TL;DR — A swap wrapper on Uniswap V2 that derives the minimum acceptable output on-chain from the current price, instead of trusting a minAmountOut computed by a frontend against stale data. Deployed to Sepolia, 20 passing Foundry tests.
Problem
Most swap UIs compute the minimum acceptable output off-chain and hand it to the router. Between quote and execution the price moves — users get sandwiched, or the transaction reverts with a confusing error. The protection is only as fresh as the frontend’s last read.
Approach
The user expresses intent as a slippage tolerance in basis points (“at most 0.5%”). At execution time the contract calls getAmountsOut on the Uniswap V2 router, derives minOut from that live quote, and only then executes the swap — so enforcement uses the actual reserves in the same transaction. A configurable protocol fee (capped at 5%) is deducted before the swap, and a previewSwap view lets the frontend show the expected output and fee up front.
Security
Every path guards first (zero amount, path length, deadline), tokens are pulled with SafeERC20, and the router is the single external call. A deadline parameter rejects stale transactions.
uint256[] memory expected = IV2Router(router).getAmountsOut(amountAfterFee, path);
uint256 expectedOut = expected[expected.length - 1];
uint256 minOut = expectedOut - (expectedOut * maxSlippageBps / BPS_BASE);
IERC20(path[0]).approve(router, amountAfterFee);
uint256[] memory amounts = IV2Router(router).swapExactTokensForTokens(
amountAfterFee, minOut, path, msg.sender, deadline
);The frontend (Next.js + wagmi) reads quotes for display only — it never participates in enforcement.