Localization Does Not Guarantee Control: Gate Failure and Partial Activation Steering in a Causal Reasoning Model

Does locating a behaviorally relevant activation site suffice to construct an effective intervention? We study this question in a frozen 25.7M-parameter transformer trained to answer interventional queries from mixed observational and interventional evidence. On a selected subset of confounded worlds, transplanting activations at mid-layer observational-record positions recovers 88.9% of a reference behavioral shift induced by evidence zeroing. We then evaluate a gated activation-editing pipeline and an ungated ablation on 50 held-out confounded worlds. The detector, calibrated with zero validation false triggers but only 0.5 recall, triggers on none of 2,400 target generations and on 6.9–7.3% of held-out in-distribution generations. Bypassing the gate enables partial correction: at the larger of two tested doses, a pooled contrast direction reduces the regression intercept from 0.3820 to 0.2637, a 31% reduction, while also reducing the gain from 0.8236 to 0.5721. An exact coefficient identity shows why: the pooled edit attenuates the whole response nearly proportionally, lowering offset and gain by the same factor instead of restoring world-specific sensitivity. These results separate failure to activate an intervention from insufficient correction after activation, and show that localization, gate coverage, and intervention efficacy must be evaluated separately.