One of the most common surprises in crypto taxes is that swapping one cryptocurrency for another — say, trading Bitcoin for Ethereum — is treated as a taxable sale by the IRS.
At the moment of the trade, you're considered to have sold your Bitcoin at its current market value, which means you owe tax on any gain over what you originally paid.
The fact that you never touched a dollar doesn't matter; the IRS sees it as two transactions happening simultaneously — a sale and a purchase.
Before 2018, some taxpayers tried to argue these were "like-kind exchanges" that didn't trigger tax under Section 1031, but the Tax Cuts and Jobs Act shut that door by limiting 1031 exchanges to real estate only.
If you've been trading between coins without tracking each swap, it's worth reviewing your transaction history and calculating your cost basis before you file — the gains can add up faster than people expect.