Beyond Perplexity: UTF-8 Validity in Byte-aware Language Models
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Computer Science > Computation and Language
Title:Beyond Perplexity: UTF-8 Validity in Byte-aware Language Models
Abstract:Byte-level tokenization enables language models to handle any Unicode input, but models can generate invalid UTF-8 sequences when encountering rare or unseen characters. We investigate the relationship between training scale and UTF-8 generation reliability with a 355M parameter model trained on 80B tokens from a balanced multilingual corpus of English, Japanese, Korean, and Chinese. We introduce multiple evaluation protocols that isolate UTF-8 structural validity from language modeling. UTF-8 validity convergence lags perplexity by a roughly a factor of two: perplexity stabilizes after 2.1B tokens, but UTF-8 validity requires 4.2B tokens. In context-free generation, rare characters achieve higher structural validity than common characters, suggesting over-specialization of frequent character representations. Through experiments, we observed that reliable UTF-8 generation is a distinct capability requiring evaluation beyond perplexity.
| Subjects: | Computation and Language (cs.CL) |
| Cite as: | arXiv:2606.14122 [cs.CL] |
| (or arXiv:2606.14122v1 [cs.CL] for this version) | |
| https://doi.org/10.48550/arXiv.2606.14122
arXiv-issued DOI via DataCite (pending registration)
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