eth_chainId

eth_chainId returns the EIP-155 chain ID of the network the endpoint is serving — for OP Mainnet, the OP Stack rollup whose gas token is ETH, that value is 0xa (decimal 10). It is the canonical way to confirm a connection is pointed at OP Mainnet and not Ethereum L1 or another Superchain member before you sign anything. Call https://optimism.therpc.io/YOUR_API_KEY and you should always get "0xa" back.

Use cases

• Pre-flight check before signing or submitting: confirm the wallet reports 0xa so the user is actually on OP Mainnet and not another Superchain network.
• Pair with eth_blockNumber for a complete "alive and on chain 10" health probe of your endpoint.
• Pick the right contract address from a multi-chain deployment map — the OP Mainnet Velodrome address differs from its Base or Ethereum counterpart.
• Guard against cross-chain replay: EIP-155 binds the signature to chain ID 10, so a transaction signed for OP Mainnet cannot be replayed on Ethereum L1.

Parameters

Este método no acepta parámetros. Pasa un array vacío [].

Response

Example request

1curl https://optimism.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 Optimism playground.

Errors & troubleshooting

Common pitfalls

• Do not confuse this with net_version: on OP Mainnet both report 10, but net_version returns a decimal network ID and on some networks the two values diverge — use eth_chainId for EIP-155 logic.
• The result is the hex string "0xa", not the number 10 — compare with BigInt(result) === 10n rather than a raw string equality check to avoid "0xa" vs "0x0a" mismatches.

Supported networks

• Mainnet — Chain ID: 10
• Sepolia — Chain ID: 11155420

See also

• eth_blockNumber
• net_version

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