BeliefNest: A Joint Action Simulator for Embodied Agents with Theory of Mind

Theory of Mind is a fundamental cognitive ability that underpins human social behavior, enabling individuals to infer the beliefs, intentions, and knowledge of others. In this paper, we propose BeliefNest, an open-source simulator designed to support research on collaborative behavior in embodied agents endowed with Theory of Mind capabilities. Recent advances in embodied agents powered by large language models (LLMs) have shown promising progress. However, there is still no platform that can explicitly represent nested belief states and integrate them with action generation mechanisms. BeliefNest addresses this gap by providing a flexible simulation framework that incorporates both hierarchical belief structures and prompt generation support. BeliefNest offers the following features: Explicit representation of nested belief states, as studied in Theory of Mind, using hierarchical simulators (see Section 3) Support for prompt generation based on each belief state, enabling the design and evaluation of methods for agent control with LLMs (see Section 5) Integration with the Minecraft environment, which is widely used in LLM agent research [1-4], and support for open-domain tasks In this paper, we describe the design and functionality of BeliefNest and demonstrate its effectiveness through experiments on false-belief tasks.