Adaptive LLM Routing for Multi-Turn Conversations with Continuously Evolving User Queries

Multi-turn conversation is the predominant form of interaction with large language models (LLMs), where user queries continuously evolve. However, existing LLM routing methods are primarily designed for single-turn interactions or fixed queries settings, overlooking the dynamic nature of multi-turn conversations and the challenge of delayed rewards, thereby limiting their ability to optimize cumulative performance. To address this challenge, we move from myopic, single-turn selection to long-horizon routing for multi-turn conversation. Accordingly, we propose ConvoRouter, which first performs MCTS to explore conversation branches induced by different LLM selections and collect trajectories with high cumulative rewards. ConvoRouter then learns a lightweight routing policy from search-derived data, augmented with retrieval-based future state approximation, enabling multi-turn routing without online search. Experiments on both open-domain and domain-specific conversation tasks across diverse candidate sets of both open-source and closed-source LLMs demonstrate that ConvoRouter significantly outperforms single LLMs and existing routing baselines in task success rate, while achieving a superior performance-cost trade-off when combined with a cost-aware reward.