In July 2025, a developer's Claude Code instance hit a recursion loop and burned through 1.67 billion tokens in 5 hours, generating an estimated $16,000 to $50,000 in API charges before anyone noticed. The agent did not crash. It did not throw an error. It just kept calling tools, getting confused, calling more tools, and silently accumulating cost. Old software crashes. LLM agents spend. This is the failure mode most teams discover the hard way. You design a clean tool interface, the agent works in your test environment, you ship it to production, and three weeks later something hits an edge case that sends the agent into a loop. The patterns below are what we have used to prevent agent loops and silent failures across production LLM systems handling thousands of tool calls per day. None of them are about better prompts. They are about better tool design. Why agent loops happen in the first place Before the patterns, it helps to understand the failure mode precisely.…