How do you actually stop an agent before it does something destructive?

I see a lot of posts about agents going wrong after the fact. Wrong account touched, a delete that shouldn't have run, a tool call that burned way more budget than it should have. What I can't find much on is stopping it before it happens.

Right now all I really have is prompt instructions telling the agent what not to do, plus manual review for anything touching production. That works, but its kind of tiring . A system prompt is just so basic, not real enforcement.

how are you devssecure your agents ( whcih primarily run my saas) not just telling it what not to do. is there any repo or software to catch prompt injection before it reaches a tool call. And do you have a spend cap that actually stops execution, not just alerts you after the fact.

if anyone has every experienced anyhting like this, how did you guys fix it or did u find it?