eth_chainId

eth_chainId returns the EIP-155 chain ID of the network behind the endpoint. For Arbitrum One — the ETH-gas Optimistic Rollup — that value is 0xa4b1, which is 42161 in decimal. Call https://arbitrum.therpc.io/YOUR_API_KEY and you should always get this exact value back; it is the canonical way to confirm a client is talking to Arbitrum One and not Ethereum L1 or another rollup before it signs anything.

Use cases

• Pre-condition check before signing or submitting a transaction — confirm the wallet is on Arbitrum One (42161) and not on Ethereum L1 or Arbitrum Sepolia.
• Combine with eth_blockNumber for a full "alive and on the right chain" probe of the endpoint.
• Select the correct contract address from a multi-chain deployment — e.g. the Arbitrum One GMX or Aave address versus the mainnet one.
• Guard against cross-chain replay attacks by baking the 42161 chain ID into the EIP-155 signature.

Parameters

Cette méthode ne prend aucun paramètre. Passez un tableau vide [].

Response

Example request

1curl https://arbitrum.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 Arbitrum One playground.

Errors & troubleshooting

Common pitfalls

• Do not confuse this with net_version — that returns a network ID string and can differ from the EIP-155 chain ID; always sign with the value from eth_chainId.
• The result is the hex string "0xa4b1", not the number 42161 — compare with BigInt(result) === 42161n rather than a raw string match to avoid case or zero-padding bugs.

Supported networks

• Mainnet — Chain ID: 42161
• Sepolia — Chain ID: 421614

See also

• eth_blockNumber
• net_version

1curl https://arbitrum.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":[]}'