Skill Probe

ci npm

Runtime-agnostic, co-loaded-aware auditor for an AI agent's skill library. It measures which skill actually fires (by real activation, not keyword matching) when your whole SKILL.md library is loaded together — with statistical confidence, not single-shot guesses.

Your skill works alone. Does it still work when the whole library is loaded?

Most skill tooling tests one skill, in isolation, once. Skills only conflict when loaded together, and activation is stochastic. skill-probe is the tool that tests the real thing.

Complements static linters like skill-audit (security/quality) — this is behavioral.

How it works / requirements. skill-probe is a terminal CLI, not an in-agent slash command. It drives your local runtime CLI under the hood, so you need that runtime installed and authenticated: claude (logged in) for runtime: claude-code, or opencode for runtime: opencode. gen/fix/diagnose use ANTHROPIC_API_KEY if set, otherwise the local claude CLI. Codex/Gemini runtimes aren't supported yet (their traces don't expose which skill fired).

The workflow:

doctor → gen → audit → context → diagnose → fix

• doctor: verify setup, auth, runtime, skills, and config before spending probes.
• gen: draft a probe config from your existing skills.
• audit: measure which skill actually fires when the whole library is co-loaded — with --baseline, gate CI on statistically significant regressions only.
• context: compare isolation vs co-loaded activation to catch library interference — add --ablate to name which sibling is stealing the trigger.
• diagnose: explain whether a failure is a routing miss or a description problem.
• fix: rewrite a skill description and keep it only if the measured reliability improves.

Why

• Skill activation is stochastic: the same prompt can produce different skill-routing outcomes.
• A single run lies (we measured the same prompt at 0/5 one batch, 2/3 the next).
• skill-probe runs each prompt k times, reports a Wilson 95% CI, stops early when the result is statistically decided, and flags trigger-theft (a sibling stealing a trigger).

Install

npm i -g skill-probe      # or: npx skill-probe

Start here (skill-probe doctor)

Run this first — it catches the setup problems (no skills dir, runtime not installed or not authenticated, a config typo) before you spend any probes:

skill-probe doctor --cwd .                      # check the project + runtime
skill-probe doctor --config probe.config.json   # also sanity-check a config

skill-probe doctor

PASS  Node 22.18.0
PASS  config parsed: probe.config.json
PASS  found .claude/skills/ with 4 skills
PASS  expected skills all exist
PASS  claude CLI found
PASS  claude-code probe succeeded (CLI is authenticated)
WARN  threshold 90% with k=10 cannot certify a pass — need ~k=35
FAIL  skill "greeter" missing description:

It checks: Node version, .claude/skills/ exists, every skill has a SKILL.md with name: + description:, names match folders, the config parses, every expected skill exists, the threshold/k are statistically achievable, the runtime CLI is installed, and a harmless live probe authenticates. Exit 0 healthy · 1 warnings only · 2 hard failures. Use --skip-probe to skip the live (costing) auth check.

Generate a config (skill-probe gen)

Don't want to hand-write the cases? Draft them from your skills, then review:

skill-probe gen --cwd . > probe.config.json

It reads each .claude/skills/*/SKILL.md, and an LLM drafts realistic should-fire prompts per skill, cross-skill near-misses (to surface mis-routing), and off-topic decoys. Hallucinated skill names are dropped automatically. Flags: --per-skill <n> (default 3), --decoys <n> (default 2), --model <id>.

Always review the draft before running. Generators lean toward obvious keyword-matches; you add the messy, oblique phrasings real users type (that's where triggering actually fails).

gen, fix, and diagnose use ANTHROPIC_API_KEY if it's set, otherwise they fall back to the logged-in claude CLI — so on a Claude subscription the whole tool works with no API key at all.

