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Don't Mix Rewards, Mix Policies: Policy Decomposition and Optimization for Multi-Reward RL

Research LLMs & Foundation Models

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TL;DR - PRISM is a multi-reward RL framework for LLM post-training that decomposes optimization into separate per-reward policies instead of blending reward signals, reducing objective conflict and adding inference-time controllability.

  • Frames the core problem as a worsened "alignment tax": combining multiple reward objectives causes trade-offs/conflicts and unstable, inefficient post-training.
  • Instead of compositing rewards, PRISM trains a set of standalone positive policies plus one global negative policy, then composes them in policy space.
  • Policy composition at inference enables flexible preference control without retraining.
  • Reported to consistently beat existing multi-reward RL baselines on scientific reasoning, tool-use reasoning, and helpfulness-safety alignment.

Sources (1)

Don't Mix Rewards, Mix Policies: Policy Decomposition and Optimization for Multi-Reward RL

arXiv cs.AI Ruiming Liang, Yi Zhong, Yizhen Yuan, Yinan Zheng, Tianyi Tan, Tianyue Wang, Haiyun Guo, Jinqiao Wang, Xianyuan Zhan 2026-07-31 arXiv:2607.29246
Public signals Semantic Scholar citations 0 · Semantic Scholar influential citations 0
Providers: Hugging Face · N/A OpenAlex · N/A Publisher · N/A Semantic Scholar · Citations 0 · Influential citations 0 X · N/A Fetched 2026-08-17 09:49:45.125529 UTC

TL;DR - PRISM is a multi-reward RL framework for LLM post-training that decomposes optimization into separate per-reward policies instead of blending reward signals, reducing objective conflict and adding inference-time controllability.

  • Frames the core problem as a worsened "alignment tax": combining multiple reward objectives causes trade-offs/conflicts and unstable, inefficient post-training.
  • Instead of compositing rewards, PRISM trains a set of standalone positive policies plus one global negative policy, then composes them in policy space.
  • Policy composition at inference enables flexible preference control without retraining.
  • Reported to consistently beat existing multi-reward RL baselines on scientific reasoning, tool-use reasoning, and helpfulness-safety alignment.
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