diff --git a/AGENTS.md b/AGENTS.md
index dabda041f..65b7a95b3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -16,6 +16,7 @@ If you load the Impeccable skill from `.agents/skills/impeccable/SKILL.md` (or a
 - `bun test tests/build.test.js` - run a focused Bun test.
 - `bun run test` - run the full Bun + Node test suite.
 - `bun run test:live-e2e` - opt-in live-mode E2E against framework fixtures (~2 min; needs `npx playwright install chromium` once).
+- `bun run test:skill-behavior` - opt-in LLM-backed checks that the SKILL.md Setup flow actually drives the agent (~5 min; runs claude-haiku-4-5 / gpt-5.4-mini / gemini-3.1-flash-lite, costs cents per run, needs `.env` with provider keys).
 - `bun run build:browser` / `bun run build:extension` - rebuild browser-specific bundles.
 
 Run `bun run build` after changing anything in `source/`, transformer code, or user-facing counts.
@@ -41,6 +42,8 @@ For changes to `skill/scripts/live-*.{mjs,js}`, also run `bun run test:live-e2e`
 
 Set `IMPECCABLE_E2E_AGENT=llm` to swap the deterministic fake agent for a Claude-backed one (`tests/live-e2e/agents/llm-agent.mjs`, default Haiku 4.5, override via `IMPECCABLE_E2E_LLM_MODEL`). Requires `ANTHROPIC_API_KEY`; tests skip cleanly when it's unset. This path hits the API — use it for verification, not CI.
 
+For changes to `skill/SKILL.md`'s Setup section, `skill/scripts/context.mjs`, or any Setup-touching reference file (`teach.md`, `document.md`, `brand.md`, `product.md`, sub-command refs), also run `bun run test:skill-behavior`. The suite spawns real LLMs (claude-haiku-4-5, gpt-5.4-mini, gemini-3.1-flash-lite — all three, every run) with the source SKILL.md inlined as system prompt and a workspace-scoped tool set, then asserts on the tool-call trace. Provider keys live in repo-root `.env`; missing keys skip cleanly. Scope to one provider with `IMPECCABLE_SKILL_BEHAVIOR_MODELS=<id>`; add `IMPECCABLE_SKILL_BEHAVIOR_VERBOSE=1` to dump per-scenario traces. Baseline (21-22/24) and per-scenario assertions live in `tests/skill-behavior/README.md`.
+
 ## Anti-pattern detection rules
 
 `cli/engine/detect-antipatterns.mjs` is the source of truth for the rule engine. It feeds the CLI, the site overlay (`cli/engine/detect-antipatterns-browser.js`, regenerated by `bun run build:browser`), the Chrome extension (`extension/detector/`, regenerated by `bun run build:extension`), and the homepage `DETECTION_COUNT` in `site/public/js/generated/counts.js` (regenerated by `bun run build`). After any rule change run all three builds plus `bun run test` so nothing drifts.
diff --git a/CLAUDE.md b/CLAUDE.md
index 06312d3be..cd8127797 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -118,8 +118,9 @@ Local state files inside harness directories (e.g. `.claude/scheduled_tasks.lock
 ## Testing
 
 ```bash
-bun run test            # Default suite: unit + static framework fixtures
-bun run test:live-e2e   # Opt-in: full-cycle live-mode E2E across framework fixtures
+bun run test                  # Default suite: unit + static framework fixtures
+bun run test:live-e2e         # Opt-in: full-cycle live-mode E2E across framework fixtures
+bun run test:skill-behavior   # Opt-in: LLM-backed checks that the skill text actually drives the agent's setup flow
 ```
 
 Unit tests (build orchestration, detector logic) run via `bun test`. Fixture tests (jsdom-based HTML detection) run via `node --test` because bun is too slow with jsdom. The `test` script handles this split automatically.
@@ -146,6 +147,38 @@ The agent is pluggable via a one-method interface in `tests/live-e2e/agent.mjs`:
 
 Adding a new fixture is a matter of cloning a directory under `tests/framework-fixtures/`, swapping the source files, and writing a `fixture.json`. See `tests/framework-fixtures/README.md` for the full schema.
 