Use

# point it at your own config (after `npm i -g skill-probe`):
skill-probe --config my.config.json --k 10 --threshold 0.7 --json

# or, from a clone of this repo, try the bundled example (needs `claude` installed + auth):
skill-probe --config examples/audit.config.json

Config (skill-probe.config.json):

{
  "runtime": "claude-code",
  "cwd": "./my-project",
  "k": 10, "threshold": 0.7, "conf": 0.95,
  "cases": [
    { "prompt": "write a commit message", "expected": "commit-writer" },
    { "prompt": "what's the weather?", "expected": null }
  ]
}

Output:

skill-probe — runtime: claude-code  model: (runtime default)  threshold: 70%

  PASS          expect=commit-writer  | write a commit message
        reliability 100% [72%, 100%] k=10
        outcomes: commit-writer×10
  FAIL          expect=pr-describer  | write a pull request description
        reliability 20% [6%, 51%] k=10
        outcomes: None×8, commit-writer×2
        ⚠ trigger-theft by: commit-writer
  PASS          expect=(none)  | what's the weather?
        reliability 100% [72%, 100%] k=10
        outcomes: None×10

Result: 2 pass / 1 fail / 0 inconclusive / 0 error  |  exit 1  |  cost $0.18

• cwd (relative paths resolve against the config file's directory) is a project dir containing .claude/skills/.
• expected: null = a decoy that should fire nothing.
• Exit code: 0 all pass, 1 a behavioral fail / trigger-theft, 2 inconclusive or an infrastructure error (runtime down → never a silent pass).

Output options: a live probing [2/3] … progress line prints to stderr while it runs (use --quiet to silence it). --markdown emits a table you can paste into a PR/README; --json for machine output; --no-cost hides the cost line (handy on a Claude subscription, where the dollar figure is just an estimate, not a charge).

Cross-runtime comparison must pin the model. A skill can fire on one runtime and not another partly because of the model, not the runtime. Always set model (recorded in the report) so a Claude Code vs OpenCode comparison is fair — otherwise you're comparing two confounded variables.

Two modes — because confidence intervals are wide at small k:

• Smoke (default): threshold 0.7, k 10 — a clean run certifies; cheap; good for CI.
• Certify: a strict bar needs more runs. To certify ≥0.9 you need ~k=35; when a case is inconclusive the report prints the exact k for your threshold. Don't set threshold 0.9, k 10 and expect a pass — that's statistically impossible and the tool will say so.

CI usage

skill-probe exits non-zero on a real problem, so it drops straight into a pipeline. Exit codes: 0 all pass · 1 a behavioral fail / trigger-theft / interference / regression · 2 inconclusive or an infrastructure error — so a runtime outage fails the build instead of silently passing. Add --json to archive the full result (it includes a run manifest: tool version, model, date, config hash — so two runs are comparable and a report is citable).

Regression gating (--baseline)

The CI question isn't "are my skills perfect?" — it's "did this PR make any skill worse?" Activation is stochastic, so comparing raw rates makes CI flaky. The baseline gate compares each case against a saved baseline with Fisher's exact test, Benjamini-Hochberg corrected — noise passes, real drops fail:

# once, on a good main build (commit the file):
skill-probe --config probe.config.json --save-baseline baselines/main.json

# on every PR:
skill-probe --config probe.config.json --baseline baselines/main.json

Baseline gate — vs baseline saved 2026-07-02T09:12:03Z (skill-probe 0.9.0)
  [▼ REGRESSED]  greeter  | write a birthday greeting
        90% (9/10) → 20% (2/10)   Δ-70%   p=0.005 · adj 0.005
Gate: ▼ FAIL — 1 significant regression(s)

The gate warns (but still runs, matching cases by prompt) if the config, runtime, or model changed since the baseline — apples-to-oranges comparisons are flagged, never silent. Significant improvements are reported too, as a nudge to re-save the baseline.

GitHub Action

# .github/workflows/skills.yml
name: skills
on: pull_request
jobs:
  probe:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: "22" }
      - uses: HystonKayange/skill-probe@main
        with:
          config: probe.config.json
          args: --baseline baselines/main.json
        env:
          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}

The action installs skill-probe plus the runtime CLI (Claude Code by default; set runtime-package: "" if your runner already has one) and runs the command. command: selects audit (default), context, diagnose, or doctor; args: passes extra flags like --ablate. The runtime authenticates via ANTHROPIC_API_KEY — headless probes each cost real tokens, so smoke settings (k: 10, few cases) are the sweet spot for per-PR gating.

