Deploy your first smart contract on Sepolia with Foundry

Foundry has become the standard toolchain for Solidity development. It is faster than Hardhat, runs tests in Solidity instead of JavaScript, and has first-class support for forking and fuzzing.

Prerequisites

• Basic Solidity knowledge
• Sepolia ETH in your wallet — see the How to get Sepolia ETH guide
• A wallet private key for deployment (use a dedicated dev wallet, never your mainnet key)

1. Install Foundry

curl -L https://foundry.paradigm.xyz | bash
foundryup

Verify the installation:

forge --version
cast --version

2. Create a new project

forge init hello-sepolia
cd hello-sepolia

Foundry creates the following structure:

hello-sepolia/
├── src/
│   └── Counter.sol       # sample contract
├── test/
│   └── Counter.t.sol     # sample test
├── script/
│   └── Counter.s.sol     # deployment script
└── foundry.toml

3. Write the contract

Replace src/Counter.sol with a simple storage contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract HelloSepolia {
    string public message;
    address public owner;

    event MessageUpdated(string newMessage, address updatedBy);

    constructor(string memory _message) {
        message = _message;
        owner = msg.sender;
    }

    function setMessage(string calldata _message) external {
        message = _message;
        emit MessageUpdated(_message, msg.sender);
    }
}

4. Write a test

Replace test/Counter.t.sol:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import {Test} from "forge-std/Test.sol";
import {HelloSepolia} from "../src/HelloSepolia.sol";

contract HelloSepoliaTest is Test {
    HelloSepolia public hello;

    function setUp() public {
        hello = new HelloSepolia("Hello, testnet!");
    }

    function test_InitialMessage() public view {
        assertEq(hello.message(), "Hello, testnet!");
    }

    function test_SetMessage() public {
        hello.setMessage("Updated");
        assertEq(hello.message(), "Updated");
    }

    function test_OwnerIsDeployer() public view {
        assertEq(hello.owner(), address(this));
    }
}

Run the tests:

forge test -vv

5. Configure the network

Create a .env file (never commit this):

SEPOLIA_RPC_URL=https://rpc.sepolia.org
PRIVATE_KEY=0xYOUR_DEV_WALLET_PRIVATE_KEY
ETHERSCAN_API_KEY=YOUR_ETHERSCAN_API_KEY

Add to foundry.toml:

[profile.default]
src = "src"
out = "out"
libs = ["lib"]

[rpc_endpoints]
sepolia = "${SEPOLIA_RPC_URL}"

[etherscan]
sepolia = { key = "${ETHERSCAN_API_KEY}", url = "https://api-sepolia.etherscan.io/api" }

6. Write the deployment script

Replace script/Counter.s.sol with script/Deploy.s.sol:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import {Script} from "forge-std/Script.sol";
import {HelloSepolia} from "../src/HelloSepolia.sol";

contract Deploy is Script {
    function run() external returns (HelloSepolia) {
        vm.startBroadcast();
        HelloSepolia hello = new HelloSepolia("Hello from Sepolia!");
        vm.stopBroadcast();
        return hello;
    }
}

7. Deploy

Load environment variables and deploy:

source .env

forge script script/Deploy.s.sol \
  --rpc-url sepolia \
  --private-key $PRIVATE_KEY \
  --broadcast \
  --verify

The --verify flag automatically verifies the contract on Sepolia Etherscan.

8. Verify the deployment

cast call <DEPLOYED_ADDRESS> "message()(string)" --rpc-url sepolia

Or visit sepolia.etherscan.io/address/<DEPLOYED_ADDRESS> and interact via the Read Contract tab.

Send a transaction

cast send <DEPLOYED_ADDRESS> \
  "setMessage(string)" "Hello from CLI!" \
  --private-key $PRIVATE_KEY \
  --rpc-url sepolia

Common errors