eth_chainId | Optimism RPC Docs

Get the chain ID for Optimism. Essential for transaction signing and network verification.

Returns the chain ID used for transaction signing on Optimism.

Why Optimism? Build on the heart of the Superchain powering 60%+ of Ethereum L2 transactions with $6B+ TVL with OP Stack modularity, Superchain interoperability, 17M+ daily transactions across 33 chains, and 20% of tokens for public goods.

When to Use This Method

eth_chainId is essential for L2 builders, DeFi developers, and teams launching Superchain-compatible apps:

EIP-155 Transaction Signing -- Include the chain ID in transaction signatures to prevent replay attacks across different networks
Multi-Chain Application Routing -- Detect which network the RPC endpoint serves and configure application logic accordingly
Wallet Integration -- Verify users are connected to the expected network before prompting transaction approval
Cross-Chain Security -- Validate chain identity before bridging assets or relaying messages on cross-chain DeFi, Superchain-interoperable dApps, and public goods funding recipients

Common Use Cases

1. Multi-Network Connection Guard

Reject connections to unexpected networks before any transaction is sent:

JavaScript

import { JsonRpcProvider, BrowserProvider } from 'ethers';

async function guardNetwork(provider, allowedChainIds) {
  const network = await provider.getNetwork();
  const chainId = Number(network.chainId);
  
  if (!allowedChainIds.includes(chainId)) {
    throw new Error(
      `Wrong network: connected to chain ${chainId}, expected one of [${allowedChainIds}]`
    );
  }
  
  console.log(`Connected to chain ${chainId}`);
  return chainId;
}

// Example: only allow Ethereum mainnet (1) and Arbitrum (42161)
await guardNetwork(provider, [1, 42161]);

2. Wallet Network Switcher

Detect the current chain and prompt wallet to switch if needed:

JavaScript

async function ensureCorrectChain(walletProvider, targetChainId, chainParams) {
  const currentChainId = await walletProvider.send('eth_chainId', []);
  const currentId = parseInt(currentChainId, 16);
  
  if (currentId !== targetChainId) {
    try {
      await walletProvider.send('wallet_switchEthereumChain', [
        { chainId: '0x' + targetChainId.toString(16) }
      ]);
    } catch (switchError) {
      // Chain not added to wallet -- add it
      if (switchError.code === 4902) {
        await walletProvider.send('wallet_addEthereumChain', [chainParams]);
      } else {
        throw switchError;
      }
    }
    
    console.log(`Switched to chain ${targetChainId}`);
  } else {
    console.log(`Already on chain ${targetChainId}`);
  }
  
  return targetChainId;
}

3. Chain-Aware Configuration Loader

Dynamically load chain-specific contract addresses and settings:

Python

from web3 import Web3

def load_chain_config(rpc_url):
    w3 = Web3(Web3.HTTPProvider(rpc_url))
    chain_id = w3.eth.chain_id
    
    configs = {
        1: {
            'name': 'Ethereum Mainnet',
            'uniswap_router': '0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D',
            'explorer': 'https://etherscan.io',
            'native_symbol': 'ETH'
        },
        137: {
            'name': 'Polygon',
            'uniswap_router': '0xa5E0829CaCEd8fFDD4De3c43696c57F7D7A678ff',
            'explorer': 'https://polygonscan.com',
            'native_symbol': 'MATIC'
        },
        42161: {
            'name': 'Arbitrum One',
            'uniswap_router': '0xE592427A0AEce92De3Edee1F18E0157C05861564',
            'explorer': 'https://arbiscan.io',
            'native_symbol': 'ETH'
        }
    }
    
    if chain_id not in configs:
        raise ValueError(f'Unsupported chain ID: {chain_id}')
    
    config = configs[chain_id]
    print(f'Loaded config for {config["name"]} (chain {chain_id})')
    return {'chain_id': chain_id, **config}

Best Practices

Use eth_chainId (not net_version) for EIP-155 transaction signing -- chain ID is the canonical signing value
Cache the chain ID at application startup -- it does not change during a session
For browser wallet dApps, use wallet_switchEthereumChain and wallet_addEthereumChain to guide users to the correct network
Some L2 chains share the same chain ID as their L1 -- always combine chain ID checks with additional endpoint verification
Return value is a hex-encoded integer -- parse with parseInt(result, 16) before using

Code Examples

net_version - Get network version
eth_syncing - Check sync status

eth_chainId - Optimism RPC Method