Testing Strategy — Core Patterns, Vitest, Playwright & pytest

Testing patterns and conventions for Vitest, React Testing Library, MSW, Playwright, and pytest. Contains 46 rules across 12 categories, prioritized by impact. Focuses on patterns that produce reliable, maintainable tests — test isolation, mock boundaries, query priorities, meaningful coverage, E2E best practices, and Python testing fundamentals.

When to Apply

Reference these guidelines when:

• Writing new test files or test cases
• Reviewing test code for correctness and maintainability
• Deciding what to mock and what to test through
• Setting up test infrastructure (MSW, factories, fixtures)
• Configuring coverage thresholds or CI test pipelines
• Refactoring flaky or brittle tests
• Writing or reviewing Playwright E2E tests
• Configuring Playwright for CI or visual regression
• Writing pytest fixtures, parametrize tests, or conftest.py files
• Reviewing pytest code for isolation, async patterns, or parallelism

Target Versions