Missing dependency. If import skore raises in this project's env, STOP. Invoke python-env-manager to detect the manager and produce the right install command (the project may not use pixi); surface the command to the user and wait for confirmation. Do not drop back to cross_val_score, cross_validate, classification_report, or hand-rolled metric prints — that silently rewrites this skill out of the project. See data-science-python-stack § "Missing dependency".
Symbol from memory is forbidden. Any skore entry point (evaluate, EstimatorReport, CrossValidationReport, ComparisonReport) and any sklearn splitter name must come from a Skill(python-api) or Skill(python-api) call in this turn. "I remember KFold(n_splits=5)" is not acceptable.
Splitter choice is data-driven, not default-driven (G-CV-SPLITTER). This is the G-CV-SPLITTER gate — owned by this skill, fired during iterate-ml-experiment § 3 (the build → evaluate → test chain, after the design note is approved at G-DESIGN), before src/<pkg>/evaluate.py is written. The splitter is NOT pre-committed in the design note. Pick from the split_kwargs content at the X marker via the table in rule 3 — never reach for KFold(5) or StratifiedKFold out of habit. If split_kwargs is empty and you cannot rule out group / temporal structure, return to build-ml-pipeline and ask before defaulting.
No Stratified* for class imbalance. It compresses across-fold variance and produces over-confident error bars. Imbalance does not change the splitter choice.
CV is necessary but not sufficient for any pipeline with history-dependent features. skore.evaluate(...) materializes the graph once with one env-dict and splits indices — it never exercises a different env-dict at predict time, which is exactly the binding shape production faces. A pipeline that loads-then-features-then-splits passes CV trivially and still silently drops cold-start rows when handed a fresh learner.predict(env₂). The structural check that catches this is the smoke test owned by smoke-test-ml-pipeline — required alongside CV for any pipeline that has a backward shift, lag, rolling window, target shift, or join with side history. If you produce a CV report and the pipeline has any such step, the matching tests/smoke/test_NN_<short_name>.py must also pass before the experiment can flip to done (enforced by iterate-ml-experiment § 4).
All Python execution goes to scratch/. Every Python command — version checks, signature lookups, walking the skore report's metrics accessors, extracting per-fold values, sanity-checking the splitter's fold geometry, multi-symbol inspect.signature(...) on skore / sklearn classes — lands in scratch/<YYYY-MM-DD>_<HHMMSS>_<short>.py and runs via pixi run python scratch/<ts>_<short>.py. Inline pixi run python -c "..." is forbidden regardless of length (see python-api § Stop conditions). The previous "2-line inline cap" is removed.
Don't filter warnings. No warnings.filterwarnings(...) around skore.evaluate(...) or the CV splitter unless the user explicitly asks. See python-code-style § Stop conditions.
skore.evaluate(...) and project.put(...) live only in experiments/NN_*.py. The experiment script is the sole producer of a report in the workspace's skore Project. Re-running evaluate from a scratch/ probe, an audit/ file, a notebook, or a one-off Python file in src/ duplicates the report under the same key and pollutes project.summarize() — the cross-experiment metrics view the audit digest draws from. Two read-only consumers of the Project share the same summarize() → get(id) → report.* discipline: scratch/<ts>_*.py probes (owned by organize-ml-workspace § "Scratch is read-only") and audit/<stem>.py files (owned by audit-ml-pipeline, executed via its bundled in-process IPython runner; output digest at scratch/audit/<stem>/audit.md). Neither calls evaluate(...) or put(...). A third consumer, iterate-from-skore, does not open the Project at all — it reads the audit's digest as text and converts the surfaced checks into Backlog candidates. The trap the two Project-side consumers share: project.get(key) raising KeyError reads as "the report is missing" but actually means "the lookup shape is wrong — get is by id, not by key". Never substitute by re-running evaluate + put. See python-api § "Lookup failure ≠ artifact missing" for the general registry-lookup discipline.
The time-ordered splitter AskUserQuestion is non-skippable, even under harness-level "no clarifying questions" instructions. When the data is temporal, the four-option pick from rule 3 is an operating-contract gate, not a clarifying question. The harness's "no clarifying questions" hint applies to agent-discretionary asks (ambiguous wording, unclear intent); it never overrides a gate a skill explicitly mandates. The same override rule applies to every other mandatory AskUserQuestion in this stack — python-env-manager § "Where does the package belong?", data-science-python-stack § Tier 2 (pandas vs polars), iterate-ml-experiment § 2 (sourcing menu), iterate-from-user § "The entry-point AskUserQuestion". When in doubt: the user's approval is the gate, not the harness's instruction text.

