Router Sensitivity Under Lightweight Fine-Tuning Identifies Prunable Experts in Mixture-of-Experts Models

arXiv:2608.07890v2 Announce Type: replace-cross Abstract: Mixture-of-Experts (MoE) models decouple parameter count from per-token compute, but deployment still requires hosting every expert in memory. Recent theory shows that experts whose router weights change least during fine-tuning can be pruned with provable accuracy preservation, yet the guarantee assumes full fine-tuning. We show that the signal can be elicited through a brief parameter-efficient adaptation. We fine-tune with a lightweight adapter, rank experts by the induced router change, and prune the least-changed experts in one shot. On Mixtral-8$\times$7B-Instruct, router-only LoRA trains 0.002% of parameters and retains 27.54% MMLU-Pro accuracy with half the experts removed, against roughly 16% for magnitude and random pruning. Signal quality improves monotonically with adapter size, reaching 28.76%, and declines as adaptation spreads beyond the router. Under their shared budget, IA3 reaches 28.04% while Houlsby reaches 25.39%. The criterion transfers to Qwen1.5-MoE fine-tuned for mathematical reasoning, retaining 49.7% mean accuracy over eleven benchmarks with half the experts removed. Structural pruning reduces memory by 49% and per-token latency by 37%. Lightweight router sensitivity therefore makes provably motivated, task-conditioned expert pruning practical at scale.

Abstract:Mixture-of-Experts (MoE) models decouple parameter count from per-token compute, but deployment still requires hosting every expert in memory. Recent theory shows that experts whose router weights change least during fine-tuning can be pruned with provable accuracy preservation, yet the guarantee assumes full fine-tuning. We show that the signal can be elicited through a brief parameter-efficient adaptation. We fine-tune with a lightweight adapter, rank experts by the induced router change, and prune the least-changed experts in one shot. On Mixtral-8$\times$7B-Instruct, router-only LoRA trains 0.002% of parameters and retains 27.54% MMLU-Pro accuracy with half the experts removed, against roughly 16% for magnitude and random pruning. Signal quality improves monotonically with adapter size, reaching 28.76%, and declines as adaptation spreads beyond the router. Under their shared budget, IA3 reaches 28.04% while Houlsby reaches 25.39%. The criterion transfers to Qwen1.5-MoE fine-tuned for mathematical reasoning, retaining 49.7% mean accuracy over eleven benchmarks with half the experts removed. Structural pruning reduces memory by 49% and per-token latency by 37%. Lightweight router sensitivity therefore makes provably motivated, task-conditioned expert pruning practical at scale.

26 pages, 8 figures, 12 tables. Camera-ready version accepted to AXIOM: Foundations of Efficient Deep Learning, NeurIPS 2026. Code: this https URL

Machine Learning (cs.LG); Artificial Intelligence (cs.AI)

arXiv:2608.07890 [cs.LG]

(or arXiv:2608.07890v2 [cs.LG] for this version)

https://doi.org/10.48550/arXiv.2608.07890 Focus to learn more arXiv-issued DOI via DataCite

arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website.

Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy. arXiv is committed to these values and only works with partners that adhere to them.

Have an idea for a project that will add value for arXiv's community? Learn more about arXivLabs.

aiqbench analysis

Router Sensitivity Under Lightweight Fine-Tuning Identifies Prunable Experts in Mixture-of-Experts Models

Related coverage