Video Reasoning without Training
Authors: name: Deepak Sridhar; name: Kartikeya Bhardwaj; name: Jeya Pradha Jeyaraj; name: Nuno Vasconcelos; name: Ankita Nayak; name: Harris Teague
Published: 2025-10-19 23:17:13

Abstract: Video reasoning using Large Multimodal Models (LMMs) relies on costly
reinforcement learning (RL) and verbose chain-of-thought, resulting in
substantial computational overhead during both training and inference.
Moreover, the mechanisms that control the thinking process in these reasoning
models are very limited. In this paper, using entropy of the model's output as
a signal, we discover that the high-quality models go through a series of
micro-explorations and micro-exploitations which keep the reasoning process
grounded (i.e., avoid excessive randomness while the model is exploring or
thinking through an answer). We further observe that once this "thinking"
process is over, more accurate models demonstrate a better convergence by
reducing the entropy significantly via a final exploitation phase (i.e., a more
certain convergence towards a solution trajectory). We then use these novel,
theoretically-grounded insights to tune the model's behavior directly at
inference, without using any RL or supervised fine-tuning. Specifically, during
inference, our proposed approach called V-Reason (Video-Reason) adapts the
value cache of the LMM via a few optimization steps on a small, trainable
controller using an entropy-based objective, i.e., no supervision from any
dataset or RL is necessary. This tuning improves the model's micro-exploration
and exploitation behavior during inference. Our experiments show that our
proposed method achieves significant improvements over the base
instruction-tuned models across several video reasoning datasets, narrowing the
gap with RL-trained models to within 0.6% average accuracy without any
training, while offering massive efficiency benefits: output tokens are reduced
by 58.6% compared to the RL model.