Correlated decoding loses the surface code's effective distance below threshold

Correlated decoding improves matching and clustering surface-code decoders by reweighting one decoding subgraph conditioned on the other’s correction. We show that standard reweighting can cause correlated decoders to fail on size-$(d-1)\mathrm{/}2$ fault configurations that a distance-$d$ code should correct, reducing error suppression deep below threshold. Under circuit-level noise, a single first-pass edge can discount several correlated edges, even though their joint occurrence requires additional independent faults, so the decoder favors an over-discounted correction. Regularizing the reweighting rule restores the nominal distance scaling and a clear correlated-decoding advantage.