Artifact or Flaw? Rethinking Prompt Sensitivity in Evaluating LLMs

ACL ARR 2025 May Submission6220 Authors

Abstract: Prompt sensitivity, referring to the phenomenon where minor variations in phrasing lead to significant changes in large language model (LLM) performance, has been widely accepted as a core limitation of LLMs. In this work, we revisit this issue and ask: Is the widely reported high prompt sensitivity truly an inherent weakness of LLMs, or is it largely an artifact of evaluation processes? To answer this question, we systematically evaluate 7 LLMs (e.g., GPT and Gemini family) across 6 benchmarks, including both multiple-choice and open-ended tasks on 12 diverse prompt templates. We find that much of the prompt sensitivity stems from heuristic evaluation methods, including log-likelihood scoring and rigid answer matching, which often overlook semantically correct responses expressed through alternative phrasings, such as synonyms or paraphrases. When we adopt LLM-as-judge evaluations, we observe a substantial reduction in performance variance and a consistently higher correlation in model rankings across prompts. Our findings suggest that modern LLMs are more robust to prompt templates than previously believed, and that prompt sensitivity may be more an artifact of evaluation than a flaw in the models.

Paper Type: Short

Research Area: Resources and Evaluation

Research Area Keywords: Large Language Models (LLMs),Prompt Sensitivity,LLM-as-a-Judge

Contribution Types: Model analysis & interpretability

Languages Studied: English

Submission Number: 6220