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LightOn converted its dense and late-interaction retrievers into sparse models that beat them, with +10.19 points on long-document retrieval

AspireAI community · Hugging Face Blog — LightOn
Sparse retrieval lets an inverted index skip documents sharing no active latent with the query, which is much cheaper than scanning a dense vector store — but sparse models usually have to be trained from scratch, which is expensive. LightOn's SlideOn and mSlideOn take a different route: sparse autoencoders pretrained on the existing DenseOn and mLateOn checkpoints supply a learned latent vocabulary, and supervised fine-tuning adapts them into sparse retrievers. SlideOn reaches 56.68 on BEIR-15 against DenseOn's 56.20, and mSlideOn beats mDenseOn by 4.76 points on multilingual retrieval (MIRACL) and 10.19 points on long-document retrieval (MLDR) while also leading on code retrieval (MTEB Code). Matryoshka-style training keeps quality as vectors are pruned, so sparsity is adjustable at inference without retraining, and the SAE checkpoints, models and training code are released under a permissive license. The limitation is the benchmark scope: these are retrieval-suite numbers on English, multilingual and code collections, not end-to-end application results.
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One distance function matched sparse-autoencoder features across layers and compressed feature circuits into supernodes
Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.
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