in terms of "AI": agent is a marketing term, it has no legal meaning
it's a piece of non-deterministic software running on someone's computer
who is responsible for its actions? hardly clear cut
The same as any other computer program: the operator of the program.
The person who chose to run it (and tell it what to do) is responsible for its actions. If you don't want to be responsible for something nondeterministic software does, then don't let nondeterministic software do that thing.