Linear Probes Transfer from Base to Distilled Language Models despite Rotation
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The paper investigates whether adding a short classification instruction after a user’s prompt improves the ability of activation probes to detect malicious inputs in large language models. Across 13 safety benchmarks and three open‑weight model families, a classification suffix consistently boosts out‑of‑distribution detection (up to ~4 AUC points) compared to no suffix, and the benefit transfers to multi‑position pooling probes used in production. The improvement stems from the classification format itself rather than the specific content of the instruction, though the optimal suffix varies with the model and readout type.
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The paper investigates how the choice of feature space affects the effectiveness of label‑only backdoor detection. By keeping the test, labels, and metric constant while swapping feature spaces, the authors show that defenses can perform very differently—e.g., a nearest‑neighbor check yields AUROC 0.546 on poisoned features but 0.995 on frozen DINOv2 features. They also demonstrate that higher detection scores do not necessarily translate to safer models and that static prototype‑based audits can be bypassed by adaptive attackers.
A 0.6B language model consistently answers YES to 1,200 logical tests, yet its behavior shows no discrimination. Linear probes reveal the correct verdict with high AUC (0.96) and transfer to unseen structures, but a single scalar readout fails due to a saturated decision threshold offset by +4.6 σ. Adjusting this threshold restores behavior accuracy from 50 % to 81 % and improves higher‑scale models, demonstrating that miscalibrated readouts, not hidden knowledge loss, drive performance gaps.