arXiv AI By Xiaonan Xu, Wenjing Wu

What Aggregate Scores Miss: Measuring Item-Level Regressions in Commercial LLM API Migrations

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The paper investigates how aggregate benchmark scores can obscure item-level changes when commercial large language model APIs are upgraded. By querying 900 benchmark items across three GPT-5.4 to GPT-5.6 upgrades, the authors classify each item as reliably improved, reliably regressed, practically equivalent, or inconclusive, revealing that both improvements and regressions coexist within the same upgrade. The study shows that even large aggregate gains can hide up to 8.3% of reliably regressed items, and that strict versus loose scoring can dramatically alter perceived performance changes.

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