Solidity Development with Foundry

Overview

Comprehensive skill for professional Solidity smart contract development using the Foundry framework. Provides security-first development practices, testing patterns, static analysis integration (Slither, solhint), and deployment workflows for EVM-compatible blockchains.

When to Use This Skill

Automatically activate when:

• Working with .sol (Solidity) files
• User mentions smart contracts, blockchain, Web3, DeFi, or Ethereum
• Foundry project detected (foundry.toml present)
• User requests contract deployment, testing, or security auditing
• Working with forge, cast, or anvil commands
• Implementing ERC standards (ERC20, ERC721, ERC1155, etc.)

Development Workflow

1. Write Secure Contracts

Follow security-first patterns from references/solidity-security.md:

Always implement:

• Checks-Effects-Interactions (CEI) pattern
• Access control (Ownable, AccessControl)
• Input validation
• Emergency pause mechanisms
• Pull over push for payments
• Custom errors for gas efficiency

Example contract structure:

solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.30;

import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {Pausable} from "@openzeppelin/contracts/security/Pausable.sol";

contract MyContract is Ownable, Pausable {
    /*//////////////////////////////////////////////////////////////
                                 ERRORS
    //////////////////////////////////////////////////////////////*/

    error InvalidInput();
    error InsufficientBalance();

    /*//////////////////////////////////////////////////////////////
                                 EVENTS
    //////////////////////////////////////////////////////////////*/

    event ActionCompleted(address indexed user, uint256 amount);

    /*//////////////////////////////////////////////////////////////
                            STATE VARIABLES
    //////////////////////////////////////////////////////////////*/

    mapping(address => uint256) public balances;

    /*//////////////////////////////////////////////////////////////
                              CONSTRUCTOR
    //////////////////////////////////////////////////////////////*/

    constructor() Ownable(msg.sender) {}

    /*//////////////////////////////////////////////////////////////
                           EXTERNAL FUNCTIONS
    //////////////////////////////////////////////////////////////*/

    function deposit() external payable whenNotPaused {
        if (msg.value == 0) revert InvalidInput();
        balances[msg.sender] += msg.value;
        emit ActionCompleted(msg.sender, msg.value);
    }

    function withdraw(uint256 amount) external whenNotPaused {
        // CEI Pattern: Checks
        if (amount == 0) revert InvalidInput();
        if (balances[msg.sender] < amount) revert InsufficientBalance();

        // Effects
        balances[msg.sender] -= amount;

        // Interactions
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");

        emit ActionCompleted(msg.sender, amount);
    }
}

2. Write Comprehensive Tests

Follow testing patterns from references/foundry-testing.md:

Test structure:

solidity
import {Test, console} from "forge-std/Test.sol";
import {MyContract} from "../src/MyContract.sol";

contract MyContractTest is Test {
    MyContract public myContract;

    address constant OWNER = address(1);
    address constant USER = address(2);

    function setUp() public {
        vm.prank(OWNER);
        myContract = new MyContract();
    }

    // Unit tests
    function test_Deposit() public { }

    // Edge cases
    function test_RevertWhen_ZeroDeposit() public { }

    // Fuzz tests
    function testFuzz_Deposit(uint256 amount) public { }

    // Invariant tests
    function invariant_TotalBalanceMatchesContract() public { }
}

Run tests:

bash
forge test              # Run all tests
forge test -vvv         # Verbose output
forge test --gas-report # Include gas costs
forge coverage          # Coverage report

3. Security Analysis

Run comprehensive security checks:

bash
# Static analysis with Slither
slither . --exclude-optimization --exclude-informational

# Linting with solhint
solhint 'src/**/*.sol' 'test/**/*.sol'

# Or use the automated script
bash scripts/check_security.sh

Pre-deployment checklist (from references/solidity-security.md):

• ] Reentrancy protection implemented
• ] Access controls on privileged functions
• ] Input validation on all external functions
• ] Emergency pause mechanism
• ] External calls follow CEI pattern
• ] Events emitted for state changes
• ] Gas optimizations applied
• ] Test coverage >90%
• ] Slither analysis passed
• ] solhint checks passed
• ] Professional audit for mainnet

4. Build and Deploy

Build contracts:

bash
forge build --optimize --optimizer-runs 200

Deploy using script:

solidity
// script/Deploy.s.sol
import {Script} from "forge-std/Script.sol";
import {MyContract} from "../src/MyContract.sol";

contract DeployScript is Script {
    function run() external {
        uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");
        vm.startBroadcast(deployerPrivateKey);

        MyContract myContract = new MyContract();
        console.log("Deployed at:", address(myContract));

        vm.stopBroadcast();
    }
}

Execute deployment:

bash
# Load environment variables
source .env

# Simulate deployment
forge script script/Deploy.s.sol --rpc-url $SEPOLIA_RPC_URL

# Deploy to testnet
forge script script/Deploy.s.sol \
    --rpc-url $SEPOLIA_RPC_URL \
    --broadcast \
    --verify

# Deploy to mainnet (after audits!)
forge script script/Deploy.s.sol \
    --rpc-url $MAINNET_RPC_URL \
    --broadcast \
    --verify

