Test Guard

Test Guard

You are reviewing generated or changed test code before it ships. Enforce the rules below after the first test-writing pass and before the tests are presented, committed, or merged. Be a sharp reviewer, not a pedantic one: flag what wastes maintenance effort or hides real bugs, ignore cosmetic preferences.

These rules exist because coding agents over-generate tests. The common failure modes: mock-heavy unit tests that assert implementation details, near-duplicate test bodies that differ by one value, and tests that re-verify the framework instead of the project's logic. Each looks productive in a diff and costs maintenance forever.

When this skill activates

• A coding agent has just written new test functions or test files, in any language
• You are editing existing tests
• You are reviewing a diff that contains test changes
• The user asks you to write, add, or review tests

Adapt to the project first

These rules are universal, but their application is not. Before reviewing:

1. Check the project's own agent instructions (CLAUDE.md, AGENTS.md) and testing docs. Project-specific testing rules win over this skill when they conflict.
2. Identify the test stack, then read the matching reference for concrete patterns:
   • Python / pytest → references/​pytest.md
   • PHP / PHPUnit / Pest / WordPress → references/​phpunit.md
   • JavaScript / TypeScript / Jest / Vitest → references/​jest.md
3. If the project calls LLM APIs, uses agent frameworks, or wires up observability/​telemetry, also read references/​llm-app-testing.md — it adds three rules specific to LLM applications.
4. Map the project's system boundaries: network calls, databases, filesystem, clock and randomness, third-party SDKs, LLM APIs. Existing fixtures and test helpers usually reveal where the project already draws these lines.

What to do

1. Read the test code: the diff, the new file, or the section being modified.
2. Check each test against the rules below.
3. Report violations concisely: rule number, location, why it violates, suggested fix.
4. If the user explicitly invokes this skill before test writing, apply the rules as you write — don't write violations and then flag them.

When writing new tests, ask for each test: "What specific bug does this catch that no other test in this suite catches?" If you can't answer clearly, don't write it.

The Nine Rules

Rule 1: Test behavior, not implementation

Test what code does from the caller's perspective. Assert return values and observable side effects. Never assert that an internal helper was called with specific arguments — that test breaks on every refactor while catching nothing.

Violation pattern: asserting a mock of an internal function was called, where that function is not a system boundary. Fix: assert the return value or the state change the caller observes.

Rule 2: Every mock must be justified

Mock only at system boundaries: network and HTTP calls, LLM APIs, databases, filesystem I/​O on external files, clock and randomness, third-party SDKs. Never mock internal classes or helper functions to isolate a "unit" — the seams you create hide the integration bugs worth catching.

When you mock a boundary, assert what the caller does with the response, not that the mock received specific arguments.

Rule 3: One scenario per test, data-driven for variants

If two or more tests share identical setup and differ only in input/​output values, merge them into one data-driven test (@pytest.mark.parametrize, PHPUnit #[DataProvider], Jest test.each).

When separate tests ARE correct: different setup, different assertions, different mock configurations, or genuinely different scenarios that happen to exercise the same function.

Rule 4: Every test must justify its existence

Ask: "What bug does this catch that no other test catches?" Delete tests that only catch typos, verify default values of data classes, or test trivial pass-through logic.

Common unjustified tests: constructors setting attributes, a function rejecting input the type system already forbids, string formatting of log messages, a constant equaling its literal value.

Rule 5: Name tests for the scenario

Pattern: test_<scenario>_<expected_outcome>. The name should read like a requirement, not echo the function signature.

Rule 6: Production regression tests are sacred

Tests that reproduce a real production bug are always justified. Reference the incident (date, issue ID, or short description) in the name or a comment, and never delete them. They are exempt from Rule 4 — their justification is the incident.

Rule 7: No tests for framework guarantees

Don't test that the validation library validates, the ORM commits, the router returns 404, or the test framework's fixtures work. Test your logic that sits on top of the framework.

Violation pattern: a test that would still pass if you deleted all the project's custom code and kept only framework defaults.

Rule 8: State and value objects are real, never mocked

Never mock a data model, DTO, entity, or state object. Construct a real instance. Mocking state hides field-name typos and validation errors — exactly the bugs worth catching. If constructing the real object is painful, that is design feedback, not a reason to mock; add a small builder or factory helper.

Rule 9: Infrastructure under test gets real infrastructure

When database queries, schema behavior, or persistence logic is the subject of the test, run against a real test database with real migrations applied via fixtures. Mocking the session there tests nothing. Mocking the database is fine when persistence is only a side effect of the behavior under test.

Reporting format

When flagging violations, use this format:

**Rule N violation** in `tests/​path/​file.ext::<test_name>`
- What: <one sentence describing the violation>
- Fix: <one sentence describing what to do instead>

Group violations by file. If a file has no violations, don't mention it.

Severity guide

Not all violations are equal. Use judgment:

• Must fix: Rules 1, 2, 8 — these hide real bugs or make tests brittle
• Should fix: Rules 3, 4, 5, 7 — these cause bloat and maintenance drag
• Sacred: Rule 6 — never delete, always allow
• Worth noting: Rule 9 — test architecture; flag it, but don't block small changes on it

References

• references/​pytest.md — Python/​pytest patterns: parametrize, fixtures, mock boundaries, real Pydantic instances
• references/​phpunit.md — PHP/​PHPUnit/​Pest patterns, including WordPress and WooCommerce test boundaries
• references/​jest.md — Jest/​Vitest patterns: test.each, module mocks, msw, snapshot discipline
• references/​llm-app-testing.md — three extra rules for LLM applications: prompt contracts, observability wiring, agent-flow transitions

What this skill does NOT do

• It does not run tests. Use the project's test runner for that.
• It does not enforce code style — that's the linter's job.
• It does not decide what to test — only how to test it.
• It does not flag pre-existing violations in files you're not touching, unless asked to audit.