Beyond Prompts: A Systematic Study of LLM-Based Machine Translation Evaluators

Dmitry Popov, Roman Bokhyan, Ekaterina Enikeeva, Artem Mekhraliev, Stepan Vysotsky, Nikolay Karpachev


Abstract
LLM-based evaluators have rapidly become a standard tool for machine translation evaluation, yet the design choices that shape their behavior remain poorly understood. We present a systematic study of how instruction design, few-shot demonstrations, backbone model choice, and multi-step workflows affect span-level and ranking-based MT evaluation. We collect new expert RATE annotations on WMT24 and WMT25 English–Russian translations, with transfer validation on MQM- and ESA-annotated benchmarks across five additional language pairs. Our experiments reveal four robust findings: instruction structure shapes evaluator behavior, high-quality demonstrations matter more than quantity, frontier backbones exhibit distinct evaluator profiles (recall-, precision-, or ranking-oriented), and fusing outputs across diverse backbones outperforms single-evaluator multi-call setups. Although evaluator design exposes a large combinatorial space, several discovered design principles transfer consistently across annotation protocols and language pairs. We synthesize these findings into ORBIT, a family of LLM-based MT evaluators. Our strongest configurations achieve expert-level performance in error span detection and in Accuracy and Fluency ranking at roughly 20× lower cost than expert human annotation.
Anthology ID:
2026.findings-emnlp.2404
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:
51648–51664
Language:
URL:
https://aclanthology.org/2026.findings-emnlp.2404/
DOI:
10.18653/2026.findings-emnlp.2404
Bibkey:
Cite (ACL):
Dmitry Popov, Roman Bokhyan, Ekaterina Enikeeva, Artem Mekhraliev, Stepan Vysotsky, and Nikolay Karpachev. 2026. Beyond Prompts: A Systematic Study of LLM-Based Machine Translation Evaluators. In Findings of the Association for Computational Linguistics: EMNLP 2026, pages 51648–51664, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Beyond Prompts: A Systematic Study of LLM-Based Machine Translation Evaluators (Popov et al., Findings 2026)
Copy Citation:
PDF:
https://aclanthology.org/2026.findings-emnlp.2404.pdf