arXiv AI By Yilong Xu, Minrui Xu, Xinyu Zhou, Xiang Long, Haochen Tan, Zhijiang Guo

Mosaic: Multi-Session Agentic Reinforcement Learning

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Mosaic introduces Multi-Session Agentic Reinforcement Learning, allowing trajectories to span an arbitrary number of sessions rather than assuming a single continuous context. It uses critic‑free hierarchical optimization, incorporating session‑usage signals into trajectory‑level advantage estimation and shaping each session’s advantage based on its marginal contribution. Experiments on BrowseComp with Qwen3.5‑4B demonstrate that Mosaic outperforms traditional broadcasting methods in both context compaction and subagent forking, achieving better session utility and allocation while using fewer sessions on average.

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