ShiQ: Bringing back Bellman to LLMs

The fine-tuning of pre-trained large language models (LLMs) using reinforcement learning (RL) is generally formulated as direct policy optimization. This approach was naturally favored as it efficiently improves a pretrained LLM, seen as an initial policy. Another RL paradigm, Q-learning methods, has received far less attention in the LLM community while demonstrating major success in various non-LLM RL tasks. In particular, Q-learning effectiveness comes from its sample efficiency and ability to learn offline, which is particularly valuable given the high computational cost of sampling with LLMs. However, naively applying a Q-learning-style update to the model's logits is ineffective due to the specificity of LLMs. Our core contribution is to derive theoretically grounded loss functions from Bellman equations to adapt Q-learning methods to LLMs. To do so, we carefully adapt insights from the RL literature to account for LLM-specific characteristics, ensuring that the logits become reliable Q-value estimates. We then use this loss to build a practical algorithm, ShiQ for Shifted-Q, that supports off-policy, token-wise learning while remaining simple to implement. Finally, we evaluate ShiQ on both synthetic data and real-world benchmarks, e.g., UltraFeedback and BFCL-V3, demonstrating its effectiveness in both single-turn and multi-turn LLM settings

@article{clavier2025_2505.11081,
  title={ ShiQ: Bringing back Bellman to LLMs },
  author={ Pierre Clavier and Nathan Grinsztajn and Raphael Avalos and Yannis Flet-Berliac and Irem Ergun and Omar D. Domingues and Eugene Tarassov and Olivier Pietquin and Pierre H. Richemond and Florian Strub and Matthieu Geist },
  journal={arXiv preprint arXiv:2505.11081},
  year={ 2025 }
}