Tool-use API design for LLMs: 5 patterns that prevent agent loops and silent failures

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 work