Lightning Weave composed two separately post-trained reasoning strengths into one model and got better accuracy with fewer tokens on both math and code
AspireAI community · arXiv
*Post-training a model for accuracy usually makes it slower, and post-training it for efficiency usually makes it worse - so how do you get both?* Lightning Weave takes the two capabilities apart, runs on-policy distillation, and recombines them. Each acquired capability is represented as the policy shift between a model before post-training and the resulting specialist, and the framework combines those aligned log-ratio shifts at shared student token states. A Tilted-Target DOPD step converts the cached signals into a stable learning target, which lets each anchor pair score its trajectories once instead of keeping several live anchor models in memory.
On Qwen3.5-4B it raises HMMT 2025 accuracy from 59.2 percent to 64.0 percent while using 10.7 percent fewer response tokens, and lifts LiveCodeBench v5 from 41.7 percent to 54.2 percent with 9.6 percent fewer response tokens. Tuning the relative strength of the anchor signals traces out an empirical accuracy-efficiency Pareto frontier. The caveat: the authors label this work in progress, and the headline results are on a single 4B student family, so the recipe is not yet demonstrated at frontier scale.
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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.