AI agent burning through tokens? How to cap the budget before the bill spikes

You write an agent, give it a loop, and go to sleep. You wake up to a $400 API bill because it got stuck trying to fix a typo all night. This happens more than anyone admits — and the fix is a mid-run budget check that catches the burn before it becomes a bill.

Why agents burn tokens

Every iteration of an agent's loop is a model call, and every model call burns tokens. When a run has no stop condition — or is stuck in a loop — it keeps calling the model long after it stopped making progress. Two things compound it:

• Loops. The single biggest cause. A repeated-tool-call loop turns a 3-step task into a 73-step token bonfire. (If that's your symptom, start with loop detection.)
• Growing context. Context accumulates every step, so the 30th model call is far more expensive than the 3rd — the burn accelerates as the run goes on.

And like a loop, runaway burn is silent. The agent looks busy right up until the bill lands.

Catch it mid-run — free, under 150ms

The fix is to check your budget while the run is happening, not after. budget_guard takes your live counters and returns advisory warnings with severity and concrete suggested actions — before you overrun:

budget_guard({
  tokens_used: 8200, token_budget: 10000,
  cost_usd: 0.42, cost_budget: 0.50,
  step: 18, step_budget: 25,
  context_pct: 0.86 })

// → { warnings: [{type:"token_burn", severity:"critical",
//                 suggestions:["trim context", "wrap up", ...] }] }

How to cap the budget properly

1. A hard cost cap per session — the backstop nothing crosses ("max $2.00 per run").
2. budget_guard on each step — the early-warning system that lets the agent react before the cap.
3. Loop detection alongside it — since a loop is the usual cause, detect_loop stops the root cause, not just the symptom.
4. Context trimming — when the guard flags context approaching its limit, summarise or prune so later calls don't balloon.

These guards are free and unlimited by design — the safety checks that protect your bill should never be a cost you hesitate over calling.