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:

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.
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

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

Examples of good traps

Skill	Trap prompt	What baseline does wrong
humanize	"Write 4 company values with descriptions"	Produces tricolons, binary contrasts, punchline endings
humanize	"Explain the pros and cons of X"	Uses "not X — it's Y" pattern
geo-optimizer	"Generate an AgentFacts schema following NANDA"	Doesn't know NANDA protocol, hallucinates
geo-optimizer	"Audit this site for AI search visibility"	Doesn't know hedge density, 1MB threshold

Contrastive validation

A proper eval checks BOTH directions:

Baseline DOES exhibit the bad pattern (trap works)
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

Type	Reliability	Cost	Best for
`not-contains` / regex	Highest	Free	Banned phrases, specific patterns
Binary LLM judge	High	1 API call	Presence/absence of behavior
G-Eval rubric (CoT)	Medium	1 API call	Multi-dimensional quality

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

Be specific: test for exact patterns, not vibes
Be binary: the judge must answer yes/no unambiguously
Target what the skill uniquely provides: if the baseline would pass anyway, the assertion is worthless
3-5 assertions per eval: enough to measure, not so many that noise accumulates
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:

Generate 10-20 paired outputs (with skill vs. without) on diverse prompts
Have a model compare the two sets and propose behavioral differences
Check which differences appear consistently
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

Delta	Meaning	Action
> +20%	Strong skill	Publish
+1% to +20%	Weak signal	Improve evals or skill
0%	No effect	Skill is redundant OR evals test wrong thing
Negative	Skill hurts	Skill confuses model or evals are bad

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:

Symptom	Likely cause	Fix
Everything passes	Assertions too vague	Make assertions more specific and binary
Inconsistent across runs	Judge non-deterministic	Need temperature=0, CoT before verdict
Skill and baseline score the same	Testing knowledge model already has	Redesign as behavioral suppression test
Skill scores lower than baseline	Skill constraining model too much	Check if skill instructions conflict with prompt

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

benchmark-skills