Verifying a smart contract allows developers to review your code from within the CeloScan Block Explorer
If you use Celo Composer all the configuration is done for you out of the box, all you need is the CeloScan API keys!

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Prerequisites

Before the installation steps you need to have your hardhat project initialized using the command 
npx hardhat init
Make sure to have dependencies installed and the hardhat config file is importing @nomicfoundation/hardhat-toolbox

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Hardhat Configuration

Add the following configuration to the config object in hardhat.config.js. 
    networks: {
        alfajores: {
            // can be replaced with the RPC url of your choice.
            url: "https://alfajores-forno.celo-testnet.org",
            accounts: [
                "<YOUR_PRIVATE_KEY>"
            ],
        },
        celo: {
            url: "https://forno.celo.org",
            accounts: [
                "<YOUR_PRIVATE_KEY>"
            ],
        }
    },
    etherscan: {
        apiKey: {
            alfajores: "<CELOSCAN_API_KEY>",
            celo: "<CELOSCAN_API_KEY>"
        },
        customChains: [
            {
                network: "alfajores",
                chainId: 44787,
                urls: {
                    apiURL: "https://api-alfajores.celoscan.io/api",
                    browserURL: "https://alfajores.celoscan.io",
                },
            },
            {
                network: "celo",
                chainId: 42220,
                urls: {
                    apiURL: "https://api.celoscan.io/api",
                    browserURL: "https://celoscan.io/",
                },
            },
        ]
    },

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Verifying Contracts

Use the following command (Make sure your contracts are compiled before verification) Alfajores Testnet 
npx hardhat verify [CONTRACT_ADDRESS] [...CONSTRUCTOR_ARGS] --network alfajores
Celo Mainnet 
npx hardhat verify [CONTRACT_ADDRESS] [CONSTRUCTOR_ARGS] --network celo