Retrieval Without Relevance: Prompt Dependence in Small Machine Translation Models

Heesu Kim, Shota Sato, Mamoru Komachi


Abstract
We study how a small machine translation model should allocate a fixed data budget between parametric training and retrieval memory. We define a shared 100M-token corpus-access ledger that charges pretraining, fine-tuning, and retrieval-datastore tokens alike. Using ≈42M-parameter models, we compare three uses of the same per-pair datastore across 12 directed pairs of English, Chinese, Japanese, and Korean: leaving it unused, training on it, or retrieving from it into the prompt. We evaluate retrieval additivity across all 12 pairs and analyze retrieval-conditioned behavior in a four-pair core set. Retrieval shows no consistent additive gain over simply not using the datastore, a result that persists under an off-budget dense retriever and pools up to approximately 1000× larger. On the four-pair core set, retrieval-trained models are nevertheless highly context-dependent: removing retrieved examples causes large score drops, while replacing them with query-unrelated, count-matched same-pair examples recovers nearly the full score. This suggests that the learned dependence is primarily on exemplar-shaped context rather than on query-specific information carried by the retrieved neighbors. Our training and evaluation code is available at https://github.com/kheesu/retrieval-without-relevance.
Anthology ID:
2026.babylm-main.9
Volume:
Proceedings of the Second BabyLM Workshop
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Leshem Choshen, Ryan Cotterell, Mustafa Omer Gul, Jaap Jumelet, Tal Linzen, Aaron Mueller, Suchir Salhan, Raj Sanjay Shah, Alex Warstadt, Ethan Gotlieb Wilcox
Venues:
BabyLM | WS
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
105–122
Language:
URL:
https://aclanthology.org/2026.babylm-main.9/
DOI:
Bibkey:
Cite (ACL):
Heesu Kim, Shota Sato, and Mamoru Komachi. 2026. Retrieval Without Relevance: Prompt Dependence in Small Machine Translation Models. In Proceedings of the Second BabyLM Workshop, pages 105–122, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Retrieval Without Relevance: Prompt Dependence in Small Machine Translation Models (Kim et al., BabyLM 2026)
Copy Citation:
PDF:
https://aclanthology.org/2026.babylm-main.9.pdf