I am unable to verify the contract below on Etherscan. No errors in Remix. I tried all of the solutions recommended in answers to similar questions, which were:

  • Remove the ^ in pragma
  • Compile contract with and without optimization turned on

enter image description here

The contract is currently deployed on address 0x113b1D84A5D25b9A921434F8131b71aDa45dAeac

Any ideas why this happens? Thanks!

// Version of Solidity compiler this program was written for
pragma solidity ^0.5.14;

// Heads or tails game contract
contract HeadsOrTails {
  address payable owner;
  string public name;

  struct Game {
    address addr;
    uint amountBet;
    uint8 guess;
    bool winner;
    uint ethInJackpot;

  Game[] lastPlayedGames;

  //Log game result (heads 0 or tails 1) in order to display it on frontend
  event GameResult(uint8 side);

  // Contract constructor run only on contract creation. Set owner.
  constructor() public {
    owner = msg.sender;
    name = "Heads or Tails dApp";

  //add this modifier to functions, which should only be accessible by the owner
  modifier onlyOwner {
    require(msg.sender == owner, "This function can only be launched by the owner");

  //Play the game!
  function lottery(uint8 guess) public payable returns(bool){
    require(guess == 0 || guess == 1, "Variable 'guess' should be either 0 ('heads') or 1 ('tails')");
    require(msg.value > 0, "Bet more than 0");
    require(msg.value <= address(this).balance - msg.value, "You cannot bet more than what is available in the jackpot");
    //address(this).balance is increased by msg.value even before code is executed. Thus "address(this).balance-msg.value"
    //Create a random number. Use the mining difficulty & the player's address, hash it, convert this hex to int, divide by modulo 2 which results in either 0 or 1 and return as uint8
    uint8 result = uint8(uint256(keccak256(abi.encodePacked(block.difficulty, msg.sender, block.timestamp)))%2);
    bool won = false;
    if (guess == result) {
      msg.sender.transfer(msg.value * 2);
      won = true;

    emit GameResult(result);
    lastPlayedGames.push(Game(msg.sender, msg.value, guess, won, address(this).balance));
    return won; //Return value can only be used by other functions, but not within web3.js (as of 2019)

  //Get amount of games played so far
  function getGameCount() public view returns(uint) {
    return lastPlayedGames.length;

  //Get stats about a certain played game, e.g. address of player, amount bet, won or lost, and ETH in the jackpot at this point in time
  function getGameEntry(uint index) public view returns(address addr, uint amountBet, uint8 guess, bool winner, uint ethInJackpot) {
    return (

  // Contract destructor (Creator of contract can also destroy it and receives remaining ether of contract address).
  //Advantage compared to "withdraw": SELFDESTRUCT opcode uses negative gas because the operation frees up space on
  //the blockchain by clearing all of the contract's data
  function destroy() external onlyOwner {

  //Withdraw money from contract
  function withdraw(uint amount) external onlyOwner {
    require(amount < address(this).balance, "You cannot withdraw more than what is available in the contract");

  // Accept any incoming amount
  function () external payable {}

Those are predeployment suggestions to reduce ambiguity at this step.

The issue is there no way to be certain how the bytecode was generated because the pragma allows flexibility.

If possible, examine the tools you used to compile, e.g. solc, Remix or truffle-config.js to work out the compiler version that must have been used.

If you cannot do that, then it's trial and error. There are no constructor arguments, so the search area is solidity 0.5.14 and higher, with and without optimization. Try them all. Start with just the official releases because no one should be using the nightly builds for production.

Etherscan will confirm the contract when it compiles the provided source code with the parameters given and it produces a bit-wise exact copy of the bytecode that is on the blockchain. In other words, it will reproduce the compilation process and compare the code you say is at that address to what is actually there.

Hope it helps.

| improve this answer | |
  • 1
    0.5.14 is the latest official release at the time of writing, thus couldn't try any later version. I succeeded with version 0.5.11 though. Thanks for your help! – René K Dec 13 '19 at 15:55
  • 1
    Glad it worked. The result implies the code was modified or is not exactly as shown here, because it should refuse to compile with < 0.5.14. Very weird. Glad you found success. – Rob Hitchens Dec 13 '19 at 16:36
  • 1
    Yeah, I changed to pragma solidity ^0.5.11; – René K Dec 13 '19 at 17:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.