Why are API costs higher for Japanese users compared to...

The primary cause is the tokenizer fertility problem. Because Japanese character patterns were underrepresented in the training corpus of standard English-centric tokenizers, they do not accumulate enough merge frequency. Consequently, Japanese text is broken down into significantly more tokens than equivalent English text. This inflation directly impacts API costs, which are billed per token. To address this, teams should measure compression ratios per language and consider vocabulary balance if rebuilding the tokenizer.