eth_chainId

eth_chainId reports the EIP-155 chain identifier that the endpoint is serving. For Ethereum mainnet — the ETH network — that value is 0x1, decimal 1. The identifier is baked into every signed transaction so a signature valid on one network can't be replayed on another; checking it before you sign is how a wallet proves it's talking to the right chain rather than Sepolia (0xaa36a7) or Holesky. Call https://ethereum.therpc.io/YOUR_API_KEY with no parameters and you'll get 0x1 back from any healthy mainnet node.

Use cases

Network guard before signing. Confirm the result is 0x1 before submitting, so a user who switched their wallet to a testnet doesn't accidentally fire a mainnet-intended transaction into the void.
Health + identity probe. Pair it with eth_blockNumber for a two-call check that the endpoint is both alive and serving chain 1, not some fork.
Pick the right deployment. In a multi-chain app, map the returned ID to the correct contract addresses — your USDC or router on mainnet differs from the one on every testnet.
Replay protection. The chain ID is the EIP-155 ingredient that stops a mainnet-signed transaction from being rebroadcast on another EVM chain; reading it lets you build that check in explicitly.

Parameters

This method takes no parameters. Pass an empty array [].

Response

Type	Description
string (hex)	Hex-encoded chain ID integer (e.g. "0x1" for Ethereum mainnet).

Example request

1curl https://ethereum.therpc.io/YOUR_API_KEY \
2  -X POST \
3  -H "Content-Type: application/json" \
4  -d '{
5    "jsonrpc": "2.0",
6    "method": "eth_chainId",
7    "params": [],
8    "id": 1
9  }'

Try it live in the Ethereum playground.

Errors & troubleshooting

Code	Message	Cause
`-32603`	`Internal error`	Node failed to retrieve chain configuration.

Common pitfalls

eth_chainId and net_version are not interchangeable. net_version returns the older network ID as a decimal string; on Ethereum mainnet both happen to be 1, but the chain ID is what EIP-155 signing actually uses, so always sign against this method's value.
The response is a hex string ("0x1"), not a number. Comparing the raw string against "1" or 1 will silently fail — normalize with BigInt(result) or parseInt(result, 16) first.

Supported networks

Mainnet — Chain ID: 1
Sepolia — Chain ID: 11155111