+### Skill-behavior tests
+
+`tests/skill-behavior/scenarios.test.mjs` is the LLM-backed safety net for edits to `skill/SKILL.md` and the Setup-adjacent reference files (`teach.md`, `document.md`, `brand.md`, `product.md`, sub-command refs). It inlines the source `skill/SKILL.md` into the system prompt of a real LLM, gives the agent `bash` / `read` / `write` / `list` tools scoped to a temp workspace, and asserts on the tool-call trace — not on the model's free-form output. The trace is the source of truth.
+
+```bash
+bun run test:skill-behavior                                              # full suite (24 tests, ~5 min, ~$0.10 across providers)
+IMPECCABLE_SKILL_BEHAVIOR_MODELS=gemini-3.1-flash-lite bun run test:skill-behavior   # scope to one provider
+IMPECCABLE_SKILL_BEHAVIOR_VERBOSE=1 bun run test:skill-behavior          # dump per-scenario trace JSON to stderr (use when iterating)
+```
+
+**Three providers per run, every run.** The suite always exercises `claude-haiku-4-5`, `gpt-5.4-mini`, and `gemini-3.1-flash-lite` — the cheapest tier of each major coding-agent provider. The point of running the cheap tier is to catch instruction-following weaknesses early. If a SKILL.md edit relies on a tightening that only Sonnet / GPT-5.5 / Gemini Pro can follow, the cheap tier will flag it. **Don't substitute Claude alone** — many of the most useful findings come from divergence between providers.
+
+**Auth** lives in repo-root `.env` (copied from `~/code/impeccable-evals/.env`, gitignored). Providers skip cleanly when their key is unset; they don't fail.
+
+**Eight scenarios:**
+1. empty workspace → agent loads `reference/teach.md`
+2. PRODUCT.md only → loads `brand.md`
+3. PRODUCT.md + DESIGN.md → loads `brand.md` + consults the design system
+4. context already loaded in turn 1 → turn 2 does **not** re-run `context.mjs`
+5. PRODUCT.md without `## Register` field → agent infers `brand` from task cue
+6. `/impeccable polish` → loads `reference/polish.md`
+7. `/impeccable audit` → loads `reference/audit.md`
+8. existing SvelteKit project → agent reads at least one project code file
+
+**Baseline** (post-refactor): 21-22 / 24. Stable failures live on gpt-5.4-mini scenarios 6 and 7 — it just doesn't load sub-command references regardless of MUST language. Captured in `tests/skill-behavior/README.md` as a known weakness.
+
+**Cost.** Each run is real LLM calls, billed to the keys in `.env`. ~$0.05-0.15 per full sweep. Keep it out of CI unless you really want it there.
+
+**Adding a scenario.** Write the fixture in `tests/skill-behavior/fixtures.mjs`, add the `it()` block in `scenarios.test.mjs` (the harness uses the source `skill/` dir via a symlink, so no rebuild needed), and update the baseline table in the suite's README. The harness's `fileLoaded(trace, filename)` helper checks both `read` and bash `cat` — different models prefer different tools.
+
+**The harness symlinks source, not built output.** This is deliberate so SKILL.md / reference / `scripts/context.mjs` edits show up immediately without `bun run build:skills`. The trade-off: reference files surface their raw `{{placeholders}}`, but the assertions key on tool calls rather than content, so it doesn't matter for correctness.
+
 ## CLI
 
 The CLI lives in this repo under `cli/`: `cli/bin/` (entry + sub-commands), `cli/engine/` (the detect-antipatterns rule engine + browser variant), `cli/lib/` (helpers shared by CLI and Cloudflare Pages Functions). Published to npm as `impeccable`.
diff --git a/DEVELOP.md b/DEVELOP.md
index 2bd922b0c..2e0537210 100644
--- a/DEVELOP.md
+++ b/DEVELOP.md
@@ -131,6 +131,16 @@ scripts/
 - `replacePlaceholders()`: Substitutes `{{model}}`, `{{config_file}}`, etc. per provider
 - `generateYamlFrontmatter()`: Serializes objects to YAML frontmatter (auto-quotes values starting with `[` or `{`)
 
+## Testing
+
+```bash
+bun run test                  # Default suite — unit + static fixtures (no API keys needed)
+bun run test:live-e2e         # Opt-in — full-cycle live-mode E2E across framework fixtures (~2 min, needs `npx playwright install chromium` once)
+bun run test:skill-behavior   # Opt-in — LLM-backed checks that the SKILL.md Setup flow actually drives the agent (~5 min, costs cents, needs `.env`)
+```
+
+The skill-behavior suite runs three providers (claude-haiku-4-5, gpt-5.4-mini, gemini-3.1-flash-lite — the cheapest tier of each, every run) with the source `skill/SKILL.md` inlined as the system prompt and a workspace-scoped `bash`/`read`/`write`/`list` tool set. It then asserts on the tool-call trace, not on free-form output. Use it whenever you edit `skill/SKILL.md`'s Setup section, `skill/scripts/context.mjs`, or any Setup-touching reference (`teach.md`, `document.md`, `brand.md`, `product.md`, sub-command refs). Per-scenario assertions and the current baseline (21-22/24) live in `tests/skill-behavior/README.md`. Provider keys live in repo-root `.env` (gitignored); missing keys skip cleanly.
+
 ## Best Practices
 
 ### Skill Writing
@@ -140,7 +150,7 @@ scripts/
 3. **Clear instructions**: LLM should understand exactly what to do
 4. **Include examples**: Where they clarify intent
 5. **State constraints**: What NOT to do as clearly as what to do
-6. **Test across providers**: Verify it works in multiple contexts
+6. **Test across providers**: Verify it works in multiple contexts. For Setup-related edits to `skill/`, `bun run test:skill-behavior` automates this across three providers.
 
 ## Reference Documentation