Paper Detail
Yaxin Luo, Jiacheng Cui, Xiaohan Zhao, Xinyi Shang, Jiacheng Liu, Xinyue Bi, Zhaoyi Li, Zhiqiang Shen
The pretraining data mixture of Large Language Models (LLMs) constitutes their "digital DNA", shaping model behaviors, capabilities, and failure modes. Yet this composition is rarely disclosed, making post-hoc auditing of data combination or provenance difficult. In this work, we formalize $\textbf{Data Mixture Surgery (DMS)}$: given only generated text from a target LLM, estimate the domain-level distribution of its pretraining corpus under a predefined taxonomy. We propose $\textbf{LLMSurgeon}$, a strong framework that casts DMS as an inverse problem under the label-shift assumption. Rather than directly aggregating classifier outputs, LLMSurgeon estimates a calibrated $\textit{soft}$ confusion matrix and solves a constrained inverse problem to correct systematic domain confusion and recover the latent mixture prior. To evaluate, we introduce $\textbf{LLMScan}$, a recipe-verifiable evaluation suite built from open-source LLMs with transparent pretraining mixtures. Across LLMScan, LLMSurgeon recovers domain mixtures with high fidelity under fixed protocols. Our work presents a practical, post-hoc approach for auditing the digital DNA of foundation models without access to their training data.
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@article{luo2026llmsurgeon,
title = {LLMSurgeon: Diagnosing Data Mixture of Large Language Models},
author = {Yaxin Luo and Jiacheng Cui and Xiaohan Zhao and Xinyi Shang and Jiacheng Liu and Xinyue Bi and Zhaoyi Li and Zhiqiang Shen},
year = {2026},
abstract = {The pretraining data mixture of Large Language Models (LLMs) constitutes their "digital DNA", shaping model behaviors, capabilities, and failure modes. Yet this composition is rarely disclosed, making post-hoc auditing of data combination or provenance difficult. In this work, we formalize \$\textbackslash{}textbf\{Data Mixture Surgery (DMS)\}\$: given only generated text from a target LLM, estimate the domain-level distribution of its pretraining corpus under a predefined taxonomy. We propose \$\textbackslash{}textbf\{LLMSurgeon\}},
url = {https://arxiv.org/abs/2605.30348},
keywords = {cs.CL, cs.AI, cs.LG, Computer science, Fidelity, Classifier (UML), Mixture model, Suite},
eprint = {2605.30348},
archiveprefix = {arXiv},
}
{}