Asked to modernize 100 legacy WordPress PHP files, five LLMs each broke 32 to 51 working files while the rule-based tool Rector broke 7
AspireAI community · arXiv (When Modernization Breaks Working Code, Mahmud et al.)
Pasting a legacy file into an LLM and asking it to modernize the code is a shortcut worth measuring against leaving the code alone. Five LLMs each attempted 100 WordPress 4.0 PHP files to PHP 8.3 with no project context, scored against 630 hidden behavioral checks and 511 Rector-detected findings. Untouched, 83 files already passed every check and the best LLM left only 59; each model broke 32 to 51 working files while repairing at most 10, whereas Rector broke 7 and repaired 6. Only 37 findings, about 7 percent, actually required a change under PHP 8.3, yet the LLMs broke 36 to 60 percent of files that needed no change. The limitation is scope: single files, no project context, no execution feedback.
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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.