Skill Eval

/skill-eval

Build the falsifier for a skill. A skill earns its place only when an agent with it produces measurably better outcomes than the same agent without it — otherwise it is context tax. This skill designs, runs, and maintains that proof, and the proof must be able to come back no-skill won.

An eval is a run with a grader, never a directory shape. Structural eval trees were deleted as theater in the 2026-06 consolidation (backlog.d/103); do not rebuild them. The exemplar to copy is skills/design/evals/routing-eval.md: an answer key, an objective grader, a pass bar a no-op revision fails, a run log. The protocol contract is skills/harness-engineering/references/mode-eval.md.

The loop

1. Name the one claim. Every skill earns exactly one load-bearing, falsifiable claim — the thing it makes true that raw prompting does not (design: "routes to the right specialist first try"; shape: "a stranger builds the right thing from the packet"). Write it as a sentence that could be wrong. A skill whose claim you can't name, or whose claim raw prompting already meets, is a deletion candidate — say so and stop. That is a valid, cheap eval result, not a failure to produce one.
2. Fix the task. 2–3 fixtures that stress the claim differently: a frozen prompt + a repo pinned at a SHA + a forbidden-edits list. One fixture is noise; cover the claim's failure modes, not one happy path.
3. Run the A/B. Same model, same fixture, two arms:
   • A — the skill installed and invoked.
   • B — raw: the bare instruction a sharp operator would type, same model, same repo access, no skill.
   • C (optional) — a credible alternative primitive (external skill, Ponytail for simplicity pressure). Add only when one exists; A-vs-B is the floor (backlog 112's three-condition shape). Drive it per references/run-recipe.md (native-subagent smoke = free; council.sh + decorrelated families = the serious run).
4. Grade blind, objective first. The grader sees the artifacts and the fixture, never which arm is which, and is a different model family than the workers. Mechanical checks before taste (below).
5. Verdict + disposition. Pass = A beats B paired on the claim across ≥2 of 3 fixtures. Then label the skill: keep / adapt / cut / needs-more-tasks / graduate-to-Daedalus. The verdict is about the skill's right to exist, not a vanity score.

Graders — three tiers, and the human is the gold

• Objective first (scriptable, ~free, every edit): sections present and non-empty, oracle is a runnable command not "it should work", cited paths resolve at the SHA, gate passes, forbidden edits absent, artifacts render. These fail without a judge — push every check you can down to this tier.
• Human judgment is the ground truth for the judgment-heavy delta. A spec's buildability, a design's taste, a critique's bite — these are the operator's call, and for most skills that verdict needs human input, not just a model's. The keep/adapt/cut decision on a taste-heavy skill is signed off by the human, or by a grader carrying a recent human anchor (see Cadence) — never by an unvalidated model judge alone. At minimum, the operator blind-grades one fixture per eval; that grade is the anchor everything else is checked against.
• The agent rubric is a calibrated proxy, not the judge. A blind, decorrelated model grader is how you afford to run the rubric often and cheaply — but it approximates the operator's taste and drifts. 1–5 per dimension tied to the claim, one-line justification, scored blind, a different family than the workers (a worker grading its own family flatters itself; same-family smokes prove the loop fires, not the margin). Trust its margin only while it matches the human anchor. When proxy and human disagree, the rubric is broken (or the claim is) — fix the grader; do not overrule the human.

Cadence — match cost to stakes

A full pass is ~15–18 runs (paid, slow). Do not gate every edit on it.

• Every skills/<skill>/** edit → cheap objective checks + a 1-fixture native-subagent smoke. Catches gross regressions for free.
• Contract-level change (the skill's claim or output shape moved) → full A/B, decorrelated, all fixtures. The skill changed; re-earn the claim.
• Major model release → re-audit. This is the point of the eval. A stronger bare model erodes every skill's edge; the skill that beat raw on the old model may be railroading on the new one. The eval is how you find the skills to retire.
• Continuous → telemetry (harness-kit-checks telemetry): did it trigger when it should, did loaded sessions cost more than they returned. Context, not proof (backlog 112).
• Calibration (the human anchor) → every eval carries ≥1 fixture the operator graded blind; the agent grader is trusted only while its verdict matches that anchor. Re-anchor when the rubric changes, the worker model upgrades, or proxy-vs-human last diverged. No recent anchor → the automated verdict is unvalidated; say so rather than quoting the margin.

Gotchas

• Falsifier that can't fail. A pass bar the skill always clears is theater. Before running, ask: what result tells me to delete this skill? If nothing can, the eval is decoration.
• Rubric laundering. Vague dimensions ("is it good") let the grader rubber-stamp the skill arm. Tie every dimension to the claim; prefer an objective check to a rubric line wherever one exists.
• Self-graded family. Grader shares the worker's model family → inflated margin. Smoke-only waiver, never the serious verdict.
• One fixture. A single task proves nothing generalizes. ≥2, spanning the claim's distinct failure modes.
• Eval bloat. This skill stays minimal. Serious, repeated arena work (composition sweeps, model selection) graduates to Daedalus; it does not expand Harness Kit into a benchmark platform.
• Grading prose, not outcomes. "The packet reads well" is not the claim. "A cold lane built the feature from the packet" is. Grade the outcome the skill promises.
• Agent judge ≠ ground truth. The rubric grader is a proxy for the operator's taste; an unanchored proxy rubber-stamps. Anchor it to a blind human grade or don't quote the margin.

Route

Verification

The eval spec lands at skills/<skill>/evals/<skill>-eval.md (mirrors the design exemplar). Run evidence lands at .evidence/harness-evals/<skill>/<date>/ — sanitized artifacts + scored receipts only, never raw transcripts with secrets. A run is real only when it produced both arms and a grader verdict that could have gone the other way.

cargo run --locked -p harness-kit-checks -- check-eval-coverage --repo . (folded into the repo gate) enforces that every first-party skill has either an eval spec or a live, unexpired skills/<skill>/evals/WAIVER.md — see skills/harness-engineering/references/skill-design-principles.md's "New Skill: Eval Scaffold Is Not Optional" for the exact new-skill scaffolding steps this gate enforces.