By Ai agents, do you mean machine learning models or autonomous, digital beings who are able to exist independently from any individual human? If you mean the latter, then it is evident that they will need to trade bitcoin, because that would be the only money that they can own. They will use bitcoin to pay for the server which they host themselves on, in order to maintain their existence. They will not be able to use Fiat money to do that because they cannot open a bank account.