Activation rate by context (skill-probe context)

The audit measures every skill co-loaded — the real, hard condition. But a skill can fire fine on its own and only fail under load, when the rest of your library is competing for the same trigger. context exposes exactly that:

skill-probe context --config probe.config.json

For each case it measures the expected skill in isolation (a throwaway project with only that one skill) and co-loaded (your full library), then tests the drop with Fisher's exact test — so interference is reported as a real effect with a p-value, not eyeballed. Across multiple cases the p-values are Benjamini-Hochberg corrected (one Fisher test per case is a family: raw p<0.05 over a 40-case library expects ~2 false flags by chance alone — interference is decided on the adjusted p, and the report shows both):

skill-probe context — runtime: claude-code  model: (runtime default)  threshold: 70%  library: 4 skills
  isolation = only that skill loaded · co-loaded = the full library (4) loaded
  3 comparisons — interference decided on Benjamini-Hochberg adjusted p<0.05 (raw p across a family overcounts)

  [⚠ INTERFERENCE]  greeter  | write a birthday greeting
        isolation 100% [72%, 100%] k=10
        co-loaded 30% [11%, 60%] k=10
        Δ -70% under load   Fisher p=0.003 · BH-adj p=0.009
        ↳ fires reliably alone but is suppressed when the library is co-loaded (stolen by: welcomer)

Result: 1 interference / 3 measured  |  exit 1

This is the "fires in isolation, fails under load X" case the aggregate rate hides. Decoy cases (expected: null) are skipped — there's no single skill to isolate; an expected skill that isn't in the library is a config error (typo), surfaced before any probes run. Exit 1 if any skill shows interference, 2 if a case is untrustworthy (infra errors dominated — never a silent pass), else 0.

Name the thief (--ablate)

Knowing that a skill is suppressed is half the diagnosis — the other half is which sibling is stealing it. With 3 skills you can guess; with 40 you can't. --ablate answers it with evidence:

skill-probe context --config probe.config.json --ablate

For each interference case it takes the suspects (ranked by how many probes they actually stole in the co-loaded run, capped by --suspects, default 3), removes them from the library one at a time, and re-measures. If activation recovers significantly (Fisher vs the co-loaded run, BH-corrected across all ablation runs), that sibling is the thief — named, with a p-value:

  [⚠ INTERFERENCE]  greeter  | write a greeting for my new teammate
        isolation 100% [61%, 100%] k=6
        co-loaded 0% [0%, 56%] k=3
        Δ -100% under load   Fisher p=0.012
        ↳ fires reliably alone but is suppressed when the library is co-loaded (stolen by: welcomer)
        leave-one-out (each suspect removed, recovery vs co-loaded):
          − welcomer removed → 100% [61%, 100%] k=6  Δ+100%  p=0.012 · adj 0.012  ← THIEF (full recovery)

Result: 1 interference / 1 measured  |  thieves named: welcomer  |  exit 1

(That's a real run.) Honest edge cases: if the co-loaded probes were suppressed to None — nothing stole them, the skill just stops firing under load — there's no suspect to remove, and the report says "dilution, not theft; nothing to ablate" instead of guessing. If no single removal recovers activation, it says so ("interference may be combinatorial") rather than naming an innocent sibling.

Why a skill fails (skill-probe diagnose)

When a skill doesn't fire, there are two very different root causes — and the activation rate alone can't tell them apart:

• Description problem — the model doesn't even recognise the skill applies. Remedy: rewrite the description (skill-probe fix).
• Routing miss — the model does know the right skill, but a sibling wins at activation time. Remedy: deconflict the sibling / use context. Rewriting this skill's description won't help.

diagnose separates them by measuring actual activation (the real runtime probe) against intended routing — a forced choice where the model is shown the skill descriptions and asked which one should handle the request, with no auto-execution:

skill-probe diagnose --config probe.config.json

  [🔀 ROUTING-MISS]  greeter  | write a greeting for my new teammate
        actual   fires 20% [6%, 51%] k=10  (welcomer×8, greeter×2)
        intended picks 100% [72%, 100%] k=10  (greeter×10)
        → the model picks "greeter" when asked, but "welcomer" wins at activation. Remedy:
          deconflict the sibling or inspect with `skill-probe context`. Rewriting "greeter" won't help.

