Title: FuseRL: Dense Preference Optimization for Heterogeneous Model Fusion
Primary area: alignment, fairness, safety, privacy, and societal considerations
Keywords: Model fusion, Large Language Models, Preference Alignment

Abstract: Heterogeneous model fusion enhances the performance of LLMs by integrating the knowledge and capabilities of multiple structurally diverse models. However, existing approaches often rely solely on selecting the best output for each prompt from source models, which underutilizes their full potential due to limited source knowledge and results in sparse optimization signals. To address this limitation, we propose FuseRL, a novel two-stage framework comprising FuseSFT and FusePO to maximize the utilization of source LLMs. FuseSFT establishes a robust initialization by integrating the strengths of heterogeneous source models through weighted supervised fine-tuning (SFT) on diverse outputs for each prompt. FusePO optimizes weighted preferences based on the outputs of multiple source models to enable superior alignment performance. Extensive experiments demonstrate the effectiveness of our framework across various preference alignment methods, including RLOO, DPO, and SimPO. Using Llama-3.1-8B-Instruct as the target model, our approach achieves competitive performance among 8B LLMs on the AlpacaEval-2 and Arena-Hard benchmarks. Further analysis suggests that FuseSFT regularizes the training to reduce overfitting, while FusePO introduces dense and diverse preference signals that enhance alignment quality.

TL;DR: We propose a novel two-stage framework to maximize source LLM utilization in heterogeneous model fusion.