An agent evaluation set starts healthy. It contains the obvious intents, a few tool failures, and the happy paths used during development. Six months later, production has changed. New tools exist. Users phrase requests differently. A fallback introduced last quarter now handles 30% of traffic. Yet CI still runs the same twelve examples and reports green. The problem is not only stale prompts. The eval set itself is rotting. A durable evaluation program needs a controlled path from ob...