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[ARTICLE · art-148068] src=simonwillison.net ↗ pub= topic=ai-tools verified=true sentiment=· neutral

ttok 1.0

Simon Willison released ttok 1.0, changing the token-counting tool's default tokenizer from GPT-4 to the GPT-5/GPT-6 family after running `uv tool upgrade ttok` and finding the old default still in place. OpenAI has not confirmed that GPT-6 shares the GPT-5 tokenizer, but a commit by William Liu reports an experiment in which all seven GPT models (5.5, 5.6 Sol/Terra/Luna, 6 Astra/Sol/Luna) returned 44,794 tokens and matched on all 31 fixtures, indicating GPT-6 introduces no input-count change on that corpus.

read1 min views1 publishedOct 9, 2026

Release: ttok 1.0 I released ttok 0.4, ran uv tool upgrade ttok, piped a file into the new version... and realized that it was defaulting to the GPT-4 tokenizer when it should very clearly default to GPT-5/GPT-6 instead!

I figured switching the default was a reasonable excuse to finally ship a 1.0.

OpenAI haven't actually confirmed that GPT-6 uses the same tokenizer as the GPT-5 family yet - there's an angry issue about it - but I found this commit by William Liu which reports on an experiment he ran confirming that the tokenizers are likely the same:

All seven GPT models (5.5, 5.6 Sol/Terra/Luna, 6 Astra/Sol/Luna) report 44,794 tokens and match each other on every one of the 31 fixtures. GPT-6 introduces no input-count change on this corpus.

Tags: projects, ai, openai, generative-ai, llms, tokenization

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