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This guide covers Hardhat 3. There is an official migration guide from the Hardhat team.
Hardhat is a smart contracts development tool, perfect if you’re familiar with Javascript/Typescript. This assumes you already have a Hardhat 3 project, setup, and you’re about to deploy and verify next.

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Install

Via npm. 
npm add --save-dev @nomicfoundation/hardhat-verify

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Config

In hardhat.config.ts, import the plugin at the top and add it to the list of plugins. Then, specify a verify config with your Etherscan API key. This key works for most supported chains, otherwise you need to define a CustomChain below. 
import { defineConfig } from "hardhat/config";
import hardhatVerify from "@nomicfoundation/hardhat-verify";

export default defineConfig({
  plugins: [
    hardhatVerify,
    // ...other plugins...
  ],
  solidity: "0.8.28",
  networks: {
    sepolia: {
      url: configVariable("SEPOLIA_RPC_URL"),
      accounts: [configVariable("SEPOLIA_PRIVATE_KEY")],
    },
  },
  verify: {
    etherscan: {
      apiKey: "YourEtherscanApiKey",
    },
  },
});

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Deploy and Verify (using Hardhat Ignition)

npx hardhat ignition deploy ignition/modules/Counter.ts --network sepolia --verify

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Verify an Existing Contract

npx hardhat verify --network sepolia 0xdCBdBAA8404554502B433106e62728B659aCfE3b

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Custom Chains

For new chains that have an Etherscan explorer but isn’t supported with Hardhat defaults, you need to add both the networks and chainDescriptor in hardhat.config.ts. 
export default defineConfig({
  // ...
  networks: {
    monadTestnet: {
      type: "http",
      chainType: "l1",
      url: configVariable("MONAD_TESTNET_RPC_URL"),
      accounts: [configVariable("MONAD_TESTNET_PRIVATE_KEY")],
    }
  },
  chainDescriptors: {
    10143: {
      name: "monadTestnet",
      blockExplorers: {
        etherscan: {
          name: "Monad Testnet Explorer",
          url: "https://testnet.monadscan.com",
          apiUrl: "https://api.etherscan.io/v2/api",
        },
      },
    }
  },
});
Use the same deploy and verify command as above, but with the updated --network. In this example for monadTestnet 
npx hardhat ignition deploy ignition/modules/Counter.ts --network monadTestnet --verify