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

• URL: http://arxiv.org/abs/2505.13360v2
• Date: Tue, 07 Oct 2025 01:41:04 GMT
• Title: What Prompts Don't Say: Understanding and Managing Underspecification in LLM Prompts
• Abstract summary: Under-specified prompts are 2x as likely to regress across model or prompt changes, sometimes with accuracy drops exceeding 20%.<n>We propose requirements-aware prompt optimization mechanisms that improve performance by 4.8% on average over baselines.
• Score: 29.524016943865337
• License: http://creativecommons.org/licenses/by/4.0/
• 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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