fork-testing

Fork Testing Patterns

Overview

Fork testing runs your tests against a snapshot of a live blockchain. This lets you test interactions with real deployed contracts (Aave, Uniswap, Chainlink) without deploying mocks.

Basic Fork Setup

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import {Test, console2} from "forge-std/Test.sol";
import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";

contract ForkTest is Test {
    uint256 mainnetFork;

    // Mainnet addresses
    address constant USDC = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;
    address constant WETH = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
    address constant AAVE_POOL = 0x87870Bca3F3fD6335C3F4ce8392D69350B4fA4E2;

    function setUp() public {
        // Pin to a specific block for reproducibility
        mainnetFork = vm.createFork(vm.envString("ETH_RPC_URL"), 19_500_000);
        vm.selectFork(mainnetFork);
    }

    function test_forkIsActive() public view {
        assertEq(vm.activeFork(), mainnetFork);
        assertEq(block.number, 19_500_000);
    }
}

RPC Configuration

In foundry.toml:

[rpc_endpoints]
mainnet = "${ETH_RPC_URL}"
arbitrum = "${ARBITRUM_RPC_URL}"
optimism = "${OPTIMISM_RPC_URL}"
base = "${BASE_RPC_URL}"

Usage: vm.createFork("mainnet", blockNumber)

Impersonating Accounts

// Impersonate a whale to get tokens
address USDC_WHALE = 0x47ac0Fb4F2D84898e4D9E7b4DaB3C24507a6D503; // Binance

function test_impersonateWhale() public {
    vm.prank(USDC_WHALE);
    IERC20(USDC).transfer(address(this), 1_000_000e6);
    assertEq(IERC20(USDC).balanceOf(address(this)), 1_000_000e6);
}

Dealing Tokens

// deal() writes directly to storage — works for standard ERC20s
function test_dealTokens() public {
    deal(USDC, address(this), 1_000_000e6);
    assertEq(IERC20(USDC).balanceOf(address(this)), 1_000_000e6);

    // For tokens with non-standard storage (e.g., USDT):
    deal(USDC, address(this), 1_000_000e6, true); // true = adjust totalSupply
}

// For complex tokens where deal() fails, impersonate a holder:
function _getTokens(address token, uint256 amount, address to) internal {
    address whale = _findWhale(token);
    vm.prank(whale);
    IERC20(token).transfer(to, amount);
}