Emil Ryd

Thanks to Adam Karvonen, Arjun Khandelwal, Arun Jose, Fabien Roger, James Chua, Nic Kruus, & Sukrit Sumant for helpful feedback and discussion.

Thanks to Claude Opus 4.5 for help with designing and implementing the experiments.

Introduction

We study to what extent LLMs can verbalize their internal reasoning. To do this, we train LLMs to solve various games and tasks (sorting lists, two-hop lookup, a custom grid-world game, and chess) in a single forward pass. After training, we evaluate them by prompting them with a suite of questions asking them to explain their moves and the reasoning behind it, e.g. “Explain why you chose your move.”, “Explain the rules of the game”).

We find that:

Models trained to solve tasks in a single forward pass are not able to verbalize a correct reason for their actions[1]. Instead, they hallucinate incorrect reasoning.
When trained to solve a very simple sorting task (sorting lists in increasing order) the models are able to verbalize the sorting rule, although unreliably. Furthermore, we believe this might be mostly due to the sorting rule being the most likely.
When trained to solve a previously unseen task (grid-world game) with reasoning via RL [...]