How to Verify and Publish Solidity Smart Contracts Source Code: Etherscan, PolygonScan, BSCScan

3 min readOct 15, 2021

Introduction

Are you the contract creator? Verify and Publish your contract source code today!

Have you ever tried “Verify and Publish” on Etherscan? Have you failed? This tutorial is for you. There is a much easier way than using the UI.

Etherscan page to verify a Smart Contract Source Code

Step 1: Get an API KEY

Once logged in, go to etherscan.io/myapikey clic on Add button and give it a name (optional).

Save the API KEY/token for later use.

Step 2: Install hardhat-etherscan

For the scope of this tutorial, I’ll be verifiying a standard OpenZeppelin ERC20 contract deployed on Ethereum Kovan testnet.

Within your proyect, install hardhat and hardhat-etherscan

npm install --save-dev "hardhat@^2.6.6"
npm install --save-dev @nomiclabs/hardhat-etherscan

Note: Even if you want to verify on a different explorer (PolygonScan, BSCScan, etc) the package will still be hardhat-etherscan, the naming can be confusing. Also, you can install hardhat with -g if you want to use it globally.

If you don’t have it, create a file named hardhat-config.js (or .ts)

If you are using JavaScript, include the following line:

require("@nomiclabs/hardhat-etherscan");

If you are using TypeScript, use this instead:

import "@nomiclabs/hardhat-etherscan";

Then, add etherscan key to your hardhat-config.js replacing the API KEY you got earlier.

module.exports = {
  networks: {
    kovan: { 
      url: "url: "https://kovan.infura.io/v3/{API_KEY}",
      accounts": "{PRIVATE_KEY}",
    }
  },
  etherscan: {
    url: "https://kovan.etherscan.io",
    apiKey: "YOUR_API_KEY"
  }
};

Note: Even if you are using any other explorer, the key should still be etherscan (confusing yes, hopefully they update it in the future). If you are using etherscan the url key is not required. If you are using another explorer, update it to the apropiate url.

Step 3: Constructor Arguments

If your contract doesn’t take any arguments, go straight to step 4.

Create a file called arguments.js (name doesn’t really matters) exporting your constructor arguments in order. In my case, my ERC20 takes name and symbol so:

module.exports = [
    "Test Token",
    "TEST",
];

Note: It must match the values you have deployed your contract with, otherwise verification will fail.

Step 4: Verification

For the last step you’ll need:

Blockchain Network
Arguments file (only if your contract constructor takes any)
Contract name (usually the same as the file).
Contract file name (relative to the directory of your command line)
Deployed contract address

Run the following command

npx hardhat verify \
--network {NETWORK} \
--constructor-args arguments.js \
--contract {CONTRACT_FILE_PATH}:{CONTRACT_NAME} \
{CONTRACT_ADDRESS}

Network: the name will vary depending on how you setup your hardhat-config.js. In this case it will be kovan

Constructor args: If your contract doesn’t take any arguments, remove this

Contract: This will be your relative path to the deployed solidity contract file, then : and the contract name (right after contract key word in solidity code)

And last but not least the address your contract was deployed on

The full command I’m going to run for my example contract is:

npx hardhat verify \
--network kovan \
--constructor-args arguments.js \
--contract contracts/token/ERC20/ERC20.sol:ERC20 \
0x70390af655c3Ce25f22a08b8480A070d9915281c

And… that’s it! Enjoy your verified badge!

Example contract: https://kovan.etherscan.io/address/0x70390af655c3Ce25f22a08b8480A070d9915281c#code

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