  Large language models (LLMs) have demonstrated powerful decision-making and
planning capabilities in solving complicated real-world problems. LLM-based
autonomous agents can interact with diverse tools (e.g., functional APIs) and
generate solution plans that execute a series of API function calls in a
step-by-step manner. The multitude of candidate API function calls
significantly expands the action space, amplifying the critical need for
efficient action space navigation. However, existing methods either struggle
with unidirectional exploration in expansive action spaces, trapped into a
locally optimal solution, or suffer from exhaustively traversing all potential
actions, causing inefficient navigation. To address these issues, we propose
ToolChain*, an efficient tree search-based planning algorithm for LLM-based
agents. It formulates the entire action space as a decision tree, where each
node represents a possible API function call involved in a solution plan. By
incorporating the A* search algorithm with task-specific cost function design,
it efficiently prunes high-cost branches that may involve incorrect actions,
identifying the most low-cost valid path as the solution. Extensive experiments
on multiple tool-use and reasoning tasks demonstrate that ToolChain*
efficiently balances exploration and exploitation within an expansive action
space. It outperforms state-of-the-art baselines on planning and reasoning
tasks by 3.1% and 3.5% on average while requiring 7.35x and 2.31x less time,
respectively.
