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Version 2 of your agent passes every case in the eval dataset. A week after release, production complaints are unchanged. What happened?

A perfect score on the eval set and no change in the field means the eval set and the field disagree about something. There are four usual suspects, and one of them means the perfect score was never evidence at all. How do you tell which?

Updated Nov 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.

A perfect score on the eval set and no change in the field means the eval set and the field disagree about something. There are four usual suspects, and one of them means the perfect score was never evidence at all. How do you tell which?

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