I've seen a few "Claude tokenizers" floating around lately with all the 4.7 chatter, but most of them just hit the count_tokens endpoint and hand you back a number. You don't actually see how your text gets split or understand the changes from 4.6 to 4.7.

I built this a while back for doing some mech interp research. It faithfully represents Claude token splitting - showing hidden tokens, real boundaries and so on. It is not cheap to run - essentially n^2 cost - you could optimise for longer sequences but you are not guaranteed a faithful representation if so.

Open Source: https://github.com/R0bk/claude-tokenizer

Feedback welcome, let me know if there are any edge cases that look wrong.

P.S. I'd expect this to face a similar fate as streaming chunk and prefill based token extraction methods did. I do worry about the ability to do independent research once it's fully closed off and would love it if there was more public frontier tokenizers.