Skill: solidity

Scope

• •Solidity 0.8.24 smart contract development with Foundry
• •EVM internals and execution environment understanding
• •Security patterns and vulnerability prevention
• •Gas optimization techniques
• •Testing strategies with Foundry (unit, fuzz, invariant)
• •Client interactions using viem v2
• •OpenZeppelin Contracts integration

Does NOT cover:

• •Frontend wallet integration (see wagmi skill)
• •Solana development (see solana skill)

Assumptions

• •Solidity 0.8.24
• •Foundry (forge, cast, anvil) for testing and deployment
• •viem v2.44.4 for client interactions
• •OpenZeppelin Contracts v5.x
• •TypeScript v5+ for client code
• •Node.js 18+

Principles

• •Use Solidity 0.8.24 + Foundry for all development
• •Follow CEI pattern (Checks-Effects-Interactions) for all state changes
• •Use custom errors instead of require strings for gas efficiency
• •Security-first mindset: validate inputs, use access control, prevent reentrancy
• •Comprehensive testing: unit, fuzz, and invariant tests
• •Optimize for gas: storage packing, calldata over memory, cache storage reads
• •Understand EVM execution model: stack, memory, storage, opcodes
• •Use viem v2 for client interactions and transaction building
• •Use OpenZeppelin Contracts for battle-tested implementations

Constraints

MUST

• •Use Solidity 0.8.24
• •Use Foundry for testing and deployment (not Hardhat)
• •Follow CEI pattern for external calls (prevents reentrancy)
• •Validate inputs (zero address checks, bounds checking)
• •Use access control on admin functions
• •Use viem v2 for client interactions
• •Validate addresses with getAddress() from viem (never cast directly)

SHOULD

• •Use OpenZeppelin Contracts via remappings (@openzeppelin/=lib/openzeppelin-contracts/)
• •Use custom errors instead of require strings (gas efficient)
• •Use EIP-1559 transactions (type 2) for predictable fees
• •Write fuzz tests for functions with numeric inputs
• •Write invariant tests for system-wide properties
• •Cache storage reads in memory when used in loops
• •Use calldata for read-only function parameters
• •Use unchecked blocks for safe math operations (loop counters, bounded increments)

AVOID

• •Storage reads in loops (cache first)
• •Unnecessary overflow checks when overflow is impossible
• •Contract size > 24KB (split into libraries if needed)
• •Relying on SSTORE refunds (EIP-3529 removed them)
• •Trusting external calls without validation
• •Using block.timestamp for critical logic (miners can manipulate)

Foundry Configuration

• •Solidity: 0.8.24
• •Optimizer: Enabled (200 runs)
• •Fuzz tests: 256 runs
• •Invariant tests: 256 runs

Patterns

Contract Development

CEI Pattern (Checks-Effects-Interactions)

Always update state before external calls to prevent reentrancy:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

contract SecureVault {
    mapping(address => uint256) public balances;

    error InsufficientBalance();
    error TransferFailed();

    function withdraw(uint256 amount) external {
        // 1. CHECKS
        if (balances[msg.sender] < amount) revert InsufficientBalance();

        // 2. EFFECTS (update state first)
        balances[msg.sender] -= amount;

        // 3. INTERACTIONS (external call last)
        (bool ok,) = msg.sender.call{value: amount}("");
        if (!ok) revert TransferFailed();
    }
}

Custom Errors Pattern

Use custom errors for gas efficiency:

error InsufficientBalance(uint256 requested, uint256 available);
error Unauthorized(address caller);
error TransferFailed();

function withdraw(uint256 amount) external {
    if (balances[msg.sender] < amount) {
        revert InsufficientBalance(amount, balances[msg.sender]);
    }
    // ...
}

OpenZeppelin Integration

Use OpenZeppelin Contracts via remappings in foundry.toml:

[profile.default]
remappings = [
    "@openzeppelin/=lib/openzeppelin-contracts/",
]

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract MyToken is ERC20 {
    constructor() ERC20("MyToken", "MTK") {}
}

Gas Optimization

Storage Packing Pattern

Pack storage efficiently to save gas:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

contract Optimized {
    // Pack into single slot (32 bytes)
    struct User {
        uint128 balance;    // 16 bytes
        uint64 lastUpdate;  // 8 bytes
        uint32 nonce;       // 4 bytes
        bool active;        // 1 byte
        // 3 bytes padding
    }

    mapping(address => User) public users;
}

Calldata vs Memory Pattern

Use calldata for read-only parameters:

