RPC method

eth_chainId on Monad

Supported on SwiftNodes

Everything below was measured on 2026-10-13 by calling eth_chainId on Monad 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.

Monad 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 Monad a legacy transaction signed without a chain ID (pre-EIP-155). Its nodes refused it: "Signer had insufficient balance". Sign with chain ID 143; every mainstream library does this by default.

eth_chainId	0x8f (143)
net_version	143

Example request

curl -s https://rpc.swiftnodes.io/rpc/monad?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 Monad before signing
import { createPublicClient, http } from "viem";

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

const id = await client.getChainId(); // 143
if (id !== 143) throw new Error(`wrong chain: ${id}`);

// Add Monad to a browser wallet
await window.ethereum.request({
  method: "wallet_addEthereumChain",
  params: [{
    chainId: "0x8f",
    chainName: "Monad",
    nativeCurrency: { name: "MON", symbol: "MON", decimals: 18 },
    rpcUrls: ["https://rpc.swiftnodes.io/rpc/monad?key=YOUR_API_KEY"],
  }],
});

Other methods on Monad

eth_chainId on other chains

Frequently asked questions

Does Monad support eth_chainId?

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

What is the Monad chain ID?

143 in decimal, 0x8f in hex, as returned by eth_chainId on 2026-10-13.

How do I add Monad to MetaMask?

Call wallet_addEthereumChain with chainId 0x8f, the native currency MON with 18 decimals, and your RPC URL; the example on this page has the full object.

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

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