CANE is a Format: Rethinking Output Formats for Few-Shot LLM-Based Named Entity Recognition

Joseph Cornelius, Kei K Ito, Fabio Rinaldi


Abstract
Output format selection is a necessary but understudied dimension of few-shot named entity recognition (NER) with large language models. Existing formats carry characteristic failure modes under autoregressive generation, yet output format has not been systematically evaluated as a primary variable in the few-shot setting across diverse domains and entity structural complexity. We evaluate 21 output formats over 17 datasets across two levels of structural entity complexity, using 9 open-weight models without fine-tuning, ranging from 1.7B to 70B parameters. Output format is the main factor affecting performance, with a 41.7 F1 points difference between the best and worst formats. This exceeds the gains from scaling models parameters within-family. Furthermore, output format accounts for approximately 49.1% of F1 variance on simple datasets and 74.9% on complex ones. We describe three mechanistically distinct failure modes that explain these observed differences; each existing format violates at least one of three properties jointly necessary for robust few-shot NER. We propose CANE (Context-Anchored Natural Expression), a format family addressing all three properties. All CANE variants ranked the highest on simple and complex datasets. This includes the GENIA dataset where all other formats had near-zero scores across nine models, indicating the necessity of the three properties. These findings highlight the importance of output format in few-shot named entity recognition, particularly for annotations involving overlapping or discontinuous entities.
Anthology ID:
2026.findings-emnlp.1398
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:
29773–29800
Language:
URL:
https://aclanthology.org/2026.findings-emnlp.1398/
DOI:
10.18653/2026.findings-emnlp.1398
Bibkey:
Cite (ACL):
Joseph Cornelius, Kei K Ito, and Fabio Rinaldi. 2026. CANE is a Format: Rethinking Output Formats for Few-Shot LLM-Based Named Entity Recognition. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 29773–29800, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
CANE is a Format: Rethinking Output Formats for Few-Shot LLM-Based Named Entity Recognition (Cornelius et al., Findings 2026)
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PDF:
https://aclanthology.org/2026.findings-emnlp.1398.pdf