// ✅ Good: calldata for read-only
function processData(bytes calldata data) external {
    // data is read-only, saves ~3 gas per byte
}

// ❌ Bad: memory for read-only
function processData(bytes memory data) external {
    // Unnecessary copy to memory
}

Cache Storage Reads Pattern

Cache storage reads in memory when used multiple times:

// ✅ Good: cache storage read
function processUser(address user) external {
    uint256 balance = balances[user]; // Cache once
    if (balance > 0) {
        // Use cached value multiple times
        process(balance);
        update(balance);
    }
}

// ❌ Bad: multiple storage reads
function processUser(address user) external {
    if (balances[user] > 0) {
        process(balances[user]); // Storage read #1
        update(balances[user]); // Storage read #2
    }
}

Unchecked Math Pattern

Use unchecked blocks for math operations where overflow/underflow is impossible:

// ✅ Good: unchecked for safe operations
function increment(uint256 x) external pure returns (uint256) {
    unchecked {
        return x + 1; // Safe: can't overflow uint256
    }
}

// ✅ Good: loop counter increment
function processBatch(uint256[] calldata items) external {
    for (uint256 i = 0; i < items.length;) {
        process(items[i]);
        unchecked {
            ++i; // Safe: loop bound prevents overflow
        }
    }
}

// ❌ Bad: unnecessary checked math
function increment(uint256 x) external pure returns (uint256) {
    return x + 1; // Unnecessary: Solidity 0.8+ checks by default
}

Security

Reentrancy Prevention

Always follow CEI pattern (see Contract Development section above). For additional protection, use reentrancy guards when needed:

import "@openzeppelin/contracts/security/ReentrancyGuard.sol";

contract Vault is ReentrancyGuard {
    function withdraw(uint256 amount) external nonReentrant {
        // CEI pattern + reentrancy guard for extra protection
    }
}

Access Control Pattern

Always protect admin functions:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

contract SecureContract {
    address public owner;

    error Unauthorized();

    modifier onlyOwner() {
        if (msg.sender != owner) revert Unauthorized();
        _;
    }

    function setAdmin(address newAdmin) external onlyOwner {
        owner = newAdmin;
    }
}

Input Validation Pattern

Validate all inputs before processing:

error ZeroAddress();
error InvalidAmount();
error OutOfBounds(uint256 value, uint256 max);

function transfer(address to, uint256 amount) external {
    // Zero address check
    if (to == address(0)) revert ZeroAddress();
    
    // Bounds check
    if (amount == 0) revert InvalidAmount();
    if (amount > balances[msg.sender]) revert InsufficientBalance();
    
    // Process transfer
    balances[msg.sender] -= amount;
    balances[to] += amount;
}

Testing

Foundry Test Setup

Use Foundry's testing framework:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "forge-std/Test.sol";
import "../src/TestToken.sol";

contract TestTokenTest is Test {
    TestToken token;
    address alice = makeAddr("alice");
    address bob = makeAddr("bob");

    function setUp() public {
        token = new TestToken("Test", "TEST", 18);
    }

    function test_Mint() public {
        token.mint(alice, 1000);
        assertEq(token.balanceOf(alice), 1000);
    }
}

Fuzz Testing Pattern

Test with random inputs to find edge cases:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "forge-std/Test.sol";

contract SecurityTest is Test {
    Vault vault;

    function setUp() public {
        vault = new Vault();
    }

    function testFuzz_Withdraw(uint256 amount) public {
        // Bound input to reasonable range
        amount = bound(amount, 1, type(uint128).max);
        
        vm.deal(address(this), amount);
        vault.deposit{value: amount}();
        
        uint256 before = address(this).balance;
        vault.withdraw(amount);
        uint256 after = address(this).balance;
        
        assertEq(after, before + amount);
    }
}

Invariant Testing Pattern

Test system-wide properties that should always hold:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

import "forge-std/Test.sol";

contract TokenInvariantTest is Test {
    Token token;
    address[] actors;

    function setUp() public {
        token = new Token();
        // Setup actors...
    }

    function invariant_BalancesMatchTotalSupply() public {
        uint256 sum = 0;
        for (uint i = 0; i < actors.length; i++) {
            sum += token.balanceOf(actors[i]);
        }
        assertEq(token.totalSupply(), sum);
    }
}

