Large language models (LLMs) have demonstrated remarkable capabilities across various tasks. However, their ability to generate human-like text has raised concerns about potential misuse. This underscores the need for reliable and effective methods to detect LLM-generated text. In this paper, we propose IRM, a novel zero-shot approach that leverages Implicit Reward Models for LLM-generated text detection. Such implicit reward models can be derived from publicly available instruction-tuned and base models. Previous reward-based method relies on preference construction and task-specific fine-tuning. In comparison, IRM requires neither preference collection nor additional training. We evaluate IRM on the DetectRL benchmark and demonstrate that IRM can achieve superior detection performance, outperforms existing zero-shot and supervised methods in LLM-generated text detection.
@article{arxiv.2604.21223,
title = {Zero-Shot Detection of LLM-Generated Text via Implicit Reward Model},
author = {Runheng Liu and Heyan Huang and Xingchen Xiao and Zhijing Wu},
journal= {arXiv preprint arXiv:2604.21223},
year = {2026}
}