@inproceedings{chiu-2026-toklens,
    title = "{T}ok{L}ens: A Multilingual Lens on Tokenizer Quality for {LLM}s",
    author = "Chiu, Guan-Ming",
    editor = "T.Y.S.S., Santosh  and
      Rodriguez, Juan Diego  and
      de Gibert, Ona",
    booktitle = "Proceedings of the 64th Annual Meeting of the {A}ssociation for {C}omputational {L}inguistics ({ACL} 2026)",
    month = jul,
    year = "2026",
    address = "San Diego, California, United States",
    publisher = "Association for Computational Linguistics",
    url = "https://preview.aclanthology.org/ingest-acl/2026.acl-srw.18/",
    pages = "188--205",
    ISBN = "979-8-89176-393-7",
    abstract = "We introduce TokLens, an open-source toolkit for evaluating tokenizer quality across languages using six intrinsic metrics: fertility, characters per token, compression ratio, normalized sequence length, single-token retention rate, and cross-lingual parity. We evaluate 24 tokenizers from major LLM families across 15 typologically diverse languages and correlate these metrics with downstream performance. Our analysis reveals stark disparities: GPT-2 produces 56x more tokens per word in Japanese than in English, while newer tokenizers like Qwen2.5 and Gemma-2 reduce this gap to under 4x. No intrinsic metric predicts English benchmark performance after controlling for model size. However, on multilingual benchmarks (MMLU-ProX), linear mixed-effects models show that tokenizer metrics significantly predict per-language performance (STRR: $\beta = +5.7$, $z = 18.5$, $p < 0.001$). A controlled experiment on the Qwen2.5 family further shows that languages with higher single-token retention rate exhibit steeper scaling slopes ($\rho = 0.91$, $p < 0.001$). These results indicate that tokenizer quality is significantly associated with multilingual LLM performance, though the evidence remains correlational and partially confounded with pretraining data composition."
}