Spent two days convinced our prompt had a bug. Prompt hadn't changed in months.

A developer spent two days troubleshooting a perceived bug in a prompt after output quality on a classification task declined. The prompt had not changed in months, and initial rewrites did not help. Investigation of API logs revealed that the model itself had implicitly changed its behavior on edge cases, leading to incorrect classifications. The issue was resolved by pinning to a specific dated model version, highlighting a systemic oversight where model changes were not visible or trackable.

This developer's account highlights the often-overlooked issue of implicit model behavior changes, unlike typical prompt engineering problems, and how pinning versions can resolve it.