Beyond Parameter Arithmetic: Sparse Complementary Fusion for Distribution-Aware Model Merging

Model merging has emerged as a promising paradigm for composing large language models directly in weight space, enabling training-free integration of specialized models. However, existing methods rely on parameter-space averaging that systematically induces generation dysregulation---a spectrum of failure modes ranging from structural repetition and termination failure to circular reasoning, and at the extreme, full semantic collapse into incoherent symbols. Even when source models exhibit near-zero dysregulation, existing methods introduce it at $>$97\% (14B/32B scales), collapsing reasoning benchmarks by up to 70 points. We propose Sparse Complementary Fusion with Reverse KL (\textbf{SCF-RKL}), a data-free merging framework that selects complementary parameters via reverse KL divergence on parameter-group proxy distributions. This structure-preserving, sparsity-inducing design maintains proxy-space geometry---theoretically motivated via entropy and subspace bounds---and empirically suppresses generation dysregulation while integrating new capabilities. Extensive experiments on 24 benchmarks across 7B--32B models spanning reasoning, instruction following, safety, and vision demonstrate that SCF-RKL achieves the best overall performance across scales while maintaining near-zero dysregulation ($<$1\%) and strong generalization.