Missing one config line cost a solo dev over $600 in excess AI agent spend

A solo developer auditing 34 days and roughly 26,000 traces of AI agent usage discovered that 22.6% of model-routing decisions deviated from his intended policy, amounting to $1,248 in excess list-price spend. The single largest cost cluster — $657 — stemmed from a missing 'model:' line in subagent definition files, which caused cheaper mechanical tasks to silently inherit and run on the more expensive parent model. He also found that live session logs from Claude Code rewrite themselves over time, meaning usage dashboards built on those files can drift; an append-only immutable snapshot resolved the discrepancy. An attempted quality comparison between expensive and cheaper model outputs was inconclusive due to methodological flaws, including context mismatch and self-recognition bias during blind judging. The developer published the full audit, including its failures, as part of an open instrumentation SDK called traceguard, designed to track model usage locally without sending data externally.