🔍 Decode Reverts: Understand Error Messages

Revert messages are your best debugging tool. When a transaction fails with "execution reverted", the contract explicitly rejected it with a reason. Pre-Solidity 0.8.4, contracts used string messages like "Insufficient balance". Post-0.8.4, custom errors save gas: revert InsufficientBalance(balance, amount). Your job: decode these messages from transaction data, understand what went wrong, and fix the root cause. Etherscan shows decoded reverts for verified contracts, but unverified contracts require manual decoding. This section teaches you to read revert data, match error selectors, extract parameters, and trace back to the failing require() statement.

// Contract code that failed
function transfer(address to, uint256 amount) public returns (bool) {
  require(balanceOf(msg.sender) >= amount, "ERC20: transfer amount exceeds balance");
  _balances[msg.sender] -= amount;
  _balances[to] += amount;
  emit Transfer(msg.sender, to, amount);
  return true;
}

// User tried: transfer(bob, 1000 tokens) with only 500 tokens balance
// Result: Transaction reverted at require() check