Opinionated Python stack for data-science / ML work — one library per job, organized into tiers (mandatory / user choice / optional / transitive). SKILL.md is the index; per-library `references/<library>.md` files carry scope, "pick this when" / "pick something else when", and pairings. TRIGGER when (any of these): (1) **a library import fails** in this stack's domain — the answer is install, not substitute (see § "Missing dependency"); (2) **a library choice has to be made** — explicitly (the user asks "which library for X?") or implicitly (code is about to introduce a new dependency, or the project is being scaffolded and the tabular library hasn't been picked yet); (3) starting a new Python data-science / ML project; (4) the user or current code reaches for a substitute outside the stack (xgboost, lightgbm, black, isort, flake8, poetry, hatch), or reaches for `mlflow` to log params/metrics, or for `cross_val_score` + handwritten reporting — redirect: tracking → `skore` Project API,

Opinionated Python stack for data-science / ML work — one library per job, organized into tiers (mandatory / user choice / optional / transitive). SKILL.md is the index; per-library `references/<library>.md` files carry scope, "pick this when" / "pick something else when", and pairings. TRIGGER when (any of these): (1) **a library import fails** in this stack's domain — the answer is install, not substitute (see § "Missing dependency"); (2) **a library choice has to be made** — explicitly (the user asks "which library for X?") or implicitly (code is about to introduce a new dependency, or the project is being scaffolded and the tabular library hasn't been picked yet); (3) starting a new Python data-science / ML project; (4) the user or current code reaches for a substitute outside the stack (xgboost, lightgbm, black, isort, flake8, poetry, hatch), or reaches for `mlflow` to log params/metrics, or for `cross_val_score` + handwritten reporting — redirect: tracking → `skore` Project API,