When Do Task Vectors Interfere? Mapping the Validity Boundaries of Weight-Space Composition
Task arithmetic composes skills by adding weight displacements, and merged models are then judged on benchmark suites. We measure when that composition is functionally additive, and find that the answer depends as much on how the model is prompted as on which tasks are merged. Across two-dimensional composition surfaces -- five model settings from 0.5B to 8B, two families, LoRA and full fine-tuning -- pairwise non-additivity is real, seed-stable, and transfers in coarse order to unseen task pairs: all eight preregistered sign predictions held. But it is input-conditioned everywhere we measured: the same merged model that shows a six-point interaction contrast on code prompts shows none on...
| Authors | Chencheng Zhu, Xiaoyang Li, Taotao Cai |
|---|---|
| Primary category | cs.LG |
| Published | 10 August 2026 |
| Last updated at source | 31 August 2026 |
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