Textual Catastrophic Forgetting in Self-Evolving Agent Skill Libraries

Self-evolving LLM agents externalize competence into textual skill libraries that grow with every solved task. We identify a previously uncharacterized failure mode—textual catastrophic forgetting (TCF): agents lose mastered competence not through weight drift but because the library itself degrades. We make three contributions. (i) We decompose TCF into three independently measurable channels—retrieval miss, semantic rot, and execution regression—operationalized in TCF-Bench, a four-phase leave-and-return protocol with coupled API and domain drift across five task domains. (ii) In accumulating libraries the dominant forgetting channel undergoes a drift-driven phase transition whose critical size shrinks with drift pressure (from ≈62 skills under mild drift to ≈33 under heavy drift; Pearson r = −0.85 over the 21 seed×severity points, Spearman ρ = −0.96 over the 7 severity means): under heavy drift a library is born already in the semantic-rot regime—the asymptotic bottleneck of uncurated growth. (iii) An ecological governance framework coordinating six load-bearing mechanisms (carrying-capacity admission control, stability shielding, niche-focused foraging, viability ranking, regeneration, and consolidation) plus prediction-error-gated reconsolidation—whose A/B-verified repair closes an autocatalytic contamination loop—reduces forgetting by 81% (TCF 0.036 vs. 0.186), attains the highest task success (0.667 vs. 0.140) and regression-probe success (0.755 vs. 0.159) against the strongest of the re-implemented published baselines (UtilityCull), and beats every baseline on all three metrics with Holm–Bonferroni-corrected paired tests (all p < 10⁻⁴). We also replicate the protocol on live inference on 5 served LLMs (6,901 API calls, 8.1M tokens, programmatic ground truth): bloat control transfers, the aggregate TCF advantage does not—a negative result reported in the external-validity section. A twenty-variant ablation and a carrying-capacity sweep confirm channel–mechanism correspondence as a graded dose-response. Our results reframe skill-library forgetting from an observed pathology into a measurable and governable process.