Two wallets for comparison:

1) Coinbase Pro wallet with a CSV 'fills' import.

2) The same Coinbase Pro Wallet but this time with just API data.

For each trade the CSV cost basis is different to the API. As far as I can tell the fee in the CSV is being interpreted incorrectly by Koinly, and is skewing the cost basis.

Example Transaction

I paid £237.25 for some ADA, and paid a 0.25% exchange fee of £0.59 on top.

In the wallet with the CSV source Koinly has taken the fiat value + exchange fee to represent the fiat value. It appears as if the fee is then added again to arrive at the cost basis. So the exchange fee is applied twice.

I think the API has it correct, but I bow to those with greater tax knowledge :) Cost basis = Fiat value + exchange fee.

Pics attached showing api vs CSV.

CSV transaction line below

ADA-GBP,BUY,2022-06-22T08:30:26.029Z,626.49,0.5931294075,-237.8448924,GBP

The CSV can be made to match the API if I amend the CSV to deduct the respective fee from each item in the 'Total' column.

My tax figures for the CSV were way out before noticing this, so it is abit of a concern. Any one else replicate this?