Smart Contract Verification on Etherscan via API: A Step-by-Step Guide?

Smart Contract Verification on Etherscan: A Step-by-Step Guide

Verifying your smart contracts on Etherscan, Bscan, Polyscan, or any EVM-compatible blockchain is crucial for building trust and transparency, especially in the financial transactions. Recently, a customer requested the ability to verify smart contracts on Etherscan when creating ERC20, ERC721, or ERC1155 tokens via BulkSendTokens.xyz.

I agreed that this was a great idea and decided to implement it.

In this guide, I'll walk you through the process of verifying a smart contract on Etherscan using Hardhat and a custom TypeScript class.

Here is architecture of the smart contracts project:

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Step 1: Compile Your Smart Contract

Let's assume you've built your ERC20 token using Hardhat. To verify it, you'll need the Solidity Standard JSON Input file generated during compilation.

Run the following command to compile your contract:

npx hardhat compile

After compilation, locate the JSON input file in the artifacts folder. This file contains the metadata required for verification.

Step 2: Create the Etherscan Verifier Class

Create a TypeScript class (etherscan.ts) to handle the verification process. This class will use the Etherscan API to submit the contract for verification.

import axios, { AxiosResponse } from "axios";
import FormData from "form-data";
import { ethers } from "ethers";

export class EtherscanVerifier {
    private apiKey: string;
    private apiUrl: string;

    constructor(apiKey: string, apiUrl: string) {
        this.apiKey = apiKey;
        this.apiUrl = apiUrl;
    }

    /**
     * ABI-encodes constructor arguments
     * @param abi - Contract ABI (Application Binary Interface)
     * @param constructorArgs - Array of constructor arguments
     * @returns ABI-encoded constructor arguments
     */
    private encodeConstructorArgs(abi: any[], constructorArgs: any[]): string {
        const iface = new ethers.Interface(abi);
        return iface.encodeDeploy(constructorArgs).slice(2); // Remove "0x" prefix
    }

    /**
     * Verifies a smart contract on Etherscan
     * @param chainId - Chain ID of the network (e.g., 1 for Ethereum Mainnet, 11155111 for Sepolia)
     * @param contractAddress - Address of the deployed contract
     * @param sourceCode - Source code of the contract in Standard JSON Input format
     * @param contractName - Name of the contract (e.g., "contracts/tokens/ERC20.sol:ERC20Token")
     * @param compilerVersion - Version of the Solidity compiler used (e.g., "v0.8.26+commit.8a97fa7a")
     * @param abi - ABI of the contract
     * @param constructorArgs - Constructor arguments (if any)
     * @param optimizationUsed - Whether optimization was used during compilation (default: true)
     * @param runs - Number of optimization runs (default: 200)
     */
    public async verifyContract(
        chainId: number,
        contractAddress: string,
        sourceCode: string,
        contractName: string,
        compilerVersion: string,
        abi: any[],
        constructorArgs: any[] = [],
        optimizationUsed: boolean = true,
        runs: number = 200
    ): Promise<void> {
        console.log("Verifying contract...");

        // Encode constructor arguments if any
        const encodedConstructorArgs = constructorArgs.length > 0
            ? this.encodeConstructorArgs(abi, constructorArgs)
            : "";

        const formData = new FormData();
        formData.append("chainId", chainId);
        formData.append("codeformat", "solidity-standard-json-input");
        formData.append("sourceCode", sourceCode); // Standard JSON Input
        formData.append("constructorArguments", encodedConstructorArgs);
        formData.append("contractaddress", contractAddress);
        formData.append("contractname", contractName);
        formData.append("compilerversion", compilerVersion);
        formData.append("optimizationUsed", optimizationUsed ? "1" : "0");
        formData.append("runs", runs.toString());

        try {
            const response = await axios.post(
                `${this.apiUrl}?module=contract&action=verifysourcecode&apikey=${this.apiKey}`,
                formData,
                {
                    headers: {
                        "Content-Type": "multipart/form-data",
                    },
                }
            );
            console.log("Verification Response:", response.data);
        } catch (error) {
            console.error("Error verifying contract:", error);
        }
    }
}

---

Step 3: Call the Verification API

Once the EtherscanVerifier class is set up, you can use it to verify your contract. Here's an example of how to call the API:

const verifier = new EtherscanVerifier(
    process.env.NEXT_PUBLIC_ETHERSCAN_API, // Your Etherscan API Key
    "https://api-sepolia.etherscan.io/api" // Etherscan API URL (Sepolia Testnet)
);

var result = await verifier.verifyContract(
    chainId, // Chain ID (e.g., 11155111 for Sepolia)
    address, // Deployed contract address
    JSON.stringify(ERC20.content), // Standard JSON Input
    "contracts/tokens/ERC20.sol:ERC20Token", // Contract name
    "v0.8.26+commit.8a97fa7a", // Compiler version
    [...ERC20.abi], // Contract ABI
    [tokenName, tokenSymbol, tokenDecimals, totalSupply] // Constructor arguments
);

you will get a data in result with the following details:

{
message: "OK"
result: "67cjtm6ptdsrikyef35bcg6pkjezzgnh5eurhckwpfmtyqjepb"
status: "1"
}

you can check the verification status on:

https://api.etherscan.io/api?module=contract&action=checkverifystatus&guid=67cjtm6ptdsrikyef35bcg6pkjezzgnh5eurhckwpfmtyqjepb&apikey=YOUR_ETHERSCAN_API_KEY

You should get:

Documentation for API Parameters

Below is a breakdown of the parameters used in the Etherscan API request, as documented in the Etherscan API Documentation:

Parameter	Description
chainId	Chain ID of the network (e.g., 1 for Ethereum Mainnet, 11155111 for Sepolia)
codeformat	Format of the source code (solidity-standard-json-input for JSON input)
sourceCode	Source code of the contract in Standard JSON Input format
constructorArguments	ABI-encoded constructor arguments (if any)
contractaddress	Address of the deployed contract
contractname	Name of the contract (e.g., contracts/tokens/ERC20.sol:ERC20Token)
compilerversion	Version of the Solidity compiler used
optimizationUsed	Whether optimization was used during compilation (1 for true, 0 for false)
runs	Number of optimization runs (default: 200)

---

Conclusion

By following this guide, you can easily verify your smart contracts on Etherscan or any EVM-compatible blockchain. This process not only enhances transparency but also builds trust with your users. Whether you're deploying ERC20, ERC721, or ERC1155 tokens, verification is a critical step in ensuring the integrity of your smart contracts.

Feel free to integrate this functionality into your projects and share your feedback! If you have any questions, drop them in the comments below.

Happy coding! 🚀