Metaharness Learn

Surfaces metaharness learn — the upstream GEPA learning harness that evolves harness policy genomes against a scored task corpus instead of hand-editing prompts. Candidates are scored on held-out slices and only measured winners promote (the shipped cand-6 genome is the first such promotion: holdout gold 2/12 → 3/12, zero regressions).

When to use

• A harness's policy prompt underperforms on a task family and you want a measured improvement loop rather than manual prompt iteration.
• Pricing a learning run before committing spend — the default dry-run resolves the slice manifest and reports cost without any model calls.
• After a learn run promotes a genome: pair with harness-gepa --op render to inspect what the promoted policy actually says.

Preconditions (upstream design)

The learning harness (GEPA + SWE-bench + Docker) is too heavy for the npm package, so learn needs a local clone:

git clone https://github.com/ruvnet/metaharness.git
node scripts/learn.mjs --repo ./metaharness --host claude-code --model haiku --slice slices/lite.json

Without a checkout the script emits {status: "checkout-required"} and exits 0 — a precondition report, not an error (distinct from degraded: true, which means the npm package itself is absent). The managed-service path (gateway-side learn jobs, no checkout) is upstream's ADR-235 follow-up and not available yet.

Algorithm

Implementation: scripts/learn.mjs.

1. Validate --repo exists when given; export it as $METAHARNESS_REPO.
2. Invoke the pinned metaharness binary (metaharness@~0.4.1, local install or one-time versioned cache — never @latest): metaharness learn --host <h> --model <m> --slice <s> [--run] via _harness.mjs (graceful degradation, hard timeout).
3. Default timeouts: 120s dry-run, 600s with --run — real runs on larger slices need an explicit --timeout-ms matched to slice size × model cost.
4. Detect the checkout-required message → structured payload, exit 0.
5. Parse the trailing JSON report when upstream emits one; otherwise return the raw report text under rawReport.

Cost note

--run is the ONLY path that spends. Everything else — dry-run, checkout probe, degraded path — is $0. The MCP tool (metaharness_learn) has a 120s subprocess budget; run real learning cycles from a terminal via ruflo metaharness learn ... --run --timeout-ms <big>.

Exit codes

• 0 — report produced (or dry-run, checkout-required, degraded)
• 1 — --alert-on-fail and the learn run reported failure
• 2 — config error (bad --repo path)