LLM Safety From Within: Detecting Harmful Content with Internal Representations
Difan Jiao Department of Computer Science, University of Toronto, Yilun Liu Ludwig-Maximilians-Universität München, Ye Yuan McGill University, McGill University and Mila - Quebec Artificial Intelligence Institute, Zhenwei Tang University of Toronto, Linfeng Du, Haolun Wu, Ashton Anderson Department of Computer Science, University of Toronto
Guard models are widely used to detect harmful content in user prompts and LLM responses. However, state-of-the-art guard models rely solely on terminal-layer representations and overlook the rich safety-relevant features distributed across internal layers. We present SIREN, a lightweight guard model that harnesses these internal features. By identifying safety neurons via linear probing and combining them through an adaptive layer-weighted strategy, SIREN builds a harmfulness detector from LLM internals without modifying the underlying model. Our comprehensive evaluation shows that SIREN substantially outperforms state-of-the-art open-source guard models across multiple benchmarks while using 250\times fewer trainable parameters. Moreover, SIREN exhibits superior generalization to unseen benchmarks, naturally enables real-time streaming detection, and significantly improves inference efficiency compared to generative guard models. Overall, our results highlight LLM internal states as a promising foundation for practical, high-performance harmfulness detection.
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Proceedings of the 64th Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers) · 2026 · 正式发表