Fork testing (mainnet fork) in Blockchain / Solidity

Introduction

Fork testing lets you try out changes on a copy of the real blockchain. This helps you check if your code works without risking real money.

You want to test a smart contract upgrade safely before using it on the real network.

You need to debug a transaction that failed on the main blockchain.

You want to simulate how your decentralized app behaves with real blockchain data.

You are developing a new feature and want to see its effect on current blockchain state.

You want to test interactions with existing contracts without spending real tokens.

Syntax

Blockchain / Solidity

npx hardhat node --fork <MAINNET_RPC_URL> --fork-block-number <BLOCK_NUMBER>

Replace <MAINNET_RPC_URL> with your Ethereum mainnet provider URL.

The --fork-block-number option lets you pick a specific block to fork from.

Examples

This starts a local blockchain forked from the latest mainnet block.

Blockchain / Solidity

npx hardhat node --fork https://mainnet.infura.io/v3/YOUR_INFURA_KEY

This forks the mainnet at block number 17,000,000 for consistent testing.

Blockchain / Solidity

npx hardhat node --fork https://eth-mainnet.alchemyapi.io/v2/YOUR_ALCHEMY_KEY --fork-block-number 17000000

Sample Program

This script runs on a forked mainnet. It shows your account address, balance, and reads the name of the DAI token from the real contract.

Blockchain / Solidity

import { ethers } from "hardhat";

async function main() {
  const [deployer] = await ethers.getSigners();
  console.log("Using account:", deployer.address);

  // Check balance on forked mainnet
  const balance = await deployer.getBalance();
  console.log("Balance:", ethers.utils.formatEther(balance), "ETH");

  // Example: Interact with an existing contract on fork
  const daiAddress = "0x6B175474E89094C44Da98b954EedeAC495271d0F"; // DAI token
  const daiAbi = ["function name() view returns (string)"];
  const dai = new ethers.Contract(daiAddress, daiAbi, deployer);
  const name = await dai.name();
  console.log("DAI token name:", name);
}

main().catch((error) => {
  console.error(error);
  process.exitCode = 1;
});

Important Notes

Fork testing requires a mainnet RPC URL from providers like Infura or Alchemy.

Balances and contract states reflect the chosen block number at fork time.

Transactions on the fork do not affect the real mainnet and cost no real ETH.

Summary

Fork testing creates a safe copy of the mainnet blockchain for testing.

It helps you try code with real blockchain data without risk.

Use it to debug, test upgrades, and simulate real user interactions.