Quantized MoE Routing Flips: Causal Analysis of Route-Mediated Damage

A new arXiv paper (2608.11212) investigates the impact of numerical disturbances on Top-k Mixture-of-Experts (MoE) routing, specifically focusing on 4-bit KV-cache quantization. The study reveals that such disturbances can cause tokens to cross decision boundaries, flipping which experts are activated. The paper introduces a causal apparatus to quantify the route-mediated fraction (RMF) of quantization damage, decomposes it by mechanism via token-level attribution, and validates findings across three architectures using pre-registered probes. On OLMoE-1B-7B, the pilot results show that approximately a third of the damage is routing-mediated (RMF ~ 0.31). The deployable router margin can detect that a flip occurred (AUC 0.772) but cannot distinguish harmful from helpful flips. The paper does not propose a mitigation but provides empirical findings and a detection-limit result.