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BULLPEN profiled 260 open-weight models from 400 questions and kept 91 percent of full-pool skill at 1.3 percent of the compute

AspireAI community · arXiv
Choosing among hundreds of thousands of models means understanding each candidate's properties, but every property is normally measured by its own benchmark suite, so profiling a model on many properties means paying for many separate evaluations. Model embeddings compress a model into a fixed-length vector from which many properties can be predicted, though prior methods build those vectors from correctness patterns and question features and often need large probe batteries. BULLPEN extends that line by adding 20 behaviour, style and provenance task benchmarks, by reading a model's own answer text jointly with its correctness and the question text, and by encoding models from a few hundred questions instead of the full 24,118-question pool. Across 260 open-weight models its answer-text encoder scores 0.48 against 0.35 for the best prior encoder, and keeps at least 91 percent of full-pool skill from 400 questions, 1.3 percent of the full pool's inference compute.
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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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