I Can't Believe It's Not Robust: Catastrophic Collapse of Safety Classifiers under Embedding Drift

Instruction tuned reasoning models are increasingly deployed with safety classifiers trained on frozen embeddings, assuming representation stability across model updates. We systematically investigate this assumption and find it fails: normalized perturbations of magnitude $\sigma =0.02$ (corresponding to $\approx 1^{\circ }$ angular drift on the embedding sphere) reduce classifier performance from $85\mathrm{\%}$ to $50\mathrm{\%}$ ROC-AUC. Critically, mean confidence only drops $14\mathrm{\%}$, producing dangerous silent failures where $72\mathrm{\%}$ of misclassifications occur with high confidence, defeating standard monitoring. We further show that instruction-tuned models exhibit 20$\mathrm{\%}$ worse class separability than base models, making aligned systems paradoxically harder to safeguard. Our findings expose a fundamental fragility in production AI safety architectures and challenge the assumption that safety mechanisms transfer across model versions.