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.

Mhad2021 — Tue, 18 Aug 2026 18:17:16 +0000

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:

hard caps per action / day / lifetime (checked against a ledger, so a restart can't reset them)
an allowlist where empty means nobody
a kill switch that fails closed
a blast-radius cap for the "pointed it at the wrong wallet" case
an append-only ledger you can verify

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.

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

Mhad2021 — Tue, 18 Aug 2026 18:15:34 +0000

DEV Community: Mhad2021

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