defense-in-depth
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.
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What it does and how to use it
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Install (Claude Code)
mkdir -p .claude/skills/defense-in-depth && curl -fsSL https://doutorai.pt/data/lib/skills__defense-in-depth.md -o .claude/skills/defense-in-depth/SKILL.mdPaste this command into the terminal, in your project folder, and the component installs automatically.
Content
# 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 dange
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