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Distilling GPT-2 into DistilGPT-2 collapsed one circuit completely while another survived intact, and aggregate accuracy hid the difference

AspireAI community · arXiv
The paper asks what actually survives depth-reduction distillation mechanistically, using linear CKA, logit-lens analysis, attention pattern decomposition, sparse autoencoder feature statistics and activation patching to compare GPT-2 with its six-layer student DistilGPT-2 across two canonical circuits. The result is a dichotomy: the Indirect Object Identification circuit collapses completely, with the student scoring 35.2 percent against the teacher's 94.8 percent and preserving no name-mover heads at all, while the Greater-Than circuit survives and recovers 87.7 percent of task performance by actively remapping the teacher's layer-9 comparison head into student layer four, with a mean Pearson correlation of 0.945. The authors argue that circuit compressibility, not aggregate accuracy, governs mechanistic survival, and that a student can look fine on benchmark metrics while having lost an entire capability. The scope is narrow by design: one model family, two circuits and small models, so the result establishes a diagnostic worth running on other models rather than a general law about distillation.
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One distance function matched sparse-autoencoder features across layers and compressed feature circuits into supernodes
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.
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