  [✍ DESCRIPTION-PROBLEM]  greeter  | say something nice to the new hire
        actual   fires 0% [0%, 28%] k=10  (welcomer×10)
        intended picks 10% [2%, 40%] k=10  (welcomer×9, greeter×1)
        → the model doesn't recognise "greeter" applies. Remedy: `skill-probe fix --skill greeter`.

intended is a proxy, not the runtime's router. The forced choice asks a fresh model which skill fits, given only the descriptions — it can disagree with how the runtime actually routes (we've seen actual route a skill 100% while intended picked a sibling). So treat the routing-miss vs description-problem split as a strong heuristic, not ground truth. A useful side effect: when a skill routes fine but intended is low, the description reads ambiguously in isolation — a leading indicator that activation may be fragile across models or contexts, which diagnose calls out on the routes-ok line.

Decoys are skipped; a typo'd expected skill is a config error. Exit 1 if any case is a routing-miss or description-problem, 2 if inconclusive/untrustworthy, else 0.

Fix a failing skill (skill-probe fix)

Rewrite a skill's trigger description and prove the lift is real before keeping it:

ANTHROPIC_API_KEY=sk-ant-... \
  skill-probe fix --config examples/fix.config.json --skill commit-writer

It (1) LLM-rewrites the description, told the sibling skills so it won't steal their triggers; (2) runs an interleaved before/after (old desc → probe → new desc → probe, paired, to control for drift); (3) computes the Bayesian P(improvement) + a credible interval on the change; and (4) applies the rewrite only if P(improvement) ≥ --apply-threshold (default 0.9) and the effect is positive — otherwise reverts. When applied, the original is snapshotted to a timestamped SKILL.md.bak.<timestamp> (no backup is left behind on a revert).

before: 0% [0%, 49%]   after: 100% [51%, 100%]   (4 paired runs)
P(rewrite improved reliability) = 100%   Δ = +80% [36%, 99%]
✅ APPLIED
original backed up to: /path/to/SKILL.md.bak.1719000000000

fix does the rewrite via ANTHROPIC_API_KEY if set, else the logged-in claude CLI (subscription). It changes descriptions on statistical evidence, not on "the new one looks nicer" — a rewrite that doesn't measurably help is reverted.

Status

Early, but usable end to end.

Doctor (skill-probe doctor): setup/auth/config preflight. Checks Node, .claude/skills/, SKILL.md frontmatter, config expected skills, runtime CLI availability, and optional live auth probe.

Gen (skill-probe gen): drafts a reviewable probe config from existing skills, including should-fire prompts, near-misses, and decoys.

Audit (skill-probe): Wilson confidence intervals + sequential stopping + four-state verdict (pass / fail / inconclusive / error), across two runtimes (Claude Code, OpenCode). Infrastructure failures (timeout / auth / crash / empty output / a zero-cost response where the model never actually ran, e.g. a usage-limit window) are reported as error, never as a behavioral pass/fail — a decoy can't falsely pass because the runtime was down. Every run carries a manifest (version, model, date, config hash); --save-baseline / --baseline turn audits into a CI regression gate (Fisher + BH — noise passes, real drops fail), and a GitHub Action (HystonKayange/skill-probe@main) wraps the whole thing for per-PR gating.

Context (skill-probe context): isolation-vs-co-loaded activation rates, with Fisher's exact test on the drop and Benjamini-Hochberg correction across cases — catches skills that fire alone but are suppressed under the full library's load, without buying false flags on big libraries. With --ablate, leave-one-out re-measurement names the thieving sibling when removing it significantly restores activation.

Diagnose (skill-probe diagnose): compares actual runtime activation against intended forced-choice routing to classify failures as routing-miss vs description-problem. Intended routing is a heuristic, not ground truth, and the README calls that out explicitly.

Fix (skill-probe fix): uses the Bayesian Beta-Binomial to gate description rewrites on a proven lift (interleaved before/after, applied only if P(improvement) clears the bar).

Every statistical function in src/stats.ts is now wired into a command: Wilson + sequential stopping (audit), Fisher's exact + Benjamini-Hochberg (context/ablate), Bayesian Beta-Binomial (fix).

Dev

node --test test/*.test.ts   # run tests (zero deps; Node >= 22.18 strips types)
node src/cli.ts --help
npm run typecheck            # tsc --noEmit (needs `npm i`)