The billing surprise problem is real, but there's a deeper one: exit code 0 with silent deviations. K9 Audit complements OTel — instead of just tracing tokens and latency, it records intent vs actual outcome for every tool call as a CIEU five-tuple with SHA256 hash chain. You know not just what the agent did, but whether it did what it was supposed to. Fully local, zero cloud required. https://github.com/liuhaotian2024-prog/K9Audit