Pre-flight — emit this checklist as visible text before any code

Before writing the evaluation call, output the following block verbatim in your response. Each box must be backed by an actual tool call or an explicit decision documented in the response.

Pre-flight (evaluate-ml-pipeline):
- [ ] Tier 1 mandatory libs importable in this env: sklearn, skrub, skore
      (per `data-science-python-stack` § "Tier 1")
- [ ] Skill(python-api) consulted for skore symbols (evaluate /
      report classes): <symbols>
      Evidence: Read scratch/api/skore/<version>/<topic>.md (this turn)
                | Write scratch/api/skore/<version>/<topic>.md (this turn)
                | "n/a — no new skore symbol introduced this turn"
      "Read python-api SKILL.md" alone is NOT evidence.
- [ ] Call site for `skore.evaluate(...)` / `project.put(...)`
      is `experiments/NN_*.py` (not `scratch/`, not a notebook,
      not `src/<pkg>/`). See Stop condition
      "`skore.evaluate(...)` and `project.put(...)` live only in
      `experiments/NN_*.py`".
      Evidence: Write experiments/<NN>_<name>.py (this turn) |
                "the call already lives in an existing experiments/ file"
- [ ] Skill(python-api) consulted for sklearn splitter: <name>
      Evidence: Read scratch/api/sklearn/<version>/cv_splitters.md
                (or topic-matching file, this turn)
                | Write of the same (this turn)
                | "n/a — splitter is one already in src/<pkg>/evaluate.py
                  and its arguments are unchanged"
      "Read python-api SKILL.md" alone is NOT evidence.
- [ ] split_kwargs at the X marker read: <groups | time | none>
- [ ] Splitter chosen via rule 3 mapping table: <name + reason>
- [ ] Data-passing form picked: <X, y> | <data={...}>
- [ ] Smoke test status (per `smoke-test-ml-pipeline`):
        passing  — CV report can be persisted and experiment can
                   flip to `done`;
        failing  — pipeline has a structural bug; route back to
                   `build-ml-pipeline` (CV report can still be
                   produced, but the experiment stays `approved`,
                   not `done`, until smoke passes);
        n/a      — pipeline has no history-dependent step (rare
                   for time-series / panel data; explain why in
                   the response).
- [ ] If a probe is needed in this turn (skore report walk,
      metric extraction, splitter fold inspection), the payload
      goes to `scratch/<ts>_<short>.py`, **not inline `pixi run
      python -c "..."`**. No inline allowance — all Python
      execution goes to scratch.

Scope

In scope: choosing the evaluation entry point, picking a cross-validator, wiring split_kwargs into the splitter, reading the report, deciding when to escalate to explicit report classes.
Out of scope: pipeline declaration, hyperparameter search, persistence, serving, multi-run tracking.

Core rules

skore.evaluate(...) is the entry point. It is a dispatcher that returns the right report for the task and splitter argument. Never hand-roll cross_val_score + manual metric prints, and don't drop back to bare sklearn for evaluation. If you see existing cross_val_score / cross_validate / classification_report / mean_squared_error calls in the diff, redirect them through skore.evaluate. Consult python-api for the exact signature.

Always pass splitter= explicitly. When splitter= is omitted, evaluate auto-selects: if the learner's DataOp was declared with mark_as_X(cv=...) it reuses that cross-validator (→ CrossValidationReport), otherwise it falls back to a single 80/20 holdout (→ EstimatorReport). This stack does not declare cv at the X marker (build-ml-pipeline § S3), so an omitted splitter= would silently produce a holdout instead of the gated CV choice. Passing splitter= explicitly is what makes the G-CV-SPLITTER decision visible, and it overrides any DataOp cv.

