# Choose the check that can catch the failure

> For agents: start with the [agent guide](https://ratstack.sh/llms.txt). Every page is Markdown by default; add `Accept: text/html` for HTML.

New on /news: [tla-ts 0.0.10 checks TypeScript models](/news).

Name the failure before choosing the check. The testing ladder starts with the fastest check that can catch that failure.
[drovr proposed this approach in issue #3](https://github.com/joelhooks/rat-stack/issues/3).

## Choose a check by what it proves

The issue proposes six verification levels:

- **Lint:** Code shape, checked by Oxlint.
- **Types:** Wiring, required services, and [contract](/systems/capabilities) drift.
- **Model and property tests:** Lifecycle behavior over sequences and generated inputs.
- **Runtime integration:** Storage, migrations, and alarm behavior under workerd, the Workers runtime.
- **Stage checks:** Deployment, configuration, bindings, and bundles in a staging environment.
- **Production canaries:** The deployed system's real paths through a dedicated test tenant.

This is a proposed verification map. It does not establish that rat-stack implements every level.
The current local gate appears below.

[The fence](/lore/the-fence) explains lint and compiler enforcement.
[Properties](/lore/property-based-testing) cover rules over inputs; [models](/lore/model-based-testing) cover state that depends on earlier actions.

## Worked example: check command-policy decisions

The command policy must block hook bypasses while allowing ordinary commands.
Its suite checks both lists against independent expected decisions:

packages/core/test/vcs-command-policy.test.ts at 66df9fc245b8f26402835a812426fb9aef06ef7e; lines 91-105; highlighted none. Title: Check both refusal and acceptance.

```typescript
  it("blocks every way to skip or disable the hooks", () => {
    expect(
      bypasses.filter(
        (command) => applyCommandPolicy(command).action !== "block"
      )
    ).toEqual([]);
  });

  it("allows ordinary commands that only look like a bypass", () => {
    expect(
      ordinary.filter(
        (command) => applyCommandPolicy(command).action !== "allow"
      )
    ).toEqual([]);
  });
```

The same suite generates commit messages and checks that their words alone do not cause a block.
That property exercises decisions over generated input. It complements the listed cases.

1. State the missing behavior: a bypass is accepted, or an ordinary command is blocked.
2. Check the existing command-policy tests for that behavior before adding another test.
3. Use a property for a rule over inputs, and plant a violation before trusting a new property.

[Repo law](https://github.com/joelhooks/rat-stack/blob/66df9fc245b8f26402835a812426fb9aef06ef7e/AGENTS.md) requires a genuine behavior-testing gap before adding a regression test.

## Keep the full gate

`pnpm check` runs typechecks, formatting checks, and type-aware lint.
Lefthook runs `pnpm check` and `pnpm test` before a commit.
A failed hook needs a fix; disabling it bypasses the fence.

Finish with `pnpm turbo run check test build` before calling the change ready.
A focused check does not replace this required gate.
Report code-ready, deployed, and behavior-verified as separate facts.

Read [tests through supplied services](/lore/tests-that-earn-their-place) next to design an independent assertion and choose a test technique.

## Sources

1. [drovr contributors. 2026. SOP and upstream ideas from drovr in production.](https://github.com/joelhooks/rat-stack/issues/3)
   GitHub joelhooks/rat-stack. Section 2 proposes a testing ladder, from code-shape checks through production canaries. Accessed 2026-11-11.

2. [rat-stack contributors. 2026. Agent instructions.](https://github.com/joelhooks/rat-stack/blob/66df9fc245b8f26402835a812426fb9aef06ef7e/AGENTS.md)
   rat-stack. Commands, commit hooks, and the property/model testing rules define the current local gate. Accessed 2026-11-11.

3. [rat-stack contributors. 2026. VCS command-policy tests.](https://github.com/joelhooks/rat-stack/blob/66df9fc245b8f26402835a812426fb9aef06ef7e/packages/core/test/vcs-command-policy.test.ts)
   rat-stack. Listed bypasses, ordinary commands, and generated commit messages exercise the command policy. Accessed 2026-11-11.

4. [rat-stack contributors. 2026. Check the actual artifact before declaring done.](https://github.com/joelhooks/rat-stack/blob/66df9fc245b8f26402835a812426fb9aef06ef7e/skills/rat-stack-mode/principles/check-the-actual-artifact.md)
   rat-stack. Code-ready, deployed, and behavior-verified are separate claims. Accessed 2026-11-11.

## Lore on this page

- [Property-based testing](/lore/property-based-testing)
- [Bindings](/lore/bindings)
- [The fence](/lore/the-fence)

## Linked from

- Agent guide → An Effect stack so pure (aspirational) Kit Langton will blush. → [Read page](https://ratstack.sh/llms.txt)

- Change log → What changed in the files served here, newest first. → [Read page](https://ratstack.sh/log)

- Full agent guide → An Effect stack so pure (aspirational) Kit Langton will blush. → [Read page](https://ratstack.sh/llms-full.txt)

- Rat Stack lore | rat-stack → Short, source-grounded notes on the ideas and decisions behind rat-stack. → [Read page](https://ratstack.sh/lore)

- Source change history (log.md) → The generated change log as Markdown. → [Read page](https://ratstack.sh/log.md)

- Tests prove behavior through supplied services → Supply services through Layers, choose properties or models for domain rules, and reject tests that cannot detect broken behavior. → [Read page](https://ratstack.sh/lore/tests-that-earn-their-place)