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While large-scale reasoning models have achieved remarkable performance gains by scaling test-time computation through extended Chain-of-Thought (CoT) sequences, their practical utility is severely constrained by protracted inference lengths and high computational latency. In this paper, we present Differentiable Hierarchical Fusion (DHF), a novel framework that merges reasoning models with efficient base models via differentiable optimization to produce concise, accurate outputs. We introduce a dual-factor adaptive weighting mechanism to capture intra-block (Attention vs. MLP) variance and inter-block (shallow vs. deep layers) importance hierarchies, thereby addressing key limitations of static merging heuristics. Specifically, DHF optimizes fusion coefficients using gradient descent on a loss function that jointly minimizes cross-entropy (for accuracy) and response length (for conciseness). Furthermore, we enhance an outlier-aware initialization strategy to seed coefficients based on activation density and construct multi-calibration datasets to improve the model’s generalization ability. Comprehensive evaluations on Qwen and LLaMA models across six reasoning benchmarks show that DHF reduces the average response length by 55% while boosting accuracy by 2.8%–6% compared to existing state-of-the-art merging methods.