Arbitrum One

Arbitrum One

eth_chainId

eth_chainId returns the EIP-155 chain ID of the network the endpoint is serving. It's the canonical "which chain am I on?" check — wallets, signers, and bridge contracts all rely on it to prevent replay attacks across chains.

Why it matters

EIP-155 mixes the chain ID into transaction signatures. Two chains with the same nonce + same sender address are forced to produce different signatures, so a transaction signed for Ethereum mainnet (chain ID 1) is invalid on Polygon (chain ID 137). Treat eth_chainId as a precondition for any write path:

1const id = await provider.send('eth_chainId', []);
2if (BigInt(id) !== 1n) throw new Error('Expected Ethereum mainnet');

Parameters

None.

Response shape

A hex-encoded integer:

1{ "jsonrpc": "2.0", "id": 1, "result": "0x1" }

0x1 = Ethereum mainnet · 0x38 = BSC · 0x89 = Polygon · 0xa4b1 = Arbitrum One · 0x2105 = Base.

See also

• net_version — older, less-reliable alternative. Some chains return the chain ID; others return a network ID. Prefer eth_chainId.
• eth_blockNumber — combine the two for a full "alive + on the right chain" probe.

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