Activation-level patches show most stated chain-of-thought steps are causally load-bearing in Qwen3-4B
This arXiv preprint introduces an activation-level causal test that patches the residual stream at token spans where a model states intermediate steps, replacing them with activations from counterfactual runs on synthetic 2–6-hop lookup tasks. For Qwen3-4B, 76.9% ± 2.8% of stated steps are causally load-bearing at the most responsive mid-network layer (random-position null 11.3%; patching the underlying prompt fact yields 83%), while a standard behavioral edit test reports 88.2% and thus overstates causal faithfulness by ~11.4 percentage points; Qwen3-1.7B is far less causally faithful overall (54.8%) and its faithfulness declines with hop depth (68% at 2 hops to 30% at 6).