EVM Internals

Storage Slot Reading Pattern

Read contract storage using viem v2:

import { createPublicClient, http, keccak256, encodePacked } from 'viem'
import { arbitrumSepolia } from 'viem/chains'

const client = createPublicClient({
  chain: arbitrumSepolia,
  transport: http(),
})

// Read mapping value: balances[address]
async function getBalance(contract: `0x${string}`, user: `0x${string}`) {
  const slot = keccak256(encodePacked(['address', 'uint256'], [user, 0n]))
  return await client.getStorageAt({ address: contract, slot })
}

EIP-1559 Transaction Pattern

Use EIP-1559 for predictable fee structure:

import { createWalletClient, http, parseEther, parseGwei } from 'viem'
import { arbitrumSepolia } from 'viem/chains'

const client = createWalletClient({
  chain: arbitrumSepolia,
  transport: http(),
})

const hash = await client.sendTransaction({
  to: '0x...',
  value: parseEther('0.1'),
  type: 'eip1559',
  maxFeePerGas: parseGwei('30'),
  maxPriorityFeePerGas: parseGwei('2'),
})

Common Vulnerabilities

Critical: Reentrancy

• •Issue: State updated after external call
• •Prevention: CEI pattern, update state before external calls, use reentrancy guards when needed

High: Missing Access Control

• •Issue: Admin functions callable by anyone
• •Prevention: Use modifiers (onlyOwner, onlyRole)

High: Unchecked Return Values

• •Issue: External calls fail silently
• •Prevention: Check return values, use SafeERC20 for tokens

Medium: Precision Loss

• •Issue: Division before multiplication loses precision
• •Prevention: Multiply first, then divide

Medium: Oracle Manipulation

• •Issue: Flash loan attacks on spot prices
• •Prevention: Use TWAP, check staleness, use multiple oracles

Security Checklist

• • CEI pattern on all external calls
• • Access control on admin functions
• • Input validation (zero address, bounds)
• • Reentrancy guards where needed
• • Event emission for state changes
• • Custom errors for gas efficiency
• • Fuzz tests for numeric inputs
• • Invariant tests for system properties
• • Oracle staleness checks (if using oracles)
• • Slippage protection (if using DEX)

Tools

Foundry (Primary)

• •Testing: forge test
• •Fuzz testing: forge test --fuzz
• •Invariant testing: invariant_* functions
• •Gas snapshots: forge snapshot
• •Coverage: forge coverage
• •Cast CLI: cast for debugging and interactions

Static Analysis

• •Slither: Fast vulnerability detection
• •Mythril: Symbolic execution
• •Semgrep: Custom security rules

Client Library

• •viem v2: Type-safe Ethereum client for TypeScript/JavaScript

Network Context

• •Testing: Arbitrum Sepolia (Chain ID: 421614) - used for contract deployments
• •RPC: https://sepolia-rollup.arbitrum.io/rpc
• •Test tokens: Deployed on Arbitrum Sepolia

Trade-offs

• •Storage packing: Saves ~20k gas per slot but requires careful type selection. Use when multiple small values fit in one slot.
• •Calldata vs memory: calldata saves ~3 gas per byte but is read-only. Use calldata for read-only parameters.
• •Custom errors vs require strings: Custom errors save ~200+ gas per revert but require error definitions. Use custom errors for production contracts.
• •Storage vs memory: Storage reads cost 2100 gas, memory reads cost 3 gas. Cache storage values in memory when used multiple times.
• •Unchecked math: Saves ~20-40 gas per operation but requires ensuring overflow is impossible. Use for loop counters and safe math operations.
• •EIP-1559 vs legacy: EIP-1559 provides predictable fees but requires base fee estimation. Use EIP-1559 for all new transactions.

Interactions

• •Complements web3-frontend for frontend integration
• •Uses OpenZeppelin Contracts for standard implementations

External Resources

• •Foundry Book - Complete Foundry documentation
• •Foundry Book - Fuzz Testing - Fuzz testing guide
• •OpenZeppelin Contracts - Standard contract implementations
• •Viem Documentation - Complete viem v2 API reference
• •EEA Security Guidelines - Enterprise Ethereum Alliance security standards
• •SCSVS - Smart Contract Security Verification Standard

Historical References

• •SWC Registry - Common weakness enumeration (not actively maintained since ~2020, retained for historical context only)