Mechanistic interpretability seeks to explain transformer behavior through circuits: sets of internal components that causally support a behavior.
However, self-repair creates a blind spot: ablating a primary component can activate a dormant backup, so a circuit that explains behavior in the intact model can become incomplete under the intervention used to test it.
We formulate this gap as conditional circuit completion: given a primary set, identify components that become causally important after its removal.
We introduce conditional co-ablation (CoAx), which ranks candidates by growth in ablation effect after primary-set removal.
We show that a perfectly dormant backup can be indistinguishable from an irrelevant component to per-unit intact-state scores, whereas its conditional effect change exactly aggregates all interaction orders linking it to the removed set.
On GPT-2-small's Indirect Object Identification (IOI) circuit
CoAx recovers the documented backup heads at 0.941 ROC-AUC, versus 0.815 for the strongest intact-state attribution baseline and 0.758 for the matched conditional-energy control.
Recovery drops to 0.40 +/- 0.13 AUC for alternative component sets matched in behavioral effect, output displacement, and depth, showing that recovery is specific to the removed circuit.
Beyond recovery, the CoAx-selected heads are causally load-bearing: freezing them after primary removal sharply reduces the IOI margin, while adding them to the incomplete circuit reduces incompleteness from 0.75 to 0.21.
More broadly
Conditional growth aligns with intervention-derived repair in 11/12 held-out instances across 4 mechanism clusters, and CoAx completions outperform matched random completions on all 8 non-GPT-2 models spanning 6 architecture families.
Together, causal explanations of self-repairing transformers must account for backup circuitry when primary components fail.