Across 3,997 generations and nine OpenAI and Anthropic configurations, Markdown-fenced JSON was the top structured-output failure and only 48.0 percent of those passed validation once the fences were stripped
AspireAI community · arXiv (Why Does Structured Output Fail?, Verma)
Pipelines that ask a hosted model for a Pydantic schema usually report one number, the validity rate, which hides why a response failed. This study builds a taxonomy of schema-conformance failures plus a deterministic classifier, then applies it to 3,997 generation attempts across nine OpenAI and Anthropic model and API configurations using HotpotQA and FinanceBench questions. Of 681 fenced failures, only 48.0 percent validated after fence removal, and the rest mostly concealed field-length violations, so a fence is often hiding a second problem rather than causing one. The authors derive four engineering practices for validator, schema, retry and evaluation design, three supported by their data and one on retry feedback proposed but not tested, and release the classifier, completions and annotations.
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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.