RPC method

eth_chainId on Optimism

Supported on SwiftNodes

Everything below was measured on 2026-10-13 by calling eth_chainId on Optimism through the same endpoint customers use, and is re-measured weekly.

What eth_chainId does

Returns the chain ID (EIP-155) as a hex string. Wallets and libraries check it before signing, and the signature commits to it, so a transaction signed for one chain cannot be replayed on another.

Optimism chain ID, as measured

net_version returns the same number as decimal text. Older tools read net_version; use eth_chainId, which is the value signatures commit to.

We sent Optimism a legacy transaction signed without a chain ID (pre-EIP-155). Its nodes refused it: "invalid sender". Sign with chain ID 10; every mainstream library does this by default.

eth_chainId	0xa (10)
net_version	10

Example request

curl -s https://rpc.swiftnodes.io/rpc/optimism?key=YOUR_API_KEY \
  -X POST -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"eth_chainId","params":[]}'

// Check you are on Optimism before signing
import { createPublicClient, http } from "viem";

const client = createPublicClient({
  transport: http("https://rpc.swiftnodes.io/rpc/optimism?key=YOUR_API_KEY"),
});

const id = await client.getChainId(); // 10
if (id !== 10) throw new Error(`wrong chain: ${id}`);

// Add Optimism to a browser wallet
await window.ethereum.request({
  method: "wallet_addEthereumChain",
  params: [{
    chainId: "0xa",
    chainName: "Optimism",
    nativeCurrency: { name: "ETH", symbol: "ETH", decimals: 18 },
    rpcUrls: ["https://rpc.swiftnodes.io/rpc/optimism?key=YOUR_API_KEY"],
  }],
});

Other methods on Optimism

eth_chainId on other chains

Frequently asked questions

Does Optimism support eth_chainId?

Yes. eth_chainId answered on the SwiftNodes Optimism endpoint when our weekly probe called it through the customer path (2026-10-13).

What is the Optimism chain ID?

10 in decimal, 0xa in hex, as returned by eth_chainId on 2026-10-13.

How do I add Optimism to MetaMask?

Call wallet_addEthereumChain with chainId 0xa, the native currency ETH with 18 decimals, and your RPC URL; the example on this page has the full object.

Optimism endpoints, live block height and WebSocket status are on the Optimism RPC page.

One key covers every chain we serve, and SwiftNodes has a free tier.