Adaptive Prompt Optimization for Open-Ended Tasks: Uncertainty Preference as a Secondary Signal

Prompt optimizers are widely used to create high-quality prompts for Large Language Models (LLMs), but their effectiveness remains unstable in practice. This instability is caused by the misalignment between conservative needs (e.g., safety compliance) and open-ended goals (e.g., creative writing). To address this, we propose a semantic-entropybased method, using task uncertainty to guide prompt optimization. Specifically, we measure the task's uncertainty level with pre-defined templates, then use this measure to direct prompt optimization: selecting high-entropy prompt candidates for creative tasks and lowentropy candidates for conservative ones. Extensive experiments across various model families demonstrate that our method consistently outperforms baselines by effectively adjusting entropy levels. Our approach requires no training, works with black-box models, and integrates easily into existing prompt optimizers. Code is available at https://github.com/ syzonline/Prompt-Evolving .