Cheap AI code reviewers miss security bugs far more often than frontier models
A code review vendor benchmarked a low-cost LLM (Luna, ~$1.20/million tokens) against a frontier model (Astra) across 50 pull requests from projects including Keycloak, Grafana, and Sentry. Luna detected bugs at roughly 20 times lower cost per verified finding, and performed comparably on routine data and logic issues. However, Luna caught only 9 of 24 security bugs versus Astra's 19, and struggled significantly on Keycloak's identity and access management code, where it also produced false positives at a much higher rate. Two critical authorization flaws — a reusable recovery code and a permission override bug — were missed by Luna entirely, as both required reasoning about the broader permission model rather than local code patterns. The study's authors recommend using cheaper models as a triage layer for routine changes while routing authentication and authorization code to more capable models, regardless of per-token cost.
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