Contract Verification

Contract Verification on Celo

This skill covers verifying smart contracts on Celo block explorers, making source code publicly readable.

When to Use

• After deploying a contract to Celo
• Publishing open-source contracts
• Enabling contract interaction via explorer UI
• Building trust with users

Verification Methods

Hardhat Verification

Configuration

Source: https://docs.celo.org/developer/verify/hardhat

// hardhat.config.js
require("dotenv").config();
require("@nomicfoundation/hardhat-verify");

module.exports = {
  solidity: "0.8.28",
  networks: {
    celo: {
      url: "https://forno.celo.org",
      accounts: [process.env.PRIVATE_KEY],
      chainId: 42220,
    },
    celoSepolia: {
      url: "https://forno.celo-sepolia.celo-testnet.org/",
      accounts: [process.env.PRIVATE_KEY],
      chainId: 11142220,
    },
  },
  etherscan: {
    apiKey: process.env.ETHERSCAN_API_KEY,
    customChains: [
      {
        network: "celo",
        chainId: 42220,
        urls: {
          apiURL: "https://api.etherscan.io/v2/api",
          browserURL: "https://celoscan.io/",
        },
      },
      {
        network: "celoSepolia",
        chainId: 11142220,
        urls: {
          apiURL: "https://api.etherscan.io/v2/api",
          browserURL: "https://sepolia.celoscan.io",
        },
      },
    ],
  },
};

Environment Variables

# .env
PRIVATE_KEY=0xYOUR_PRIVATE_KEY
ETHERSCAN_API_KEY=your_celoscan_api_key

Get an API key from Etherscan or Celoscan.

Verify Commands

Mainnet:

npx hardhat verify --network celo <CONTRACT_ADDRESS> <CONSTRUCTOR_ARGS>

Testnet:

npx hardhat verify --network celoSepolia <CONTRACT_ADDRESS> <CONSTRUCTOR_ARGS>

Example with Constructor Arguments

# Contract with constructor: constructor(string memory name, uint256 value)
npx hardhat verify --network celo 0x1234...5678 "MyToken" 1000000

Complex Constructor Arguments

For complex arguments, create a file:

// arguments.js
module.exports = [
  "MyToken",
  "MTK",
  1000000,
  "0x1234567890123456789012345678901234567890",
];

npx hardhat verify --network celo 0x1234...5678 --constructor-args arguments.js

Foundry Verification

Source: https://docs.celo.org/developer/verify/foundry

Configuration

# foundry.toml
[profile.default]
src = "src"
out = "out"
libs = ["lib"]
solc_version = "0.8.28"

[rpc_endpoints]
celo = "https://forno.celo.org"
celoSepolia = "https://forno.celo-sepolia.celo-testnet.org"

Environment Setup

export ETHERSCAN_API_KEY=<your_etherscan_api_key>

Verify Commands

Mainnet (Chain ID 42220):

forge verify-contract \
  --chain-id 42220 \
  <CONTRACT_ADDRESS> \
  src/MyContract.sol:MyContract \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --watch

Testnet (Chain ID 11142220):

forge verify-contract \
  --chain-id 11142220 \
  <CONTRACT_ADDRESS> \
  src/MyContract.sol:MyContract \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --watch

With Constructor Arguments

forge verify-contract \
  --chain-id 42220 \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  <CONTRACT_ADDRESS> \
  src/MyContract.sol:MyContract \
  --constructor-args $(cast abi-encode "constructor(string,uint256)" "MyToken" 1000000) \
  --watch

Deploy and Verify in One Command

forge create \
  --rpc-url https://forno.celo.org \
  --private-key $PRIVATE_KEY \
  --etherscan-api-key $ETHERSCAN_API_KEY \
  --verify \
  src/MyContract.sol:MyContract \
  --constructor-args "MyToken" 1000000

Celoscan UI Verification

1. Go to your contract on Celoscan
2. Click the Contract tab
3. Click Verify & Publish
4. Select compiler settings:
   • Compiler Type (Solidity Single/Multi-file)
   • Compiler Version
   • License Type
5. Paste your source code
6. Add constructor arguments (ABI-encoded)
7. Complete CAPTCHA
8. Click Verify and Publish

Blockscout Verification

Celo has a Blockscout explorer at celo.blockscout.com that supports contract verification.

