Defense-in-Depth Validation Overview When you fix a bug caused by invalid data, adding validation at one place feels sufficient. But that single check can be bypassed by different code paths, refactoring, or mocks. Core principle: Validate at EVERY layer data passes through. Make the bug structurally impossible. Why Multiple Layers Single validation: "We fixed the bug" Multiple layers: "We made the bug impossible" Different layers catch different cases: - Entry validation catches most bugs - Business logic catches edge cases - Environment guards prevent context-specific dangers - Debug loggin…