Most enterprises can instrument their agents but can't prove they work: 89 percent have adopted observability while only 52 percent run evaluations, and quality is now the top barrier to getting agents into production. This article frames evaluation as the engineering discipline that closes that gap, covering what to measure across the end-to-end, trajectory, and component altitudes, how the offline-online-optimize lifecycle runs in practice, and how the native eval tooling from AWS, Google Cloud, and Microsoft compares to the third-party ecosystem.