Runaway while-loop in Claude agent burns ~$47,000 in API tokens overnight
anonymous (engineering post-mortem) · Dec 1, 2025 · General agent
What happened
An engineering post-mortem titled 'The Success Tax' detailed a rogue while-loop in a Claude-based AI agent that ran unbounded overnight and consumed roughly $47,000 in API tokens. The write-up became a flagship example used to argue for 'AI circuit breakers' and FinOps-style cost controls on agents.
Causal vector
Unbounded agent loop with no token/cost ceiling or kill-switch
Source
Reported by Post-mortem reshared on LinkedIn. Sourced from a secondary summary; flagged pending a stronger primary source.
This is the textbook runaway loop. A velocity cap (‘max N calls per minute’) and a daily total cap (‘max $X/day’) return BLOCKED on the call that would push spend over the ceiling, stopping the loop cold.
The six rule types that contain this class of failure
Per-transaction cap
Any single spend above your ceiling is BLOCKED before it moves.
Daily total
Cumulative spend across all agent calls, bounded per day.
Velocity limit
Stops runaway retry loops — the #1 cause of overnight losses.
Merchant allowlist
Only approved destinations can ever receive funds.
Category rules
Flag high-risk classes (crypto, infra, refunds) for review.
Approval threshold
Above a value, the action waits for a human.
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