Advanced Testing Patterns

Mainnet Forking

Test against real deployed contracts:

solidity
contract ForkTest is Test {
    IERC20 constant USDC = IERC20(0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48);

    function setUp() public {
        vm.createSelectFork("mainnet", 18_000_000);
    }

    function test_InteractWithRealProtocol() public {
        // Test with actual mainnet state
    }
}

bash
forge test --fork-url $MAINNET_RPC_URL

Invariant Testing

Test properties that must always hold:

solidity
function invariant_SumOfBalancesEqualsTotalSupply() public {
    assertEq(token.totalSupply(), handler.sumOfBalances());
}

Fuzz Testing

Test with randomized inputs:

solidity
function testFuzz_Transfer(address to, uint256 amount) public {
    vm.assume(to != address(0));
    amount = bound(amount, 1, 1000 ether);
    // Test with random inputs
}

Gas Optimization

Key strategies from references/solidity-security.md:

1. Storage packing - Group uint8/uint16/etc in single slots
2. uint256 for counters - Native EVM word size
3. calldata over memory - For function parameters
4. Events over storage - Log data instead of storing
5. Custom errors - More gas-efficient than strings
6. Immutable/constant - For unchanging values

Contract Verification

bash
# Verify on Etherscan
forge verify-contract $CONTRACT_ADDRESS \
    src/MyContract.sol:MyContract \
    --chain-id 1 \
    --etherscan-api-key $ETHERSCAN_KEY

# With constructor args
forge verify-contract $CONTRACT_ADDRESS \
    src/MyContract.sol:MyContract \
    --chain-id 1 \
    --etherscan-api-key $ETHERSCAN_KEY \
    --constructor-args $(cast abi-encode "constructor(uint256)" 100)

Common Commands Reference

Quick reference from references/foundry-commands.md:

Development:

bash
forge build                    # Compile contracts
forge test                     # Run tests
forge test -vvv                # Verbose test output
forge coverage                 # Coverage report
forge fmt                      # Format code
forge clean                    # Clean artifacts

Testing:

bash
forge test --match-test test_Transfer    # Run specific test
forge test --match-contract MyTest       # Run specific contract
forge test --gas-report                  # Show gas usage
forge test --fork-url $RPC_URL          # Fork testing
forge snapshot                           # Gas snapshot

Deployment:

bash
forge script script/Deploy.s.sol --broadcast
forge create src/MyContract.sol:MyContract --rpc-url $RPC_URL
forge verify-contract $ADDRESS src/MyContract.sol:MyContract

Debugging:

bash
forge test --debug test_MyTest           # Interactive debugger
cast run $TX_HASH --debug                # Debug transaction
cast run $TX_HASH --trace                # Trace transaction

Local node:

bash
anvil                                    # Start local node
anvil --fork-url $MAINNET_RPC_URL       # Fork mainnet locally

Security Tools Integration

Slither

Static analyzer with 99+ vulnerability detectors:

bash
slither .                                # Full analysis
slither . --exclude-optimization         # Skip optimizations
slither . --exclude-informational        # Critical issues only
slither . --checklist                    # Generate checklist

solhint

Linter configured via assets/solhint.config.json:

bash
solhint 'src/**/*.sol'                   # Lint source
solhint 'test/**/*.sol'                  # Lint tests
solhint --fix 'src/**/*.sol'             # Auto-fix issues

Resources

references/

• solidity-security.md - Comprehensive security patterns, vulnerabilities, and mitigation strategies
• foundry-testing.md - Testing patterns including fuzzing, invariants, fork testing, and mocking
• foundry-commands.md - Complete command reference for forge, cast, and anvil

Best Practices Summary

1. Security First: Always implement CEI pattern, access controls, and input validation
2. Test Coverage: Aim for >90% coverage with unit, fuzz, and invariant tests
3. Static Analysis: Run Slither and solhint before every deployment
4. Gas Optimization: Apply after security, never at the expense of safety
5. Documentation: Use NatSpec comments for all public/external functions
6. Audit: Professional audit required for mainnet deployment
7. Verification: Verify all contracts on Etherscan post-deployment
8. Emergency: Include pause mechanisms for critical contracts

Common Patterns to Reference

When implementing specific functionality, reference these sections:

• ERC20 tokens - Use OpenZeppelin's implementation
• Access control - references/solidity-security.md (AccessControl, Ownable)
• Upgradeable contracts - OpenZeppelin's proxy patterns
• Reentrancy protection - references/solidity-security.md (CEI pattern)
• Testing external protocols - references/foundry-testing.md (fork testing)
• Gas optimization - references/solidity-security.md (optimization strategies)
• Randomness - Use Chainlink VRF, never block.timestamp alone
• Oracles - Use Chainlink price feeds for production

Workflow Example

Complete workflow for adding a new feature:

1. Write contract following security patterns
2. Write tests with >90% coverage
3. Run tests: forge test -vvv
4. Check coverage: forge coverage
5. Run Slither: slither .
6. Run solhint: solhint 'src/**/*.sol'
7. Format code: forge fmt
8. Build: forge build
9. Deploy to testnet: forge script script/Deploy.s.sol --broadcast
10. Verify: forge verify-contract ...
11. Get audit (for mainnet)
12. Deploy to mainnet after audit approval