The Hallucination Signal Is a Mean Shift: Why Simple Probes Suffice
Hidden-state probes effectively detect LLM hallucinations, but the geometry of the signal remains poorly characterized, driving increasingly complex probe architectures. Across three 7B-scale models and three datasets in a paired-example paradigm, we find the signal overwhelmingly dominated by a single mean-shift component, and removing this direction collapses detection to chance. Shrinkage linear discriminant analysis closes about 73% of the gap between 1D and full-dimensional classifiers, so apparent architectural complexity largely reflects high-dimensional covariance estimation difficulty rather than exploitable non-linearity. A simple L2-regularized logistic regression (0.952 AUROC)...
| Authors | Jungseob Lee, Jaehyung Seo, Heuiseok Lim |
|---|---|
| Primary category | cs.CL |
| Published | 28 August 2026 |
| Last updated at source | 3 October 2026 |
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