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I am experimenting with Uniswap's swapExactTokensForTokensSupportingFeeOnTransferTokens method by attempting to swap USDC -> WETH -> UNI-v2 using ethers.js just for the sake of understanding how the function works. My transaction fails with error TransferHelper: TRANSFER_FROM_FAILED that in my experience is almost always caused by not calling the approve method before the swap. Surely enough, when I check the parity trace on Goerli's etherscan.io I see that the error returned from the last call in my transaction is (ERC20: transfer amount exceeds allowance). My question is, which contract must I approve here? Approving Uniswap's v2 router did not work.

Here is such a failed transaction: https://goerli.etherscan.io/tx/0x677fef980f6e7ee813282811cb2edcf8bc71ceb6f61a9969574506084e89b112

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  • Yes, the allowance must be the issue. Looking at your Tx history, looks like you have succeed approving and swapping now?
    – kfx
    Oct 25, 2022 at 16:00
  • Indeed I have, I will write the answer shortly
    – astralmaster
    Oct 25, 2022 at 16:30

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The issue was that I was approving UniswapV3's contract to spend balance of the token in question but swapExactTokensForTokensSupportingFeeOnTransferTokens method was called on Uniswap's V2 router contract. Once I gave V2 Router contract the approval to spend, the transaction worked fine.

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