Simon Willison’s Weblog
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.
Recent articles
A new feature for my blog, built using my voice - 9th October 2026
Claude Haiku 5.5 - 7th October 2026
We're going to need default hard budget caps on pretty much everything - 3rd October 2026
OpenAI DevDay 2026 live blog - 29th September 2026
This is a beat by Simon Willison, posted on 9th October 2026 .
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