zip2zip: Inference-Time Adaptive Vocabularies for Language Models via
  Token Compression
Authors: name: Saibo Geng; name: Nathan Ranchin; name: Yunzhen yao; name: Maxime Peyrard; name: Chris Wendler; name: Michael Gastpar; name: Robert West
Published: 2025-06-01 17:03:02

Abstract: Tokenization efficiency plays a critical role in the performance and cost of
large language models (LLMs), yet most models rely on static tokenizers
optimized for general-purpose corpora. These tokenizers' fixed vocabularies
often fail to adapt to domain- or language-specific inputs, leading to longer
token sequences and higher computational costs. We introduce zip2zip, a
framework that enables LLMs to dynamically adjust token vocabulary at inference
time, allowing for fewer generated tokens and thus faster inference. zip2zip
consists of three key components: (1) a tokenizer based on Lempel-Ziv-Welch
(LZW) compression that incrementally compresses tokens into reusable
"hypertokens" on the fly; (2) an embedding layer that computes embeddings for
newly formed hypertokens at runtime; and (3) a causal language modeling variant
that trains the model to operate on hypertokenized, compressed sequences. We
show that an existing LLM can be zip2zip-fied in 10 GPU-hours via
parameter-efficient finetuning. The resulting zip2zip LLMs effectively learn to
use hypertokens at inference time, reducing input and output sequence length by
20-60\%, with significant improvements in inference latency.