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.
