MutantPrompt: Prompt Optimization via Mutation Under a Budget on Modest-sized LMs

Arijit Nag, Animesh Mukherjee, Niloy Ganguly, Soumen Chakrabarti


Abstract
Prompts serve as a critical instruction interface to unlock the diverse capabilities of Large Language Models (LLMs), thus directly influencing the quality of their outputs. While prompt engineering has shown great promise, identifying optimal prompts remains a significant challenge, particularly for low-resource languages, which often face higher computational costs due to increased token generation and limited gold standard task data. In response, we propose MutantPrompt, a framework that leverages multi-armed bandit algorithms to efficiently identify optimal prompts tailored to low-resource languages. By framing prompt selection as an exploration-exploitation problem under a fixed computational budget, the framework dynamically balances exploring new prompts with exploiting known high-performing ones. We demonstrate the framework’s effectiveness across multiple low-resource Indic language tasks, including classification, question-answering and causal reasoning using three small parameter-size LLMs. The results highlight the cost efficiency of the search method in finding optimal prompts and resulting performance improvements.
Anthology ID:
2025.findings-acl.1139
Volume:
Findings of the Association for Computational Linguistics: ACL 2025
Month:
July
Year:
2025
Address:
Vienna, Austria
Editors:
Wanxiang Che, Joyce Nabende, Ekaterina Shutova, Mohammad Taher Pilehvar
Venues:
Findings | WS
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
22082–22092
Language:
URL:
https://preview.aclanthology.org/acl25-workshop-ingestion/2025.findings-acl.1139/
DOI:
Bibkey:
Cite (ACL):
Arijit Nag, Animesh Mukherjee, Niloy Ganguly, and Soumen Chakrabarti. 2025. MutantPrompt: Prompt Optimization via Mutation Under a Budget on Modest-sized LMs. In Findings of the Association for Computational Linguistics: ACL 2025, pages 22082–22092, Vienna, Austria. Association for Computational Linguistics.
Cite (Informal):
MutantPrompt: Prompt Optimization via Mutation Under a Budget on Modest-sized LMs (Nag et al., Findings 2025)
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PDF:
https://preview.aclanthology.org/acl25-workshop-ingestion/2025.findings-acl.1139.pdf