Collaborative Multi-Agent Test-Time Reinforcement Learning for Reasoning
Authors: name: Zhiyuan Hu, name: Yunhai Hu, name: Juncheng Liu, name: Shuyue Stella Li, name: Yucheng Wang, name: Zhen Xu, name: See-Kiong Ng, name: Anh Tuan Luu, name: Xinxing Xu, name: Bryan Hooi, name: Cynthia Breazeal, name: Hae Won Park
Organization: Massachusetts Institute of Technology

Multi-agent systems have evolved into practical LLM-driven collaborators for many applications, gaining robustness from diversity and cross-checking. However, multi-agent RL (MARL) training is resource-intensive and unstable: co-adapting teammates induce non-stationarity, and rewards are often sparse and high-variance. Therefore, we introduce Multi-Agent Test-Time Reinforcement Learning (MATTRL), a framework that injects structured textual experience into multi-agent deliberation at inference time. MATTRL forms a multi-expert team of specialists for multi-turn discussions, retrieves and integrates test-time experiences, and reaches consensus for final decision-making. We also study credit assignment for constructing a turn-level experience pool, then reinjecting it into the dialogue. Across challenging benchmarks in medicine, math, and education, MATTRL improves accuracy by an average of 3.67\% over a multi-agent baseline, and by 8.67\% over comparable single-agent baselines. Ablation studies examine different credit-assignment schemes and provide a detailed comparison of how they affect training outcomes. MATTRL offers a stable, effective and efficient path to distribution-shift-robust multi-agent reasoning without tuning.