FLEXITOKENS: Flexible Tokenization for Evolving Language Models
Authors: name: Abraham Toluase Owodunni; name: Orevaoghene Ahia; name: Sachin Kumar
Published: 2025-07-17 01:55:41

Abstract: Language models (LMs) are challenging to adapt to new data distributions by
simple finetuning. This is due to the rigidity of their subword tokenizers,
which typically remain unchanged during adaptation. This inflexibility often
leads to inefficient tokenization, causing overfragmentation of
out-of-distribution domains, unseen languages, or scripts. In this work, we
develop byte-level LMs with learnable tokenizers to make tokenization adaptive.
Our models include a submodule that learns to predict boundaries between the
input byte sequence, encoding it into variable-length segments. Existing
tokenizer-free methods train this boundary predictor using an auxiliary loss
that enforces a fixed compression rate across the training corpus, introducing
a new kind of rigidity. We propose FLEXITOKENS, a simplified training objective
that enables significantly greater flexibility during adaptation. Evaluating
across multiple multilingual benchmarks, morphologically diverse tasks, and
domains, we demonstrate that FLEXITOKENS consistently reduces token
over-fragmentation and achieves up to 10\% improvements on downstream task
performance compared to subword and other gradient-based tokenizers. Code and
data for our experiments will be released at
https://github.com/owos/flexitokens