Detecting a Route Flip Is Easier Than Knowing Whether to Fix It: Causal Route-Mediated Damage in Quantized Mixture-of-Experts

It is a change in which experts are activated due to a small numerical disturbance (like quantization), altering the computation path and affecting performance.

Quantization can push the router across decision boundaries, flipping selected experts, causing additional damage that is not easily fixable.

The study found that detecting a flip is possible (AUC 0.772), but no local statistic can distinguish harmful from helpful flips.

It implies that improving quantization requires addressing routing stability, not just weight precision, especially when deploying large models with limited resources.