Individual skill
Defense-in-Depth Validation
Validate at every layer data passes through to make bugs impossible
skills/problem-solving/defense-in-depth/SKILL.md
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# 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: