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LLM-text detectors broke under rewriting, so MCRP probed how a frozen model responds when a matched condition is added or removed

AspireAI community · arXiv (MCRP)
Detectors for LLM-written text lean on lexical, syntactic and local discourse cues that are weakened the moment output is rewritten, polished, expanded or lightly edited. MCRP measures something different: a frozen probe LLM first writes a short semantic condition, then token-level losses and prompt-target attention are compared across matched, null and control conditions, and the resulting conditioned-alignment, counterfactual-sensitivity and attention-coupling features feed a lightweight random-forest classifier with the probe LLM frozen throughout. Under rewriting, polishing and expansion it reaches 93.97% F1 and 96.24% ROC-AUC, ahead of the strongest evaluated baseline by 19.50 and 25.77 percentage points. A condition-control study shows matched conditions beating random, mismatched and corrupted alternatives, which is the evidence that the semantic matching itself is doing work rather than the feature set.
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