in-place-tokenizer-expansion-for-pre-trained-llms-d04c59dd·1 events·first seen Aliases: In-Place Tokenizer Expansion for Pre-trained LLMs
Researchers present a method for expanding a pre-trained LLM's tokenizer without retraining from scratch, using continued BPE merges on a multilingual corpus and a two-stage adaptation procedure (embedding-only training followed by full continued pre-training). Applied to LFM2-8B-A1B (a Mixture-of-Experts model) to produce LFM2.5-8B-A1B with a 128K tokenizer, the approach achieves 2.4–2.6× token compression for Hindi and Vietnamese (up to 4× for Thai), translating to an estimated 2.2–3.7× per-character decode speedup on reference devices. The technique is particularly relevant for on-device models where embedding and LM-head matrices constitute a material fraction of decode bandwidth, making vocabulary size a real efficiency constraint.