Harmful Intent as a Geometrically Recoverable Feature of LLM Residual Streams

Abstract

Harmful intent is geometrically recoverable from large language model residual streams: as a linear direction in most layers, and as angular deviation in layers where projection methods fail. Across 12 models spanning four architectural families (Qwen2.5, Qwen3.5, Llama-3.2, Gemma-3) and three alignment variants (base, instruction-tuned, abliterated), under single-turn, English evaluation, we characterise this geometry through six direction-finding strategies. Three succeed: a soft-AUC-optimised linear direction reaches mean AUROC 0.98 and TPR@1\%FPR 0.80; a class-mean probe reaches 0.98 and 0.71 at <1ms fitting cost; a supervised angular-deviation strategy reaches AUROC 0.96 and TPR of 0.61 along a representationally distinct direction ($73^\circ$ from projection-based solutions), uniquely sustaining detection in middle layers where projection methods collapse. Detection remains stable across alignment variants, including abliterated models from which refusal has been surgically removed: harmful intent and refusal behaviour are functionally dissociated features of the representation. A direction fitted on AdvBench transfers to held-out HarmBench and JailbreakBench with worst-case AUROC 0.96. The same picture holds at scale: across Qwen3.5 from 0.8B to 9B parameters, AUROC remains $\geq$0.98 and cross-variant transfer stays within 0.018 of own-direction performance This is consistent with a simple account: models acquire a linearly decodable representation of harmful intent as part of general language understanding, and alignment then shapes what they do with such inputs without reorganising the upstream recognition signal. As a practical consequence, AUROC in the 0.97+ regime can substantially overestimate operational detectability; TPR@$1\%$FPR should accompany AUROC in safety-adjacent evaluation.