Test Harness

Test the Claude Code harness — the hooks, skills, settings, and CLAUDE.md that steer an agent — as the assembled machine it ships as. vigiles gives three tiers, cheapest first; this skill picks the right one, writes the test, and runs it.

The guiding rule: start at the cheapest tier that can answer the question, and climb only when it genuinely can't. Two of the three tiers need no model and no API key, so they run on every commit for free — reach for the paid real-model tier only when the question actually requires a real model.

Step 0 — Pick the tier (the judgment call)

Match what you're testing to the cheapest tier that can answer it:

| What you're testing | Tier | Cost | API | | -------------------------------------------------------------------------------------------------------------------------------------- | ----------------- | ------------------------------------------------ | --------------------------------------------------------------------------------------------- | | "Does this hook block/allow event X?" — pure hook logic, every event type (incl. Edit/Write, PreCompact, SessionEnd, SubagentStop) | Unit | free, milliseconds, no claude | runHook | | "Is the hook actually wired into the assembled plugin and does it fire in a real session?" | Deterministic | free, no API key (real claude + scripted mock) | runHarnessTest + scriptModel | | "Did the injected context (a SessionStart hook, a /command) actually reach the model?" | Deterministic | free, no API key | runHarnessTest → trace.modelRequests / assertRequestContains | | "Does this skill's description trigger when it should (recall) and stay quiet when it shouldn't (precision)?" | Eval | paid (real model) | measureTriggerRate (+ irrelevantPrompts) → assertTriggerRate({ min, maxFalsePositive }) | | "Is this exact skill's output any good?" — absolute quality, no on/off baseline (the default for testing one skill) | Eval | paid (real model) | measure({ checks: [judged(rubric)] }) → assertRates({ min }) | | "Does this harness change move what the agent does, relative to off?" — A/B lift, regression, signal vs noise | Eval | paid (real model) | runEval (arms) + assertSignificant |

Most harness questions — block/allow, wired-in, context-landed — never need a model. Only "does the model trigger / behave differently" needs the eval tier.

If the unit and deterministic tiers can both answer it, prefer unit: it's faster and reaches events the deterministic mock can't drive.

Step 0.5 — Set honest expectations (what's testable, and at what cost)

Be explicit with the user about which bucket each surface falls into — never let "we'll test it" hide whether that's free, sub-priced, or needs a container. Every surface sorts into one of three buckets:

• A — Free & deterministic (no model, runs in CI on every commit): a hook's

block/allow decision (runHook), a tool-contract / "did NOT call the forbidden tool" check, structural facts (vigiles audit), and record-replay of any tool a skill shells out to (record the real result once, replay it via a PATH stub).

• B — Model-gated, on your subscription (real model, no metered API): does a

skill's description fire (measureTriggerRate, recall + precision) and does its guidance actually produce good output (score it directly: measure({ checks: [judged(rubric)] }) + assertRates — the absolute oracle; use a runEval A/B on-vs-off only when you need the relative lift). This is the half a prose / guidance skill lives in — its worth is behavioral, so only a model can judge it. That is not "uncovered" and not free: it's fully testable on the sub. State it that way.

• C — Needs a real service (a real browser / DB / redis / a11y runtime): vigiles

composes with a container here; it does not fake real semantics. Name the service and hand off — don't pretend a cheap tier substitutes for it.

So a prose-skill library is roughly ~100% testable (some free, most on your sub), ~0% needs-a-container — not "poorly covered." An accessibility/browser plugin is the worst case, with a large bucket C. When you report coverage, give two numbers: "% testable at all (free + sub)" vs "% that needs a container", and say which surfaces are free vs sub-priced. The model-gated half is the point of the eval pillar (affordable on the sub), not a gap — and testing a prose skill's behavior requires a real model for everyone (promptfoo, the SDKs, all of it); vigiles just does it on your subscription instead of metered API.

Step 1 — Ensure vigiles is installed

Check whether vigiles is a dependency (package.json), and install it as a dev dependency if not:

npm i -D vigiles    # or: pnpm add -D vigiles / yarn add -D vigiles

The deterministic tier additionally needs the claude CLI on PATH (no API key): npm i -g @anthropic-ai/claude-code. The eval tier needs model auth. If the claude CLI is missing, you can still write and run unit-tier tests.

Step 2 — Locate the harness surface to test

Find what the project actually ships, in this order:

1. .claude/settings.json / .claude/settings.local.json — inline hooks.
2. .claude-plugin/plugin.json — a plugin manifest (hooks, skills, agents, mcpServers).
3. hooks/hooks.json — the plugin hooks convention (e.g. obra/superpowers).
4. skills//SKILL.md, agents/.md, commands/.md.

Pick one concrete thing to pin down — a specific PreToolUse hook, a specific SessionStart injection, a specific skill.

Step 3 — Write the test for the chosen tier

Unit (runHook) — hand a hook a synthesized event, assert the decision:

import { runHook, assertHookBlocked } from "vigiles/testing";

const r = runHook(hookCommand, {
  hook_event_name: "PreToolUse",
  tool_name: "Bash",
  tool_input: { command: "git commit --no-verify" },
});
assertHookBlocked(r); // exit 2 / decision:"block" / permissionDecision:"deny"

Testing a hook you didn't write (a vendored third-party script)? Mark it { trusted: false } and it runs confined under bubblewrap by default (read-only host, cleared env, no network egress). Add { recordEgress: true } to also record what it tries to reach — r.egress plus assertNoEgress(r) / assertEgressOnly(r, [...]) — the supply-chain check for "what does this skill phone home to / install from?". When the hook's setup needs a real install, { egress: { allow: ["registry.npmjs.org"] } } lets it reach only that allowlist (a packet-layer nft wall, so a raw socket off-list is dropped too) → r.egress (allowed hosts) + r.egressDropped. Be precise about the boundaries: see [docs/sandboxing.md](../../docs/sandboxing.md) (it blocks destruction and egress, but does NOT isolate reads of host files, and only under bwrap).

