A small, phase-structured set of attention heads does the label selection in a reasoning model, and it can be distilled
AspireAI community · arXiv
Reasoning-trained language models can select zero-shot a small set of relevant labels from thousands to hundreds of thousands of candidates, and this paper asks mechanistically how and whether the mechanism can be distilled. Each decision is treated as a token-level event scored by the model's own decision margin, and attribution, exact mean-ablation, knock-in into another example's context and a null calibration give individual attention heads causal standing. On clinical coding of MIMIC-IV discharge summaries with all 5,651 candidate codes in context, a small, global, phase-structured set of heads was necessary and sufficient on essentially every summary, with distinct head families reading the candidate region and the earlier near-miss, and blocking what they read removes their effect. The structure recurs in a second reasoner, and the MISTILL method distills it rather than copying chain-of-thought, also supervising the student's pooled attention. The paper is at v4 as of October 29, so the head-level account has been iterated.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.