Using API billing would be a bummer because, as far as I know, there's no way to set up a spend cap or pre-pay the API key, correct?

They finally allowed for a hard spend cap that's easy to set a few weeks ago: https://aistudio.google.com/spend

If you're happy with API billing then just use OpenRouter: https://openrouter.ai/google/gemini-3.8-flash

Makes it easy to switch between models and I like it for exactly the reason that you're saying - I prepay and so can't accidentally spend my food budget.

Aside from the other reply, pi makes it _really easy_ to build your own usage tracking and limit machinery.

I would normally advise against such efforts for a variety of reasons (such as inaccurate tracking, etc), but specifically under pi, this mechanism has been extremely well behaved and accurate for me.

It is trivial to set up a spending cap on Google Cloud and also on Google AI Studio.

That said, it is probably easier to use APIs from FireWorks.ai, Ollama Cloud, Open Router, etc.