Post 1/8: At 3 AM, an AI agent I built made 21 API calls to a premium LLM endpoint.
Each call cost $133.
That's $2,800 gone in 60 seconds while I was asleep.
The budget alert email arrived at 6:14 AM. Four hours too late.
So I built a firewall. 🧵
Post 2/8: The problem: AI agents don't know they're spending money.
They make autonomous decisions about token consumption. Retry loops, context accumulation, tool call spirals — each one costs real dollars that the agent has zero awareness of.
Post 3/8: Existing tools don't help:
❌ API rate limits protect the provider, not you ❌ Budget alerts arrive after the damage ❌ Cost dashboards show what happened, not what's about to happen ❌ Monitoring doesn't scale past 3 agents
All reactive. None preventive.
Post 4/8: I built AgentShield — a per-transaction spend firewall.
Every API call is evaluated against rules YOU set before it executes:
• Transaction limits ($500 max per call) • Daily caps ($2,000 max per day) • Velocity detection (max 10 calls/hour) • Merchant allowlists (only approved APIs)
Post 5/8: The evaluation takes <1ms. Pure Python stdlib. Zero dependencies.
If a call violates a rule, it's BLOCKED before it reaches the API provider. The agent gets a structured error. Your wallet stays closed.
Post 6/8: Two rules that came from production experience:
1️⃣ session_budget — Daily caps miss the "2 AM cron burst" where one session eats the whole day. Session-scoped budgets with decay tightening catch it.
2️⃣ cascade_cost — A $0.50 call with 30% failure rate and $5 retry = $2.00 expected cost. The rule blocks calls that look cheap but compound on failure.
Post 7/8: 56 labeled test scenarios. 7 composable rule types. MIT licensed.
The eval gym is a universal benchmark — use it to test YOUR spend-control implementation, not just ours.
Live eval: https://agentshield.fly.dev/eval
Post 8/8: AgentShield is live now: 📦 pip install agentshield 🌐 https://agentshield.fly.dev 🔧 https://github.com/kindrat86/agentshield
Free to self-host. $19/mo managed. $299 professional audit (money-back guarantee if we don't find $299 in waste).
Built because budget alerts shouldn't arrive by email.