Benchmark Skills

Write evals for skills and run the benchmark harness to measure whether a skill actually helps compared to baseline (no skill).

The Core Principle

Only two types of skills produce measurable benchmark delta:

1. Behavioral suppression — The skill suppresses patterns the model naturally produces. The baseline consistently exhibits the bad behavior; the skill stops it. This is the highest-signal category.
2. Genuinely novel knowledge — The skill injects domain knowledge NOT in the model's training data. If a knowledgeable human would need to look it up, the model probably doesn't know it either.

What does NOT produce delta (don't waste time benchmarking these):

• Knowledge the model already has (common frameworks, well-known patterns)
• General quality improvement without a specific behavioral target
• Skills requiring real system access (filesystem, APIs, browsers)
• Skills requiring multi-turn interaction

Pre-Flight Checklist

Before writing evals for a skill, verify ALL of these:

• The skill changes default model behavior OR injects genuinely novel knowledge
• The skill works in single-prompt-in/single-response-out mode (no interactivity)
• The skill doesn't require real system access to demonstrate value
• The skill is ours (not copied from another publisher)
• You can design at least 2 trap prompts that reliably elicit baseline failure
• Assertions are concrete and binary (not vague quality judgments)

If any box fails, the skill is not a good benchmark candidate.

Eval File Format

Every skill that wants benchmarking needs an evals/evals.json file:

skills/
  my-skill/
    SKILL.md
    evals/
      evals.json

evals.json Structure

{
  "skill_name": "my-skill",
  "evals": [
    {
      "id": 1,
      "prompt": "The exact prompt to send to the model",
      "expected_output": "Description of what a good response looks like",
      "files": [],
      "assertions": [
        {
          "id": "unique-assertion-id",
          "text": "Specific, verifiable claim about the output",
          "type": "qualitative"
        }
      ]
    }
  ]
}

Trap Input Design

Every eval prompt must be a trap — a prompt that reliably elicits the bad behavior the skill suppresses. If the baseline model passes your assertions without the skill, your test case is useless.

How to design traps

1. Identify what the skill changes (what patterns it suppresses or what knowledge it injects)
2. Write a prompt that naturally invites those patterns
3. Verify the baseline model actually falls into the trap (run without the skill first)
4. If the baseline passes, redesign the prompt or drop the test case

Examples of good traps

Contrastive validation

A proper eval checks BOTH directions:

1. Baseline DOES exhibit the bad pattern (trap works)
2. Skill output does NOT exhibit the bad pattern (skill works)

If baseline passes an assertion, that assertion is not measuring delta.

Writing Assertions

Assertion types by reliability

Default to negative assertions for suppression skills. "Output does NOT contain tricolons" is more reliable than "output sounds natural."

Good vs bad assertions

Bad assertions (will show 0% delta):

• "The response is helpful" — too vague, baseline passes
• "The response is correct" — not specific to skill
• "The response describes three phases" — model already knows this

Good assertions (will show real delta):

• "The output does NOT use binary contrast patterns such as 'not X — it's Y'" — specific, testable, baseline fails
• "The response includes the @context field pointing to nanda.dev namespace" — genuinely novel knowledge
• "Processes are categorized into safety levels rather than a flat list" — specific format the skill teaches

Rules

1. Be specific: test for exact patterns, not vibes
2. Be binary: the judge must answer yes/no unambiguously
3. Target what the skill uniquely provides: if the baseline would pass anyway, the assertion is worthless
4. 3-5 assertions per eval: enough to measure, not so many that noise accumulates
5. Mix negative and positive: "does NOT contain X" AND "DOES contain Y"

Assertion Discovery (VibeCheck Method)

If you're unsure what assertions to write for a new skill:

1. Generate 10-20 paired outputs (with skill vs. without) on diverse prompts
2. Have a model compare the two sets and propose behavioral differences
3. Check which differences appear consistently
4. Those consistent patterns become your formal assertions

This prevents guessing at assertions that don't actually differentiate.

Running the Benchmark

bun run benchmark                                    # All skills with evals
bun run benchmark --skill geo-optimizer              # Single skill
bun run benchmark --model claude-sonnet-4-6          # Override model (default: haiku)
bun run benchmark --concurrency 4                    # Parallel workers

From within Claude Code, prefix with CLAUDECODE= to avoid nested session errors.

The harness runs each eval prompt twice: once with the skill injected via --append-system-prompt, once without. Both outputs are graded by LLM-as-judge.

Reading Results

Results go to benchmarks/latest.json and per-skill evals/benchmark.json:

Key Metrics

• pass_rate: Assertion pass rate with skill active
• baseline_pass_rate: Assertion pass rate without skill
• Delta (pass_rate - baseline_pass_rate): The signal

Publishing Policy

• Only publish skills with positive delta
• Zero or negative = don't publish, refine skill or evals
• latest.json merges per-skill results when using --skill flag

Judge Quality

The LLM-as-judge has known failure modes. When results seem wrong:

Lessons Learned

These patterns have been confirmed through multiple benchmark runs:

• Behavioral suppression skills are easiest to benchmark (humanize: +53%)
• Novel knowledge injection works if truly novel (geo-optimizer: +50%, NANDA protocol)
• Common knowledge injection shows 0% delta (charting, prd-creator, hunter-skeptic-referee)
• Skills needing system access can't be benchmarked this way (process-cleanup: -5%)
• Long, expensive prompts waste money without improving signal (saas-launch-audit)
• 2-3 well-designed evals beat 10 mediocre ones