Blockscout URLs

UI Verification

1. Go to celo.blockscout.com (or celo-sepolia.blockscout.com for testnet)
2. Search for your contract address
3. Click Code tab
4. Click Verify & Publish
5. Select verification method:
   • Via flattened source code - Single file with all imports inlined
   • Via standard input JSON - Compiler standard JSON input
   • Via Sourcify - Decentralized verification
6. Fill in compiler settings (version, optimization, EVM version)
7. Submit verification

API Verification

Blockscout provides a REST API for programmatic verification.

Flattened Source Code:

curl -X POST "https://celo.blockscout.com/api/v2/smart-contracts/0xYOUR_ADDRESS/verification/via/flattened-code" \
  -H "Content-Type: application/json" \
  -d '{
    "compiler_version": "v0.8.28+commit.7893614a",
    "source_code": "// SPDX-License-Identifier: MIT\npragma solidity ^0.8.28;\n...",
    "is_optimization_enabled": true,
    "optimization_runs": 200,
    "contract_name": "MyContract",
    "evm_version": "paris",
    "license_type": "mit"
  }'

Standard JSON Input:

curl -X POST "https://celo.blockscout.com/api/v2/smart-contracts/0xYOUR_ADDRESS/verification/via/standard-input" \
  -F "compiler_version=v0.8.28+commit.7893614a" \
  -F "contract_name=MyContract" \
  -F "license_type=mit" \
  -F "files[0]=@standard-input.json"

API Endpoints:

• Flattened: POST /api/v2/smart-contracts/{address}/verification/via/flattened-code
• Standard JSON: POST /api/v2/smart-contracts/{address}/verification/via/standard-input
• Multi-part: POST /api/v2/smart-contracts/{address}/verification/via/multi-part
• Sourcify: POST /api/v2/smart-contracts/{address}/verification/via/sourcify

Source: https://docs.blockscout.com/devs/verification

Sourcify Verification

Sourcify provides decentralized contract verification that works across multiple explorers.

Source: https://docs.celo.org/developer/verify/remix

Benefits

• Decentralized storage of verified source code
• Works with Blockscout and other explorers
• Metadata hash verification ensures exact source match

Using Remix Sourcify Plugin

1. Open Remix IDE
2. Go to Plugin Manager (plug icon)
3. Search for "Sourcify" and activate it
4. Deploy your contract to Celo
5. In the Sourcify plugin:
   • Select the deployed contract
   • Choose the network (Celo Mainnet: 42220, Sepolia: 11142220)
   • Click Verify
6. Contract will be verified on Sourcify and visible in Blockscout

Programmatic Sourcify Verification

curl -X POST "https://sourcify.dev/server/verify" \
  -F "address=0xYOUR_ADDRESS" \
  -F "chain=42220" \
  -F "files[0]=@MyContract.sol" \
  -F "files[1]=@metadata.json"

Remix Verification

Verify contracts directly from Remix IDE using the Etherscan plugin.

Source: https://docs.celo.org/developer/verify/remix

Setup

1. Open Remix IDE
2. Go to Plugin Manager
3. Activate Etherscan - Contract Verification plugin

Verification Steps

1. Deploy your contract to Celo via Remix
2. Open the Etherscan plugin (checkmark icon)
3. Enter your Celoscan API key
4. Select the contract to verify
5. Choose network:
   • Celo Mainnet (Chain ID: 42220)
   • Celo Sepolia (Chain ID: 11142220)
6. Click Verify

Configuration for Celo

In Remix settings, add custom network:

• Network Name: Celo Mainnet
• Chain ID: 42220
• API URL: https://api.celoscan.io/api
• Browser URL: https://celoscan.io

Troubleshooting

"Unable to verify"

Causes:

• Compiler version mismatch
• Optimizer settings mismatch
• Wrong constructor arguments

Solutions:

• Match exact compiler version used in deployment
• Match optimizer settings (enabled, runs)
• Verify constructor args are ABI-encoded correctly

"Already verified"

Contract is already verified. Check the explorer to see the source code.

"Contract not found"

Causes:

• Wrong contract address
• Contract not yet indexed

Solutions:

• Double-check the address
• Wait a few minutes for indexing

Proxy Contracts

For proxy contracts, verify both:

1. The implementation contract
2. The proxy contract

Then link them on Celoscan:

1. Go to proxy contract
2. Click "More Options"
3. Select "Is this a proxy?"
4. Follow verification steps

API Endpoints

Dependencies

For Hardhat:

{
  "devDependencies": {
    "@nomicfoundation/hardhat-verify": "^2.0.0",
    "hardhat": "^2.19.0"
  }
}

Additional Resources

• verification-config.md - Complete configuration examples