Before the Last Token: Diagnosing Final-Token Safety Probe Failures
Final-token safety probes monitor a single hidden state after prompt prefill, but jailbreak prompts can contain probe-visible unsafe evidence distributed across earlier user-token representations that is missed by this readout. We study this prefill-time failure mode using SafeSwitch-style probes trained only on clean harmful and benign prompts across three instruction-tuned LLMs. The probes achieve high recall on clean harmful prompts, but miss many jailbreaks and can produce false positives on safety-adjacent benign prompts. Subspace analyses suggest that missed jailbreaks differ from clean benign prompts along directions that are poorly captured by the probe's representational subspace...
| Authors | Shravan Doda |
|---|---|
| Primary category | cs.LG |
| Published | 12 May 2026 |
| Last updated at source | 9 July 2026 |
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