This post is for people who file taxes in the U.S. and want to be in compliance.
For those of you who refuse to have anything to do with taxes and laws, that's fine.  But this post isn't for you.
Some of you still may not be able to refrain from replying ph*ck the IRS bitcoin's gonna burn their shit down, or words to that effect.  Whatever.

The IRS has made a very big change in the calculation of crypto capital gains, which will phase in completely in 2026.
2025 is a transition year that the IRS calls "safe harbor."  The process of transitioning to the new capital gains tax calculation regime by way of this safe harbor is pretty complicated, but what did you expect from the IRS?
First you have to choose an allocation method for assigning basis to your coin holdings, and it has to be done wallet by wallet -- no longer are your coins in one big heap, so to speak, but every wallet has to have the capital gains on the coins sold from it calculated separately.
The two methods of assigning basis to your sells are:  global allocation, and specific identification.  You choose one allocation method and it applies to all your wallets.  A "wallet" can be a private wallet ("unhosted") or a custodial account like an exchange.
If you use global allocation, each wallet has to have its capital gains/losses calculated by FIFO.  So you'll need a strategy, like putting coins with a high cost basis in one wallet and coins with a low cost basis in another wallet.  That way you can sell from the wallet with the high basis coins, in order to lower your taxes.  If all your coins were in one wallet, the FIFO requirement would draw on the oldest basis, without regard to whether it's high or low.  And since older coins might have lower cost basis, you could end up paying more tax on them.
If you use specific allocation, you choose the cost basis for the tax calculation of every transaction.  And you have to declare the allocation before you sell.
The extra control this gives you might sound good, but it could make doing your taxes a monster headache if you have more than a just few transactions.

This is how I understand it.  Do your own due diligence.