So I've been struggling to get a working swap contract that swaps an exact token for a token via uniswap.

I have multiple versions at this point, as I've tried several different approaches. After watching videos, reading articles, reviewing solidity & uniswap docs, scouring forums, I'm coming up on various errors on each attempt at this.

For my current attempt, I wrote the below contract, deployed on Kovan using remix, passed the address of test USDC (which I have in my wallet), 10 for amount in, 1 for amount min. I set "message value" to 100000000 gwei, gas limit 3000000.

Failed TX: https://kovan.etherscan.io/tx/0xcb9c3dc323a051e39ab6db06d7bcf848253fb92ed83a40a1bb803a276e29895f

I'm not sure what the fail error is here.

Below is the code for the most recent iteration I've been working on, any guidance, suggestions, or advice would be deeply appreciated!

Here is my code:

// SPDX-License-Identifier: Unlicensed
pragma solidity ^0.7.0;
//this is my main swap for now
interface IUniswap {
   function swapExactTokensForTokensSupportingFeeOnTransferTokens(
        uint amountIn,
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline)
        returns (uint[] memory amounts);
        function WETH() external pure returns(address);
    function swapExactTokensForETH(uint amountIn, uint amountOutMin, address[] calldata path, address to, uint deadline)
        returns (uint[] memory amounts);
interface IERC20 {
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
    function approve(address spender, uint256 amount) external returns (bool);
contract Swap {
    IUniswap uniswap;
    address internal constant UNISWAP_ROUTER_ADDRESS = 0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D;
    address private DAI = 0x4F96Fe3b7A6Cf9725f59d353F723c1bDb64CA6Aa;
    address private USDC = 0xb7a4F3E9097C08dA09517b5aB877F7a917224ede;
    address private WETH = 0xd0A1E359811322d97991E03f863a0C30C2cF029C;
    constructor() {
        uniswap = IUniswap(UNISWAP_ROUTER_ADDRESS);
    function swapFeeTokensforTokens(
        address token,
        uint amountIn,
        uint amountOutMin)
        external {
            IERC20(token).transferFrom(msg.sender, address(this), amountIn);
            address[] memory path = new address[](3);
            path[0] = USDC;
            path[1] = uniswap.WETH();
            path[3] = DAI;
            IERC20(token).approve(address(uniswap), amountIn);
    // important to receive ETH
     receive() payable external {}

1 Answer 1


To my knowledge, swapExactTokensForTokensSupportingFeeOnTransferTokens does not have a return value in contrast to swapExactTokensForTokens

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