Ignore the KL Penalty! Boosting Exploration on Critical Tokens to
  Enhance RL Fine-Tuning
Authors: name: Jean Vassoyan; name: Nathanaël Beau; name: Roman Plaud
Published: 2025-02-10 14:56:25

Abstract: The ability to achieve long-term goals is a key challenge in the current
development of large language models (LLMs). To address this, pre-trained LLMs
can be fine-tuned with reinforcement learning (RL) to explore solutions that
optimize a given goal. However, exploration with LLMs is difficult, as a
balance has to be struck between discovering new solutions and staying close
enough to the pre-trained model, so as not to degrade basic capabilities. This
is typically controlled with a Kullback-Leibler (KL) penalty. In this paper, we
investigate the exploration dynamics of a small language model on a simple
arithmetic task. We show how varying degrees of pre-training influence
exploration and demonstrate the importance of "critical tokens" which have a
dramatic impact on the final outcome. Consequently, we introduce a simple
modification to the KL penalty that favors exploration on critical tokens,
increasing the efficiency of the RL fine-tuning stage.