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This research investigates the structural implications of jointly optimizing tokenization strategies within language modeling pipelines, specifically comparing subword learning against tokenizer-free architectures like SSLMs and H-Nets across diverse linguistic typologies. The findings demonstrate that removing fixed vocabularies allows models to prioritize contextual efficiency over rigid morphological alignment, yielding distinct but effective representations for downstream NLP tasks. For practitioners in foundational model development, this highlights the significant performance gains achievable by rethinking tokenization as a learnable component rather than a static preprocessing step.
What Tokens are Learned when Tokenization is Optimized Jointly with Language Modeling?
arXiv:2608.17325v1 Announce Type: new Abstract: Tokenization is a fundamental component of language modeling pipelines. Despite its importance, it is often fixed, even though it significantly impacts model performance across languages. In this work, we analyze what tokens are learned when tokenization is jointly optimized with language modeling. We compare tokenizer-free approaches such as SSLMs and H-Nets with fixed tokenizers across 18 typologically and script-diverse languages. Our results show that joint optimization fundamentally alters token structure. SSLMs recover morphologically aligned and contextually efficient tokens, whereas H-Nets prioritize byte-level efficiency, producing longer tokens with very low overlap with standard subword vocabularies. We further show that tokenization behavior varies across language typologies. Agglutinative languages exhibit more dynamic segmentation patterns while learning. Through downstream evaluation, with pretrained-then-finetuned BERT models, we find that SSLM-based pretokenization consistently reduces language modeling perplexity and achieves competitive downstream performance despite distinct vocabularies. Overall, tokenizer-free approaches optimize for contextual and computational efficiency rather than strict morphological structure, resulting in fundamentally different yet effective vocabularies for downstream NLP.
Title:What Tokens are Learned when Tokenization is Optimized Jointly with Language Modeling?
Abstract:Tokenization is a fundamental component of language modeling pipelines. Despite its importance, it is often fixed, even though it significantly impacts model performance across languages. In this work, we analyze what tokens are learned when tokenization is jointly optimized with language modeling. We compare tokenizer-free approaches such as SSLMs and H-Nets with fixed tokenizers across 18 typologically and script-diverse languages. Our results show that joint optimization fundamentally alters token structure. SSLMs recover morphologically aligned and contextually efficient tokens, whereas H-Nets prioritize byte-level efficiency, producing longer tokens with very low overlap with standard subword vocabularies. We further show that tokenization behavior varies across language typologies. Agglutinative languages exhibit more dynamic segmentation patterns while learning. Through downstream evaluation, with pretrained-then-finetuned BERT models, we find that SSLM-based pretokenization consistently reduces language modeling perplexity and achieves competitive downstream performance despite distinct vocabularies. Overall, tokenizer-free approaches optimize for contextual and computational efficiency rather than strict morphological structure, resulting in fundamentally different yet effective vocabularies for downstream NLP.
Computation and Language (cs.CL)
arXiv:2608.17325 [cs.CL]
(or arXiv:2608.17325v1 [cs.CL] for this version)
https://doi.org/10.48550/arXiv.2608.17325 Focus to learn more arXiv-issued DOI via DataCite (pending registration)
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