When a new low-cost model appears, launch-day benchmarks tend to dominate the discussion. Developers rarely need a global leaderboard; they need to know whether the model breaks a critical parse function, emits an unsafe shell command, or changes behavior on a repo-specific bug fix. That answer cannot come from a vendor chart. This article presents a small regression harness for comparing a candidate model with a baseline on a handful of private, deterministic tasks. The harness is useful whe...