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Method to extract tokenization oracles from chat APIs described

A Reddit post outlines a technique to reconstruct a closed-source LLM tokenizer using two oracles derived from standard chat APIs: a token length oracle and a prefix token oracle. The prefix oracle resolves merge order ambiguities by leveraging the model's text repetition capability. This method could enable researchers to analyze tokenization without direct access to the tokenizer.

1 engagement·1 source·Sat, Jul 11, 2026, 06:17 AM
The post describes extracting two oracles from chat APIs: (1) a token length oracle that returns the number of tokens for any given string, and (2) a prefix token oracle that returns the decoded string from the first n tokens of the tokenized input. The prefix oracle is constructed by having the model repeat text in a controlled manner, allowing the user to infer token boundaries. This approach can resolve ambiguities in merge order, such as distinguishing between segmentations like (a, bc) and (ab, c). The technique is significant for researchers and developers who need to understand tokenization behavior of closed-source models without direct access to the tokenizer.

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tokenization oracle(concept)token length oracle(concept)prefix token oracle(concept)

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