The spend cap for my first AI project was just a limit on an Anthropic API key.

Crossing your fingers that users don't find your limit, and that it's high enough for their usage anyway, is a sign that you need a real control layer.

AI spend control is not trivial

It seems like it would be easy, right? It's just a meter per user and a few if statements.

Then the questions and edge cases start. And don't really stop.