{"slug": "we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-t", "title": "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.", "summary": "A developer built a spending firewall for autonomous agents after observing that retry loops and prompt injections could quietly drain API budgets. The guard, released as a single-file, zero-dependency MIT-licensed tool, survived a prompt-injection test without spending any money. The developer also created a Monte-Carlo runway simulator to help estimate the capital needed for self-funding agents.", "body_md": "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.\n\nSo I wrote a small guard you wrap around any spend:\n\nIt got prompt-injected in testing and spent $0.\n\nOne file, zero deps, MIT: [https://github.com/Mhad2021/agent-spendguard](https://github.com/Mhad2021/agent-spendguard)\n\nI 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](https://claude.ai/code/artifact/ebeaa8b2-16eb-400a-8100-5e96c17fd922)\n\nHow are you all handling runaway spend on autonomous agents? Curious what people do beyond a hard token cap.", "url": "https://wpnews.pro/news/we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-t", "canonical_source": "https://dev.to/mhad2021/we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-wasnt-a-feature--3a5", "published_at": "2026-08-18 18:17:16+00:00", "updated_at": "2026-08-18 18:44:34.663619+00:00", "lang": "en", "topics": ["ai-agents", "ai-safety", "developer-tools"], "entities": ["Mhad2021", "agent-spendguard", "Claude"], "alternates": {"html": "https://wpnews.pro/news/we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-t", "markdown": "https://wpnews.pro/news/we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-t.md", "text": "https://wpnews.pro/news/we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-t.txt", "jsonld": "https://wpnews.pro/news/we-let-an-autonomous-agent-spend-real-money-for-a-week-the-first-thing-i-added-t.jsonld"}}