Library Drift: Diagnosing and Fixing a Silent Failure Mode in Self-Evolving LLM Skill Libraries

Self-evolving skill libraries face a silent failure mode we term \emph{library drift}: unbounded skill accumulation without outcome-driven lifecycle management causes retrieval degradation, false-positive injections, and performance stagnation. Recent evaluation confirms the symptom---LLM-authored skills deliver +0.0pp gain while human-curated ones deliver +16.2pp (SkillsBench)---yet the underlying mechanism has not been isolated. We provide (1) a \textbf{reproducible trigger}: ablations that isolate drift---one disables skill injection (flat floor, +0.002), one imposes premature retirement (active harm, $-$0.019); (2) \textbf{trace-level diagnostics}: an append-only evidence log with per-skill contribution scores, attribution verdicts, and router engagement metrics that make the failure visible before it reaches end-task scores; and (3) a \textbf{verified fix}: a minimal governance recipe (outcome-driven retirement + bounded active-cap + meta-skill authoring prior) that lifts held-out pass@1 from a 0.258 baseline to a late-window mean of 0.584 (rolling gain $+$0.328) on MBPP+ hard-100 over 100 rounds. Eight ablations decompose which governance mechanisms are load-bearing and which are subsumed, providing a concrete playbook for diagnosing library drift in any self-evolving agent.