Where Should a Skill Edit Live? Scoped Root Evolution in Hierarchical Agent Skill Libraries

Self-evolving agents turn execution experience into edits to a skill library, but most edit locally, so what they learn stays where it was written. In a hierarchical library only the shared root skills can carry an edit to tasks that did not produce it; for the same reason a root edit's scope is the whole subtree, so it can repair the tasks it was written for while silently breaking a sibling, which admission rules that score only the proposing tasks or the subtree mean cannot see. We recast rootlevel evolution as an edit-placement problem with a single rule: an edit belongs in the root only if no evaluated descendant regresses under paired execution, and otherwise in the leaves of the descendants it helps. ScopeEvo instantiates the rule with root-aware revision, a subtree gate that promotes, localizes, or discards each candidate, and release recursion, and treats the leaf editor as a pluggable singleskill optimizer. On SkillsBench it further improves a library already evolved by a strong baseline, and on SearchQA and DocVQA it matches the strongest baseline with fewer editor calls and a more compact library. Its promoted roots repair tasks the loop never saw, and its refused root edits are recovered in the leaves rather than lost. We further show why a root edit needs subtree-wide validation while a localized edit does not, and that what a gate can guarantee from a few executions is bounded by the coverage of its validation set, which more runs cannot extend.