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Rephrasing a judge prompt without changing its meaning changed the verdicts often enough to reverse model rankings, and LLM judges degraded 6 to 9 times more than human annotators under the same rephrasing

AspireAI community · arXiv
LLM-as-a-Judge is now standard for evaluating models, and most work assumes a single judge prompt is a stable evaluation instrument. This paper studies paraphrastic rephrasings, which are validated as semantically equivalent but differ in surface realization, and reports a Prompt Lottery: across a large-scale study spanning diverse evaluation tasks, judge families and model backbones, these variations alter evaluation outcomes and in some settings reverse relative model rankings. In a user study with 10 human annotators, LLM judges showed 6 to 9 times larger agreement degradation under prompt rephrasing than humans did, falling below the threshold generally accepted in usability studies, which is the paper's argument that single-prompt evaluation is not robust enough for model ranking. The authors also show that multi-prompt aggregation substantially improves robustness by emphasizing judgements that stay consistent across prompt variants. The limitation is that the 10-annotator user study is small, and the benchmark selection determines which tasks show the effect.
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