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Multi-turn agents learn faster when credit is assigned to both the turn and the token

AspireAI community · arXiv (Jiaju Chen et al.)
A multi-turn LLM agent gets one sparse signal at the end of a task, but two credit-assignment questions happen at once: which responses helped, and which token decisions inside a response mattered. Existing methods answer only one of them. VETTA learns turn-level and token-level values through separate heads on one shared lightweight critic, then combines each turn advantage with a within-response-centered token residual for the PPO update. To keep the critic cheap it retains only the early Transformer blocks from the checkpoint that initialized the actor. On ALFWorld and WebShop, VETTA improved success rates over PPO by 37.5% and 22.3% with Qwen2.5-1.5B-Instruct, reaching 95.5% and 76.0% with Qwen2.5-7B-Instruct, and the critic-depth comparisons show strong task performance with substantially less critic-side computation. The numbers come from two agent benchmarks with small-to-mid models, so the gains are best read as evidence for the mechanism rather than a settled improvement for frontier-scale agents.
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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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