A Geometric Information Bottleneck for Activation Steering

Activation-based steering methods for large language models often induce broad, entangled changes in model behavior, inadvertently altering capabilities unrelated to the intended behavior, which limits their reliability for fine-grained behavioral control. We address this limitation by reframing behavioral intervention through a geometric information bottleneck (IB) perspective, in which effective steering corresponds to selectively modifying task-relevant information while preserving the geometric structure of orthogonal representational subspaces. Building on this view, we propose a disentanglement-based intervention framework, termed IB-ACT, that identifies both where and how to intervene by exploiting the layer-wise geometry of representation spaces to isolate behavior-relevant information without disturbing other dimensions. Our method introduces a layer-selection mechanism determined prior to intervention, rather than relying on post hoc sparsity or regularization losses, and applies geometrically constrained transformations that target behavior-relevant subspaces in activation space while preserving orthogonal structure. We provide theoretical justification showing that interventions at these layers reduce unintended information leakage under an IB-style objective. Empirically, we evaluate IB-ACT on toxicity control and hallucination reduction in large language models and demonstrate consistent improvements over recent baselines, while analyzing the spectral structure of behavior-relevant representations for jailbreak mitigation. Overall, our findings suggest that selectively intervening at structurally appropriate layers is critical for controllable and disentangled behavioral steering in large language models.