AAMAS Conference 2026 Conference Paper
MENSA: Leveraging Mental Simulation for In-Context Policy Improvement in LLM Agents
- Chung-Che Chang
- Erick Chandra
- Jane Yung-jen Hsu
- Yen-Ling Kuo
Large Language Model (LLM) powered agents have shown promise in sequential decision-making tasks in interactive environments. However, prior agent frameworks usually rely on advanced LLM capabilities such as planning or instruction following to carry out tasks successfully. Effectively improving the performance of an LLM agent without assuming these capabilities remains challenging. To address this issue, we propose MENtal Simulation Agent (MENSA), anovelmodel-basedapproachthatenhancesLLMagents without fine-tuning. MENSA leverages the fundamental ability of any LLMs, text completion, to generate forecasts of action-state pairs(i. e. ,transitions)forfuturetimesteps. Theseforecastsareused toconstructasetofrelevantpastexperiences, whichareprovidedto the LLM agent in context to improve its decision-making behavior. We evaluate MENSA in two challenging interactive environments, ScienceWorld and NetHack, and show that MENSA improves performance across various sizes of LLMs. Using large models (e. g. , GPT-4o-mini), MENSA outperforms previous state-of-the-art methodsby+15. 8pointsinScienceWorldandby+40. 0pointsinNetHack. Even with smaller models like Phi-3-mini, MENSA achieves a gain of +11. 9 points in ScienceWorld. Our results further suggest that MENSA is less affected by an LLM’s limitations in instructionfollowing and planning compared to baselines. Project page and code are available at: https: //roger0426. github. io/MENSA.