EXTCODESIZE (0x3b)

address (uint160 as uint256)

codeSize (uint256, in bytes)

import { extcodesize } from '@tevm/voltaire/evm/context';

const contractAddr = Address('0x...');
const frame = createFrame({ stack: [BigInt(contractAddr)] });

const err = extcodesize(frame, host);

console.log(frame.stack[0]); // Code size in bytes

function isContract(address account) public view returns (bool) {
    uint256 size;
    assembly {
        size := extcodesize(account)
    }
    return size > 0;
}

// INSUFFICIENT: Can be bypassed during construction
modifier onlyEOA() {
    uint256 size;
    assembly { size := extcodesize(caller()) }
    require(size == 0, "Contracts not allowed");
    _;
}

// Attack: Call from constructor where extcodesize returns 0

function verifyImplementation(address impl) internal view {
    uint256 size;
    assembly { size := extcodesize(impl) }
    require(size > 0, "Implementation must be contract");
}

contract Vulnerable {
    modifier noContracts() {
        uint256 size;
        assembly { size := extcodesize(caller()) }
        require(size == 0);
        _;
    }

    function restricted() public noContracts {
        // Attacker can call from constructor!
    }
}

contract Attacker {
    constructor(Vulnerable v) {
        v.restricted(); // extcodesize(this) == 0 in constructor!
    }
}

modifier onlyEOA() {
    require(msg.sender == tx.origin, "Only EOA");
    _;
}