Large Language Models (LLMs) are typically shipped with tokenizers that deterministically encode text into so-called canonical token sequences, to which the LLMs assign probability values.One common assumption is that the probability of a piece of text is the probability of its canonical token sequence.However, the tokenization of a string is not unique: e.g., the Llama2 tokenizer encodes Tokens as [Tok,ens], but [Tok,en,s] also represents the same text.In this paper, we study noncanonical tokenizations.We prove that, given a string, it is computationally hard to find the most likely tokenization for an autoregressive LLM, as well as to compute the marginal probability over all possible tokenizations.We then show how the marginal is, in most cases, indistinguishable from the canonical probability.Surprisingly, we then empirically demonstrate the existence of a significant amount of signal hidden within tokenization space.Notably, by simply aggregating the probabilities of noncanonical tokenizations, we achieve improvements across a range of LLM evaluation benchmarks for a variety of architectures, including transformers and state space models.

@inproceedings{GehEMNLP24,
  author    = {Geh, Renato Lui and Zhang, Honghua and Ahmed, Kareem and Wang, Benjie and Van den Broeck, Guy},
  title     = {Where is the signal in tokenization space?},
  booktitle = {Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing (EMNLP)},
  url       = "https://starai.cs.ucla.edu/papers/GehEMNLP24.pdf",
  slides    = "https://starai.cs.ucla.edu/slides/GehEMNLP24.pdf",
  code      = "https://github.com/RenatoGeh/where-is-tokenization",
  month     = 11,
  year      = {2024},
  keywords  = {conference,selective},
  annotation = "(Oral full presentation, acceptance rate 198/6105 = 3.2\%)"
}