LLM generation novelty through the lens of semantic similarity

Philipp Davydov, Ameya Prabhu, Matthias Bethge, Elisa Nguyen, Seong Joon Oh


Abstract
Generation novelty is a key indicator of an LLM’s ability to generalize, yet measuring it against full pretraining corpora is computationally challenging. Existing evaluations often rely on lexical overlap, failing to detect paraphrased text, or do not consider the full pretraining corpus. We frame novelty as a semantic retrieval problem. This framing enables us to address novelty with modern embedding and indexing pipelines, allowing for efficient analysis at pre-training scale. Specifically, we propose a three-stage framework that retrieves semantically similar samples, reranks them at varying subsequence lengths, and calibrates scores using a human novelty reference for interpretability. We apply this framework to the SmolLM model family and report three key findings: (1) models draw on pretraining data across much longer sequences than previously reported; (2) some task domains systematically promote or suppress generation novelty; and (3) instruction tuning not only alters style but also increases novelty. These results highlight the value of semantic novelty analysis for studying generalization. To support reproducibility and further research, we release ∼ 20 TB of corpus chunks and index artifacts upon acceptance.
Anthology ID:
2026.findings-emnlp.1942
Volume:
Findings of the Association for Computational Linguistics: EMNLP 2026
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
Findings
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
41411–41431
Language:
URL:
https://aclanthology.org/2026.findings-emnlp.1942/
DOI:
10.18653/2026.findings-emnlp.1942
Bibkey:
Cite (ACL):
Philipp Davydov, Ameya Prabhu, Matthias Bethge, Elisa Nguyen, and Seong Joon Oh. 2026. LLM generation novelty through the lens of semantic similarity. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 41411–41431, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
LLM generation novelty through the lens of semantic similarity (Davydov et al., Findings 2026)
Copy Citation:
PDF:
https://aclanthology.org/2026.findings-emnlp.1942.pdf