Two data-passing forms — pick the one that matches the estimator:
- sklearn-style: skore.evaluate(estimator, X, y, splitter=...) for any estimator whose fit is (X, y).
- env-dict-style: skore.evaluate(learner, data={"X": X, "y": y, ...}, splitter=...) for a skrub SkrubLearner (its fit takes a single environment dict mapping skrub.var(name=...) names to values). This is the right form for the pipelines produced by build-ml-pipeline.
X/y and data are mutually exclusive. The same split applies to CrossValidationReport(...); EstimatorReport(...) uses train_data= / test_data= for the env-dict equivalent of X_train / y_train / X_test / y_test. The full interop pattern (env-dict-style vs sklearn-style, how data={...} keys map to skrub.var roots, key conventions in the Project store) is in python-api/references/skrub_interop.md; for exact signatures, look them up via python-api against the installed skore version.
Escalate to explicit report classes only when evaluate is too coarse. The escalation order:
- EstimatorReport — single fit on a held-out set (no CV); use when CV is wasteful (e.g., evaluating the final model on all data after CV has already been done).
- CrossValidationReport — k-fold over one learner with access to per-fold artifacts.
- ComparisonReport — two or more learners side-by-side.
See references/reports.md for the escalation table; defer all API details to python-api.
Pick the cross-validator from the structural facts of the data — not by default (the G-CV-SPLITTER gate). The data tells you what splitter is correct. The structural facts arrive at the X marker through split_kwargs (set by build-ml-pipeline at declaration time). Mapping rules:

split_kwargs content Splitter

groups GroupKFold

temporal ordering ask the user (see "Time-ordered data" below)

none KFold (or RepeatedKFold for small / noisy data)

Imbalanced classification does not change the choice — use plain KFold / GroupKFold. See "Avoid by default" below.

Avoid by default:
- Stratified variants (StratifiedKFold, StratifiedGroupKFold, StratifiedShuffleSplit, RepeatedStratifiedKFold) — they reduce across-fold variance by construction, producing over-confident error bars on the score. Don't reach for them on imbalance.
- LeaveOneOut / LeaveOneGroupOut / LeavePGroupsOut — high per-fold variance; aggregate hides the noise. Use KFold / GroupKFold with 5–10 splits instead.
See references/cross-validation.md § "Avoid" for the reasoning. Wiring details: references/metadata-routing.md.

Time-ordered data — AskUserQuestion is mandatory. When the data is temporal, fire AskUserQuestion before picking a splitter, with four explicit options:
1. TimeSeriesSplit(gap=horizon) — growing-window train, contiguous test, embargo equal to the forecast horizon. The safe default for any horizon-h forecasting task: it prevents the train tail from leaking into the test head by up to one horizon. Follow up to surface n_splits / test_size / max_train_size.
2. TimeSeriesSplit(gap=0) — only on the user's explicit pick. Warn in the option description that with horizon h > 0, the last h rows of every training fold predict values whose target time is inside the test fold; the reported metric is optimistic.
3. Custom splitter — purged-and-embargoed (finance), blocked calendar windows, walk-forward with refit cadence. Pick this when the time structure has more shape than TimeSeriesSplit captures. See references/custom-splitter.md.
4. KFold ignoring time — only when the user confirms the temporal structure shouldn't drive splitting (e.g. the time column is a covariate but the task is treated as IID). The skill should not recommend this option on time-ordered data without an explicit user reason.
No silent default. Even if the data looks "obviously TimeSeriesSplit", the user picks via AskUserQuestion. The gap parameter is the one most often wrong by default — TimeSeriesSplit(n_splits=5) from memory uses gap=0, which silently leaks for any non-trivial horizon. The structured pick exists to make that visible. Ambiguous free text ("just pick something", "you decide") routes to a clarifying AskUserQuestion; don't infer.

Separately, ask whether the time column should stay as a covariate or be dropped from the feature matrix (encoders can extract calendar patterns from a timestamp; the user's call). This is a follow-up question, not a substitute for the splitter pick.

