We let an autonomous agent spend real money for a week. The first thing I added wasn't a feature — it was a spending firewall.

I kept giving agents real API keys and budgets, and the scary part was never a bad answer — it was a retry loop or a prompt injection quietly spending money while I wasn't watching.

So I wrote a small guard you wrap around any spend:

It got prompt-injected in testing and spent $0.

One file, zero deps, MIT: https://github.com/Mhad2021/agent-spendguard I also built a free Monte-Carlo "runway simulator" to answer how much capital a self-funding agent needs before it breaks even: https://claude.ai/code/artifact/ebeaa8b2-16eb-400a-8100-5e96c17fd922

How are you all handling runaway spend on autonomous agents? Curious what people do beyond a hard token cap.