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arXiv · 2505.13360

What Prompts Don't Say: Understanding and Managing Underspecification in LLM Prompts

Chenyang Yang·Yike Shi·Qianou Ma·Michael Xieyang Liu·Christian Kästner·Tongshuang Wu
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Abstract

Prompt underspecification is a common challenge when interacting with LLMs. In this paper, we present an in-depth analysis of this problem, showing that while LLMs can often infer unspecified requirements by default (41.1%), such behavior is fragile: Under-specified prompts are 2x as likely to regress across model or prompt changes, sometimes with accuracy drops exceeding 20%. This instability makes it difficult to reliably build LLM applications. Moreover, simply specifying all requirements does not consistently help, as models have limited instruction-following ability and requirements can conflict. Standard prompt optimizers likewise provide little benefit. To address these issues, we propose requirements-aware prompt optimization mechanisms that improve performance by 4.8% on average over baselines. We further advocate for a systematic process of proactive requirements discovery, evaluation, and monitoring to better manage prompt underspecification in practice.

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Chenyang Yang, Yike Shi, Qianou Ma, Michael Xieyang Liu, Christian Kästner, Tongshuang Wu. 2025-05-19. What Prompts Don't Say: Understanding and Managing Underspecification in LLM Prompts. https://arxiv.org/abs/2505.13360

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