internal/ethapi: add note about eth_chainId compatibility with EIP-69…

…5 (geth #25168)

rpc/namespaces/ethereum/eth/api.go

@@ -328,6 +328,11 @@ func (e *PublicAPI) MaxPriorityFeePerGas() (*hexutil.Big, error) {
 }
 
 // ChainId is the EIP-155 replay-protection chain id for the current ethereum chain config.
+//
+// Note, this method does not conform to EIP-695 because the configured chain ID is always
+// returned, regardless of the current head block. We used to return an error when the chain
+// wasn't synced up to a block where EIP-155 is enabled, but this behavior caused issues
+// in CL clients.
 func (e *PublicAPI) ChainId() *hexutil.Big { //nolint
 	e.logger.Debug("eth_chainId")
 	return e.backend.ChainID()