RPC method

eth_chainId on Polygon

Supported on SwiftNodes

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

Polygon 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 Polygon a legacy transaction signed without a chain ID (pre-EIP-155). Its nodes refused it: "only replay-protected (EIP-155) transactions allowed over RPC". Sign with chain ID 137; every mainstream library does this by default.

eth_chainId	0x89 (137)
net_version	137

Example request

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

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

const id = await client.getChainId(); // 137
if (id !== 137) throw new Error(`wrong chain: ${id}`);

// Add Polygon to a browser wallet
await window.ethereum.request({
  method: "wallet_addEthereumChain",
  params: [{
    chainId: "0x89",
    chainName: "Polygon",
    nativeCurrency: { name: "POL", symbol: "POL", decimals: 18 },
    rpcUrls: ["https://rpc.swiftnodes.io/rpc/polygon?key=YOUR_API_KEY"],
  }],
});

Other methods on Polygon

eth_chainId on other chains

Frequently asked questions

Does Polygon support eth_chainId?

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

What is the Polygon chain ID?

137 in decimal, 0x89 in hex, as returned by eth_chainId on 2026-10-13.

How do I add Polygon to MetaMask?

Call wallet_addEthereumChain with chainId 0x89, the native currency POL with 18 decimals, and your RPC URL; the example on this page has the full object.

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

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