On the Representation Geometry of LoRA Model Merging
Paper · 2026 · Chenyang Lu, Jiaru Li, Jinman Zhao et al.
Summary
Combining several task-specific LoRA adapters into one model without further training is hard because their low-rank updates tend to occupy misaligned subspaces. Leading training-free approaches such as KnOTS align updates through a shared SVD basis, which assumes that a single global subspace is enough. This paper tests that assumption by examining the geometry of LoRA weight updates, using the adapters released with KnOTS. Subspace-overlap and energy analyses show that SVD bases and column-selection CUR bases (CUR keeps the columns with the largest ℓ2 norms) capture different directions. SVD favours structure common to all tasks, while CUR keeps localized, task-specific components. The authors build on this trade-off with a training-free merging procedure that blends SVD-aligned and CUR-aligned merged updates. The method outperforms existing gradient-free LoRA merging baselines on ViT-B/32 across eight image classification datasets and on Llama-3 8B across NLI tasks, including a harder joint-task evaluation. The study's broader lesson is that merging quality depends on the representation basis, not only on the rule used to combine updates.
Key points
- Core finding: SVD and CUR bases are systematically misaligned at the subspace level and have complementary strengths, which exposes a trade-off between preserving shared structure and preserving task-specific detail.
- Language results: On Llama-3 8B NLI tasks, the method reaches the best average normalized accuracy among compared methods, at 93.5%.
- Vision results: Experiments use ViT-B/32 on Cars, DTD, EuroSAT, GTSRB, MNIST, RESISC45, SUN397 and SVHN.
- In the joint-task setting, the eight label sets are merged into 748 unique classes.
- The method leads all baselines on every Hits@k metric.
- Ablations:
- Interpolating between pure CUR (α=0) and pure SVD (α=1) gives a unimodal curve that peaks near α≈0.6.
- Merging in the CUR "UR" space beats merging in the reduced "R" space, especially on MNIST and SVHN.
- Adding LoRA-LEGO did not bring consistent further gains.
- Code and limitations:
- Code is available at github.com/lcytoronto/knots_cur.
- The analysis covers weight space only and supervised fine-tuning only; whether the findings hold for other adaptation paradigms, such as reinforcement learning, is untested.
Institutions
Fields, topics and concepts
📚 Shelves 5The field of science this work belongs to (OpenAlex fields).Computer Science · 16,667 worksArtificial Intelligence · 16,194 worksArts and Humanities · 16,039 worksLanguage and Linguistics · 15,891 worksComputer Vision and Pattern Recognition · 2,869 works
💡 Concepts 1Key ideas and methods explained on our concept pages.Latent semantic indexing · 29 works
Cite
Cite as Chenyang Lu, Jiaru Li, Jinman Zhao, Xinran Chen, Yining Wang, Renyi Cai, Yuchen Li and Chao He. 2026. On the Representation Geometry of LoRA Model Merging. Findings of the Association for Computational Linguistics: ACL 2026.
@inproceedings{lu2026representation,
title = {{On the Representation Geometry of LoRA Model Merging}},
author = {Chenyang Lu, Jiaru Li, Jinman Zhao, Xinran Chen, Yining Wang, Renyi Cai, Yuchen Li and Chao He},
booktitle = {Findings of the Association for Computational Linguistics: ACL 2026},
year = {2026},
doi = {10.18653/v1/2026.findings-acl.261},
url = {https://doi.org/10.18653/v1/2026.findings-acl.261}
}References
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