Finetuning multi-vector models involves several components: the model itself, datasets, loss functions, training arguments, evaluators, and the trainer class. I'll have a look at each of these components, accompanied by practical examples of how they can be used for finetuning strong multi-vector models.

Lastly, in the Evaluation section, I'll show you that my finetuned multi-vector-encoder/mLateOn-medical model, trained in 14.5 hours on a single RTX 3090 alongside this blogpost, easily outperforms every general-purpose retrieval model I could find on my medical retrieval evaluation: dense, sparse, lexical, and multi-vector alike.

If you're interested in finetuning dense embedding models, sparse embedding models, or rerankers instead, then consider reading through my prior Training and Finetuning Embedding Models, Training and Finetuning Sparse Embedding Models, and Training and Finetuning Reranker Models blogposts.

This blogpost is about training multi-vector models. If you want to learn how to use them, from loading and encoding to indexing in vector databases, see the companion Multi-Vector (Late Interaction) Embedding Models with Sentence Transformers blogpost.

A dense embedding model compresses a whole text into a single vector, and similarity is one dot product between two such summaries. A multi-vector model (also called a late-interaction or ColBERT-style model) skips that compression. It keeps one small vector per token and scores a query against a document with the MaxSim operator, where every query token finds its best-matching document token and the scores are summed. Token-level matching preserves exactly the fine-grained signals that a single vector has to average away, which usually means stronger retrieval, at the cost of a bigger index.

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