Claude code does utilize both the full Sonnet model and the lighter Haiku model in an automatic way. When you exit a Claude code session, it gives you the stats (tokens, cost, etc). I expect there’s a way to get this in-session as well.

I believe you can just hit /cost within a session for this

Only if you're using it without a subscription, and without one it doesn't take much usage to get to $20 in a month