baseline — freeze splits, score the seed

baseline is the first phase that touches data, so it owns the run's one irreversible decision: the split. It writes splits.json once (seeded), scores the unmodified seed capability on val, and records that score as the bar every algorithm must beat.

Run implement-and-check first. baseline re-runs that check itself and exits non-zero before creating a run dir if it is red — a split frozen against a broken adapter poisons every number measured afterwards.

Why it matters

• Fair comparison point. Every algorithm hill-climbs against the baseline val score; a candidate that does not beat it is not progress.
• Headroom. The printed JSON carries headroom (1 - val) and headroom_verdict: saturated means the seed is already at the ceiling and further iterations buy noise — stop; floor (val at 0) usually means a mis-wired adapter rather than a hard task — re-check before spending budget; ok means proceed. The same verdict is logged as a headroom event so the orchestrator can stop on it with no human reading the number.

Splitting choices

• Seeded ratio split (default 0.5 / 0.25 / 0.25): deterministic given --seed. Reproducible runs partition identically.
• Pinned split (--split-ids): a JSON {train,val,test} of ids — use a benchmark's official split, or set all three equal to fit the whole set with no holdout (the test number is then a fit metric, not a held-out result; the run dir records a splits_warning saying so).
• A ratio split that leaves val or test empty is refused — the gate would have nothing to decide on and the sealed test number would cover no tasks. Below 5 val tasks baseline warns: the gate's bar is optimistic at that n, and a candidate that improves exactly one val task cannot reliably clear it at all (issue #351), so size val with the decisions it has to make in mind.

Reusing a prior baseline (--reuse-baseline PRIOR_RUN_DIR)

Re-scoring the seed is wasteful when the split + seed are unchanged. --reuse-baseline <prior run_* dir> (spec key reuse_baseline) copies that run's splits.json, baseline.json, seed snapshot and seed val rollouts into the fresh run dir and skips the baseline eval; the copied test_used flag is reset so this run can still finalize on test exactly once. --resume is the same-run variant: reopen an existing run dir, skip the eval when baseline.json is already there. Budget flags (--max-iterations, --stall, --max-usd, …) are accepted here because the run dir owns the budget and later phases read it from there.

Runs standalone (/cap-evolve:baseline) or headlessly via cap-evolve run; same scripts/run.py either way.

How to run

python scripts/run.py --base .capevolve --project .capevolve/project \
    --capability seed_capability --seed 0 --ratios 0.5,0.25,0.25 \
    --max-iterations 10 --stall 2

Prints the run-dir path (used by the algorithm + finalize), the split sizes, the baseline val and the headroom verdict. Use --n-trials ≥ 3 for stochastic targets so the baseline carries a real stderr rather than 0.

The one failure mode nothing later can repair is re-splitting after this phase: a task migrating out of test leaks the held-out set, and every later number — including the sealed test score — becomes unfalsifiable.

References

• references/concepts.md — why the split is frozen once and seeded, how to read the headroom verdict, and why no-holdout runs are fit metrics, with sources.