Getting less tokens in removeLiquidity call than what were added in addLiquidity call #8

dt6120 · 2024-10-08T06:27:26Z

dt6120
Oct 8, 2024

Hi everyone,

I created a test function to play around with adding and removing liquidity using the Router V2 contract on a forked ETH mainnet rpc. I observed that the amount of tokens I received on removing liquidity were less than what were added during the add liquidity call. Both calls are happening in the same transaction, so I'm assuming pool reserves ratio is also not changing. Also, I think there was no fees for both these calls. Any idea what could be the reason here?

Test snippet

address user = makeAddr("user");

uint256 INITIAL_ETH_BALANCE = 10 ether;
uint256 INITIAL_TOKEN_BALANCE = 10000e18;

function test_removeLiquidity() public {
        deal(user, INITIAL_ETH_BALANCE);
        deal(DAI, user, INITIAL_TOKEN_BALANCE);

        vm.startPrank(user);
        IERC20(DAI).approve(UNISWAP_V2_ROUTER_02, type(uint256).max);
        (uint256 amountAs, uint256 amountBs, uint256 liquidity) = routerV2
            .addLiquidityETH{value: 1 gwei}({
            token: DAI,
            amountTokenDesired: 3000e9,
            amountTokenMin: 0,
            amountETHMin: 0,
            to: user,
            deadline: block.timestamp
        });
        vm.stopPrank();

        console.log("\n===   ADD LIQUIDIY   ===");
        console.log("DAI added", amountAs);
        console.log("ETH added", amountBs);
        console.log("Shares minted", liquidity);

        uint256 startingEthBalance = user.balance;
        uint256 startingDaiBalance = dai.balanceOf(user);

        vm.startPrank(user);
        wethDaiPair.approve(UNISWAP_V2_ROUTER_02, type(uint256).max);
        // token, liquidity, amountTokenMin, amountETHMin, to, deadline
        (uint256 amountAe, uint256 amountBe) = routerV2.removeLiquidityETH({
            token: DAI,
            liquidity: liquidity,
            amountTokenMin: 0,
            amountETHMin: 0,
            to: user,
            deadline: block.timestamp
        });
        vm.stopPrank();

        uint256 endingEthBalance = user.balance;
        uint256 endingDaiBalance = dai.balanceOf(user);

        assertEq(wethDaiPair.balanceOf(user), 0);
        assertEq(endingDaiBalance - startingDaiBalance, amountAe);
        assertEq(endingEthBalance - startingEthBalance, amountBe);

        console.log("\n===   REMOVE LIQUIDIY   ===");
        console.log("DAI returned", amountAe);
        console.log("ETH returned", amountBe);

        console.log("\n===   NET DIFFERENCE   ===");
        console.log("DAI difference", amountAs - amountAe);
        console.log("ETH difference", amountBs - amountBe);
    }

Output

===   ADD LIQUIDIY   ===
  DAI added 2427090181494
  ETH added 1000000000
  Shares minted 25049450063

===   REMOVE LIQUIDIY   ===
  DAI returned 2427090181462
  ETH returned 999999999

===   NET DIFFERENCE   ===
  DAI difference 32
  ETH difference 1