Fidelity Is Not Safety: Gently-Compressed LLMs Pass Every Data-Free Quality Guard Yet Invent Procedure Steps in Agentic Execution

TL;DR - Gently compressed LLMs can pass standard quality and fidelity checks yet invent procedural steps during agentic execution. A data-free compression-error screen identifies risky low-rank builds before deployment.

• SVD truncation caused invented SOP steps across three model families; perplexity-matched magnitude pruning did not.
• Perplexity, MMLU, and representation-based fidelity tests failed to predict this behavior.
• Risk correlated with compression-error coherence multiplied by error rate, not overall damage magnitude.
• Fixed thresholds for coherent-error fraction and error rate flagged failing builds across architectures.