function etch(address who, bytes calldata code) external;


Sets the bytecode of an address who to code.


bytes memory code = address(awesomeContract).code;
address targetAddr = makeAddr("target");
vm.etch(targetAddr, code);
log_bytes(address(targetAddr).code); // 0x6080604052348015610010...

Using vm.etch for enabling custom precompiles

Some chains, like Blast or Arbitrum, run with custom precompiles. Foundry is operating on vanilla EVM and is not aware of those. If you are encountering reverts due to not available precompile, you can use vm.etch cheatcode to inject mock of the missing precompile to the address it is expected to appear at.

pragma solidity 0.8.10;

import "forge-std/Test.sol";

// Firstly, we implement a mock emulating the actual precompile behavior
contract YieldMock {
    address private constant blastContract = 0x4300000000000000000000000000000000000002;

    mapping(address => uint8) public getConfiguration;

    function configure(address contractAddress, uint8 flags) external returns (uint256) {
        require(msg.sender == blastContract);

        getConfiguration[contractAddress] = flags;
        return 0;

    function claim(address, address, uint256) external pure returns (uint256) {
        return 0;

    function getClaimableAmount(address) external pure returns (uint256) {
        return 0;

contract SomeBlastTest is Test {
    function setUp() public {
        // Deploy mock of the precompile
        YieldMock yieldMock = new YieldMock();
        // Set mock bytecode to the expected precompile address
        vm.etch(0x0000000000000000000000000000000000000100, address(yieldMock).code);

    function testSomething() public {
        // Now we can interact with Blast contracts without reverts

Injecting mocks of precompiles might be tricky as such mocks will not fully emulate the actual precompile behavior on-chain.

Mock in the case above will not cause the actual yield to be accrued if any yield mode is configured.


Forge Standard Library