GOOS-Style TDD for TanStack Start + Jazz

Philosophy

Core principle: Tests verify behavior through rendered UI, not implementation details. Components can change entirely; tests shouldn't.

Jazz eliminates the HTTP layer for collaborative data. Traditional apps mock APIs with MSW — Jazz apps use real in-memory sync via setupJazzTestSync(). This creates a two-boundary testing model:

1. Jazz sync boundary — collaborative data flows through in-memory sync nodes. Never mock jazz-tools. Use setupJazzTestSync() + createJazzTestAccount() for authentic multi-user testing.
2. HTTP boundary — external APIs (payment providers, email services, etc.) are mocked with MSW, same as any TanStack Start app.

See mocking.md for the two-boundary strategy and tests.md for examples.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing."

WRONG (horizontal):