funds)

RIGHT — use wallet_addEthereumChain for precise configuration:

await window.ethereum.request({
    method: "wallet_addEthereumChain",
    params: [{
        chainId: "0xaa36a7",
        chainName: "Sepolia Testnet",
        rpcUrls: ["https://rpc.sepolia.org"],
        nativeCurrency: { name: "ETH", symbol: "ETH", decimals: 18 },
        blockExplorerUrls: ["https://sepolia.etherscan.io"],
    }],
});

Fix 5: Chain ID Mismatch in Hardhat

// hardhat.config.js:
module.exports = {
    networks: {
        sepolia: {
            url: "https://sepolia.infura.io/v3/YOUR_KEY",
            chainId: 11155111,  // must match the actual chain ID
            accounts: [PRIVATE_KEY],
        },
    },
};

Fix 6: Common Chain ID Reference

Network Chain ID Hex
Ethereum Mainnet 1 0x1
Sepolia 11155111 0xaa36a7
Goerli 5 0x5
Polygon 137 0x89
BNB Smart Chain 56 0x38
Avalanche C-Chain 43114 0xa86a
Arbitrum One 42161 0xa4b1
Optimism 10 0xa

Use DodaTech's Chain Manager to configure, switch, and verify network connections in your dApp, preventing chain ID mismatches.

Prevention

  • Always use hex string format for chain IDs in MetaMask RPC.
  • Verify RPC endpoint returns the expected chain ID.
  • Handle chain switching failure (code 4902) gracefully.
  • Test network switching on multiple wallets.
  • Display the connected network name in the dApp UI.

Common Mistakes with chain id

  1. Using return to exit a function early instead of wrapping a pure value in the monad
  2. Mixing let bindings with <- bindings in do notation, producing type errors
  3. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors

These mistakes appear frequently in real-world METAMASK code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

### Why does MetaMask say "This chain ID is already in use"?

MetaMask allows only one network per chain ID. If you are trying to add a network with a chain ID that matches an existing network, it will skip the addition. Use wallet_switchEthereumChain instead.

Can a dApp force MetaMask to switch networks?

Yes, but the user must approve the switch via the MetaMask popup. The wallet_switchEthereumChain RPC triggers a confirmation dialog. If the user rejects it, handle the error with .catch() and show a manual switching guide.

What happens if I send a transaction on the wrong chain?

The transaction is broadcast to the network corresponding to the chain ID in the transaction. If you are on Ethereum mainnet but meant to use Sepolia, the transaction uses real ETH. Always verify the chain ID before sending value transactions.

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