Microsoft, Shanghai Jiao Tong University, Tongji University, and Fudan University researchers developed SkillOpt, a text-space optimizer that trains natural-language skill documents while keeping the target model frozen. The system uses an optimizer model to propose bounded edits based on scored rollouts, exporting the result as a single `best_skill.md` file.
- Skills trained on GPT-5.4 transfer to smaller variants with retention rates ranging from 16% to 82% of in-domain gains.
- A skill optimized in Codex lifted Claude Code's SpreadsheetBench score from 22.1 to 81.8, slightly exceeding the 80.4 achieved by training directly on Claude Code.
- Cross-harness transfer for LiveMath was asymmetric, with only 10% retention when moving from Codex to Claude Code.
- The artifact size ranges from 379 to 1,995 tokens, built from one to four accepted edits, and adds zero inference-time calls at deployment.
The shared `best_skill.md` format allows procedural skills to move across different tool APIs, enabling optimization in the cheapest environment for deployment in another.