If split_kwargs is empty and you cannot confirm there's no structure (from build-time checks or from the user), do not silently default. Return to build-ml-pipeline and ask the user first.
Trust skore's metric defaults; override only on explicit user request. skore.evaluate picks task-appropriate metrics automatically (regression: MSE/RMSE/MAE/R²; binary: accuracy, precision, recall, F1, ROC-AUC; multiclass: macro/micro variants; multilabel: per-label + averages). Override only when the user says so — e.g., "use RMSE", "report ROC-AUC". Don't pre-emptively pin metrics or pass a scoring=... argument unless asked.
Custom splitter — only when sklearn doesn't have it. Examples that justify one: purged-and-embargoed time-series CV (finance), blocked spatial CV. The contract is small: split + get_n_splits. See references/custom-splitter.md. Otherwise, prefer the sklearn built-in.

`split_kwargs` content	Splitter
`groups`	`GroupKFold`
temporal ordering	ask the user (see "Time-ordered data" below)
none	`KFold` (or `RepeatedKFold` for small / noisy data)

Decision flow

Is the goal to score one learner, or to compare ≥ 2?
- One → skore.evaluate(...) (default), escalate to CrossValidationReport or EstimatorReport only if needed.
- ≥ 2 → ComparisonReport.
Read split_kwargs at the X marker.
Map to a splitter using the table in rule 3.
Pick the data-passing form (rule 1): data={"X": X, "y": y, ...} for a SkrubLearner, positional X, y otherwise.
Pass the splitter via splitter=... to the chosen entry point (always explicit — never rely on the omitted-splitter default, which would holdout-or-DataOp-cv; an explicit splitter= overrides any DataOp cv).
Inspect the report; override metrics only on explicit user request.

Companion skills

python-api — every skore symbol used here. Mandatory before naming evaluate, EstimatorReport, CrossValidationReport, ComparisonReport. Don't guess from memory. Cache hits first: check scratch/api/skore/<version>/ before WebSearching for narrative pages; cache new findings back there (per python-api Shape 0/3).
python-api — every splitter used here. Mandatory before naming KFold, GroupKFold, TimeSeriesSplit, etc. Cache hits first: check scratch/api/sklearn/<version>/ before WebSearching.
build-ml-pipeline — upstream pipeline shape and where structural metadata is attached via split_kwargs. Return there if the metadata you need at evaluation time isn't wired in, or if the smoke test (below) fails on row count — that's a graph-topology bug owned by build-ml-pipeline (rule 2, early-mark_as_X).
smoke-test-ml-pipeline — the structural check CV cannot do by construction: predict on a different env-dict from the one used at fit, assert the prediction count matches the predict-grid row count exactly. Required alongside CV for any pipeline with a history-dependent step. The CV report and the smoke test are independent artifacts — both must be in place before an experiment can flip to done.
audit-ml-pipeline — read-only consumer of the report this skill's skore.evaluate(...) produced. The experiment script puts the report; the audit file loads it via project.summarize() → project.get(id) and renders a markdown digest for the agent (no evaluate, no put). Fires at iterate-ml-experiment § 4 record-outcome.
test-ml-pipeline — router for tests/. Owns layout and the stem pairing between an experiment and its smoke test.
python-env-manager — detection + install commands for the project's environment manager (pixi / uv / poetry / hatch / conda / pip+venv). Invoke whenever the Stop condition on import skore fires, or whenever any other dependency is missing from the env. Don't infer the manager or hand-craft the install command — that skill owns it.
python-code-style — must be invoked after writing or editing src/<pkg>/evaluate.py (and, if a custom splitter is authored, the module that holds it). Running pixi run ruff check directly without invoking this skill silently drops the NumPyDoc docstring convention this stack expects: ruff's D-rules pass on a one-line summary, but only the skill body teaches the parameter-shape-in-type-slot, Parameters / Returns / Yields sections, and the imperative one-line summary.

evaluate-ml-pipeline

Evaluate ML Pipeline

Stop conditions — read before anything else