CoAx locates backup heads at 0.941 AUC, still unreplicated
Conditional co-ablation ranks candidates by ablation-effect growth to find backup heads, but all numbers are author-reported.
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Readers doing circuit analysis or safety evaluation now have a new idea to use: conditional co-ablation (CoAx), which after ablating a primary component set ranks candidates by growth in ablation effect to find backups the intervention activates — the authors report 0.941 ROC-AUC recovering known backup heads on GPT-2's IOI circuit.
Previously, recovering backup heads masked by primary components relied on intact-state attribution, whose strongest baseline reached only 0.815 ROC-AUC on GPT-2-small's IOI circuit and could not directly complete an incomplete circuit.
The authors' reported measurement: CoAx recovers the documented backup heads on that IOI circuit at 0.941 ROC-AUC, and adding the selected heads to the incomplete circuit cuts incompleteness from 0.75 to 0.21; they also report it beats matched random completions on 8 non-GPT-2 models across 6 architecture families.
All numbers are first-party evaluations, backup-head recovery is mainly validated on the known-answer IOI setting, and nothing has been independently reproduced; preprint arXiv 2607.01940 (v3, 23 Sep 2026).