Evaluating Agent Skills for Version-Specific Plugin Migration: A Retrospective Study

Agent skills package version-specific maintenance knowledge for coding agents, but a higher diagnostic score does not by itself show that the resulting migration advice satisfies the target version's contract. We study a shipped plugin-upgrade skill through an archive of 64 reports on 16 static migration tasks, with two attempts per condition and 328 criterion decisions. With the skill, mean recorded reward rises from 93.83 to 98.75, a gain of 4.92 points (95% task-bootstrap interval [0.31, 10.86]); the gain is concentrated in one task, and eight task pairs are at the ceiling. Tracing every decision to its contract domain and reviewing ten reports in depth exposes grading errors that favor either arm; in one, a containment predicate that accepts the parent directory still receives full credit. Executable probes confirm this defect and show that a working teardown repair is excluded only by a narrower lifecycle rubric. Replacing the reviewed decisions keeps the estimate positive (4.61 to 5.39 points) but moves its interval to or across zero. Re-grading all 64 reports with judges from two other model families, without arm labels or prior scores, agrees with the original judge on 91.8% and 95.7% of decisions (weighted kappa=0.64 and 0.72) and gives gains of 10.63 and 6.09 points. The study contributes a traceable evaluation that connects aggregate reward to contract-level evidence and judge sensitivity, together with concrete review checks for migration advice. Executable end-to-end repairs, independent human annotation, and other frameworks are left to future work.