invariant

/invariant — generate invariant tests

Property-based testing in Foundry. The single highest-leverage test artifact you can produce — invariants catch bugs no example-based test does.

Procedure

Step 1 — Identify invariants

Dispatch invariant-writer subagent. It reads the contract and proposes invariants based on protocol type:

Token / ERC-20:

• totalSupply() == sum(balanceOf(a) for a in all_addrs)
• balances never overflow
• transfers preserve totalSupply (no minting / burning)

Vault / ERC-4626:

• totalAssets() >= sum(convertToAssets(balanceOf(a)) for a in all_holders)
• previewDeposit + previewWithdraw round-trip ≤ initial value
• totalSupply == 0 ⇒ totalAssets == 0
• shares only mintable via deposit/mint, only burnable via withdraw/redeem

AMM:

• K = reserve0 * reserve1 non-decreasing after fee-paying swaps
• LP token supply ratio matches reserves ratio over time

Lending:

• sumDebt <= sumCollateralValue * MAX_LTV (modulo bad debt)
• borrow + repay is value-preserving
• no negative balances

Governance:

• vote weight is snapshotted; can't be modified after proposal creation
• timelock delay always elapses before execution

Step 2 — Generate Foundry invariant harness

contract VaultInvariants is Test {
    Vault vault;
    Handler handler;   // ← actor that calls arbitrary fns with bounded random inputs

    function setUp() public {
        vault = new Vault();
        handler = new Handler(vault);
        targetContract(address(handler));
    }

    function invariant_TotalAssetsCoversShares() public {
        uint256 sum;
        address[] memory users = handler.getActors();
        for (uint i; i < users.length; ++i) {
            sum += vault.convertToAssets(vault.balanceOf(users[i]));
        }
        assertGe(vault.totalAssets(), sum, "vault would be insolvent");
    }
}

Plus a Handler contract that has deposit, withdraw, transfer, etc. — each bounded to realistic random inputs.