“Shorter Tokens Are More Likely” by Brendan Long

I was thinking about LLM tokenization (as one does) and had a thought: We select the next output token for an LLM based on its likelihood, but shorter tokens are more likely.

Why? Shorter common tokens are (correctly) learned to be higher-probability because they have the combined probability of any word they could complete. However, standard generation techniques will only consider a subset of probabilities and scale the largest probabilities. Both of these will take the highest probabilities and increase them further, meaning short/common tokens become significantly more likely to be generated just because they're shorter.

I ran an experiment to investigate this, showing that the first-character distribution of words generated by nanoGPT[1] is similar regardless of tokenization without top-K or temperature scaling, but if we use common settings (top-K=200 and temperature=0.8), we can increase the likelihood that a word starts with 'c' from 4% up to 10% just [...]

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Outline:

(01:36) Why?

(02:27) Top-K Sampling

(02:52) Temperature 1.0

(04:19) The Experiment

(05:42) Results

(06:47) Shakespeare Experiment

(07:08) Results

(08:15) Why Does It Matter?

The original text contained 4 footnotes which were omitted from this narration.

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First published:

August 24th, 2025

Source:

https://www.lesswrong.com/posts/iZPKuuWsDXAcQWbLJ/shorter-tokens-are-more-likely

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Narrated by TYPE III AUDIO.

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