arXiv Machine Learning
Sep 29

From Weak Data to Strong Policy: Q-Targets Enable Provable In-Context Reinforcement Learning

The paper introduces Q-Target Pretrained Transformers (QTPT), a method that replaces supervised behavior cloning with a Bellman-style Q‑target objective for in‑context reinforcement learning. QTPT retains the context‑conditioned Transformer architecture but learns to estimate action values using rewards and transitions from the context, rather than merely imitating offline actions. The authors provide theoretical analysis in stochastic linear bandits and finite‑horizon MDPs, demonstrating improved robustness to weak or suboptimal data, and empirically show gains over supervised pretraining on controlled RL benchmarks and extensions to D4RL Kitchen and AntMaze.

By Yichen Lin, Xuyuan Xiong, Xue Wang, Xiangfu Meng, Mike Mingcheng Wei, Tao Yao
arXiv Machine Learning
Oct 7

Reinforcement Learning for Hierarchical Reasoning Rewards: Minimax-Optimal Rates with Transformers

The paper studies reinforcement learning (RL) for post‑training language models on reasoning tasks, focusing on a hierarchical reward structure where each component becomes relevant only after the previous ones are resolved. It demonstrates that a Transformer‑based actor–critic algorithm that alternates between KL‑regularized policy sampling, critic fitting, and policy updates achieves minimax‑optimal rates in query budget and regularization strength, and is optimal for a fixed number of prompts. In contrast, sampling from a fixed reference distribution, as used in offline reward modeling, only yields a logarithmic regret decay, highlighting the advantage of on‑policy exploration in concentrating on high‑reward regions.

By Naoki Nishikawa, Taiji Suzuki