Deterministic (runHarnessTest) — load the real plugin, drive a scripted mock model, assert the hook fired (or the context landed):

import {
  runHarnessTest,
  scriptModel,
  assertHookFired,
  assertRequestContains,
} from "vigiles/testing";

const r = await runHarnessTest({
  pluginDir: "./", // or { settings: { hooks: {...} } }
  transcript: true,
  model: scriptModel([{ text: "ok" }]),
});
assertHookFired(r, "SessionStart");
assertRequestContains(r, "expected injected text"); // did it actually land?

Eval — absolute (measure + judged) — testing one skill, the usual case: score its output directly against a rubric. No on/off baseline — this is the "is it any good?" oracle (what promptfoo/DeepEval lead with), and the right default when there's nothing to compare against:

import { measure, judged, skill, assertRates } from "vigiles/testing";

const report = await measure({
  pluginDir: "./",
  task: "…a task the skill should handle…",
  checks: [
    skill("my-plugin:my-skill"), // it fired
    judged("the answer correctly does X and avoids Y"), // …and the output is good
  ],
  trials: 6,
});
assertRates(report, { min: 0.8 }); // each check passes ≥ 80% of trials

Eval — relative (runEval + assertSignificant) — when the question is lift over no-skill (regression, or proving a change isn't noise): A/B the change on vs off and gate on significance, not eyeballing:

import { runEval, assertSignificant } from "vigiles/testing";

const report = await runEval({
  arms: { off: {}, on: { pluginDir: "./" } },
  task: "…a task the harness change should affect…",
  measure: (ctx) => ({ ok: /* a bare predicate over the trace */ true }),
  trials: 6,
  cache: "readwrite",
});
assertSignificant(report, { baseline: "off", arm: "on", metric: "ok" });

Step 4 — Run it

In a runner (node:test / vitest / jest) the tests are plain async functions. Or use the zero-setup CLI, which discovers and runs the files:

npx vigiles test                 # *.harness.{mjs,ts} — unit + deterministic, no API key
npx vigiles eval --trials=6      # *.eval.{mjs,ts} — real model (local / nightly, not CI)

Unit-tier runHook tests need no claude and always run — write and run them even with no claude installed. A tier that genuinely can't run reports a loud ⊘ SKIPPED (tallied separately, never a fake ✓); a standalone script emits one via skip(reason) from vigiles/testing. A skip passes by default, but in a CI job that asserts the capability is present, run vigiles test --no-skip so a skipped tier fails — a green-with-skips is untested surface. Keep unit + deterministic tests in CI (free); run evals locally or on a schedule with auth.

After a real-model run: TELL THE USER WHAT IT SPENT

Whenever you run a real-model eval (runEval / measureArms / measureTriggerRate / measure), surface the spend to the user in your reply — don't let a paid run be silent. runEval prints a cost block to stderr and every report carries usage (report.arms[*].usage: totalCostUsd + token counts). Relay, in plain words:

• tokens spent and the API-equivalent $ (total_cost_usd — what it would

cost at metered API rates);

• how it was billed — "on your Claude subscription ($0 metered)" if you're

logged in, or a ⚠ warning if ANTHROPIC_API_KEY is set (that run was billed per token — tell them to unset it and claude login to run free).

We do not show "% of your subscription" — Anthropic doesn't expose a plan's quota, so any percentage would be invented. Tokens + API-equivalent $ + the billed-to line is the honest, complete picture. Keep the user's cost visible, always.

Step 5 — Lock the eval so CI stays honest (you do this automatically)

Real-model evals run on the user's subscription — locally, never in CI. So as part of writing an eval, you keep its result fresh for them. Do these two things without being asked:

1. Give every eval a name. That's what the lock keys on.
2. After running it, commit the lock:

``bash vigiles eval --update # records the result → .vigiles/eval-locks/.lock.json ``

Then commit that file. CI runs vigiles eval --check (no model) to verify it still matches the inputs — so a later edit that forgets to re-eval fails loud instead of shipping stale numbers.

When you later change a skill's description or prompts, re-run vigiles eval --update and commit the updated lock — the change altered what the eval measures. (vigiles also nudges you: when a lock exists, a SKILL.md edit triggers a non-blocking reminder.)

Why it's cheap: --check only hashes inputs (skill text, prompts, model). A threshold change in the test re-uses the saved numbers (no model); only an input change needs a fresh --update. Full mechanics: [docs/harness-testing.md](../../docs/harness-testing.md#keep-eval-results-fresh-in-ci-the-lock).

When the user didn't say what to test

Don't ask them to specify — pick something real and demonstrate. Scan the harness surface (Step 2), choose the cheapest meaningful test, write it, run it, and show the result. Good default picks, in order:

1. A PreToolUse hook → unit-test that it blocks the thing it's meant to block (and allows a safe sibling).
2. A SessionStart hook that injects context → deterministic test that the text actually reaches the model (assertRequestContains).
3. A skill → deterministic test that it resolves via pluginDir, then offer the paid measureTriggerRate eval as a follow-up.

Then say which tier you used and why, and offer to climb a tier if the cheaper test can't fully answer their question.

Reference

The full guide — every tier, testing skills for real, "fired ≠ landed", the safe-by-default sandbox, the coverage matrix, and how it compares to promptfoo — is in [docs/harness-testing.md](../../docs/harness-testing.md).

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: zernie
• Source: zernie/vigiles
• License: MIT
• Homepage: https://vigiles.sh

Install and usage instructions live in the source repository linked above.