Debugging ReBasin: What Limits Symmetry-Based Model Merging?

Weights-merge accuracy is the standard measure of model-merging success, but it does not reveal whether failures come from poor alignment, architectural constraints, folding error, or normalisation. We introduce a representation-merge diagnostic that applies the same alignment maps directly at runtime, then progressively constrains this idealised merge to isolate contributors to the final weights-merge result. On CIFAR-100 with VGG-16-BN and ResNet-20, this decomposition shows that narrow-ResNet failures are dominated by a structural constraint: one residual-stream alignment must work at multiple positions within a stage. Trained encoder--decoder maps outperform closed-form alignments at narrow width, but this gap closes as width grows. Together, these results show that symmetry-based merging is limited both by the ability to find good alignments and by the ability of the architecture to realise them.