test-implementer

name	test-implementer
description	Implement and run tests adhering to valid architecture and best practices.

Test Implementer Instructions

Use this skill when you need to implement or run tests. Follow these strict guidelines to ensure architectural compliance.

1. Test Location & Structure

• Location: Test code MUST be placed inside a test folder that is a direct child of the SUT (Subject Under Test) production code folder.
  • Example: If code is in src/feature/my_service.ts, test MUST be in src/feature/test/my_service.test.ts.
• Micro-architecture:
  • Do NOT keep all tests in a single file if it grows too large (>200 lines).
  • Use abstraction to separate concerns:
    • *_test.py or *.test.ts: The actual test cases.
    • *_fixtures.py / *.fixtures.ts: Test data setup and fixtures.
    • *_mocks.py / *.mocks.ts: Mock definitions.
  • Separation: Test code must be strictly separated from production code.

2. Naming Conventions

• SUT Instance: Variable name for the service/class under test instance MUST be sut.
• Test Files: Must end in _test.py (Python) or .test.ts (TypeScript).
• Exceptions: E2E tests (Playwright) are handled by the e2e-implementer skill and live in apps/e2e. They use .spec.ts.

3. Coding Best Practices

• Abstraction: Avoid duplicated code. Extract common setup, teardown, and helper logic into specialized test files (see Structure above).
• Constants: Reuse production code constants. Do NOT duplicate string literals or magic numbers in tests; import them from the production code.
• SOLID/KISS: Keep tests simple and focused.
• Parameterized Tests: Use @pytest.mark.parametrize (Python) or test.each (Vitest) to avoid duplicate test code when testing the same function with multiple inputs/outputs. Each case must have a descriptive id= or name.
• Performance: Unit tests MUST execute quickly (under 500ms each).
• Mocking: To achieve the performance goal, all external layers (database, network, file system, etc.) and dependencies MUST be properly mocked. Do not rely on real I/O operations in unit tests.
• Handling State Updates (act): Always wrap code that triggers React state updates in act(...).
  • Use await waitFor(() => { ... }) for asynchronous updates in standard tests.
  • Fake Timers: When using vi.useFakeTimers(), use await act(async () => await vi.advanceTimersByTimeAsync(MS)) to advance time and flush microtasks. Avoid waitFor with fake timers unless time is advanced manually.
  • Initial Check on Mount: If a hook/component performs async work on mount, use await vi.advanceTimersByTimeAsync(0) (with fake timers) or await waitFor(...) (with real timers) to ensure it completes before asserting or clearing mocks.
• TanStack Query (React Query): Ensure ALL query functions used by the SUT are mocked. If using vi.mock(), explicitly mock setiap function with mockResolvedValue to avoid "Query data cannot be undefined" errors, as React Query v5+ does not allow undefined returns.

4. Architecture Verification

Refuse to complete the task without verifying architecture compliance.

• CommonLib: Run poetry run pytest apps/commonlib/commonlib/test/architecture_test.py (adjust path as needed from CWD).
• Web: Run npx vitest run apps/web/src/test/architecture.test.ts (adjust path as needed from CWD).

Usage

• When creating a new unit test for MyService:
  1. Create test/MyService_test.py (or .test.ts).
  2. Instantiate sut = MyService().
  3. Run tests.
  4. Run architecture tests to enforce rules.