Test Doubles Design is a community-contributed Claude Code skill in the testing sub-category. It ships as a SKILL.md file that Claude Code auto-discovers under ~/.claude/skills/test-doubles-design/ and loads when your prompt matches the skill's trigger.
When to invoke it: Use when designing or reviewing test doubles — the stand-in objects that replace real collaborators in a test: the five-kind taxonomy (dummy, stub, spy, fake, mock) from Meszaros's xUnit Test Patterns, the difference between state verification (Detroit-school, classicist) and interaction verification (London-school, mockist) and how it determines which doubles fit, the cost of doubles (fragility, false confidence, divergence from real behavior), the role of fakes as the under-used middle ground, the verify_with relationship to test-driven-development, and the heuristics for when to use a real collaborator instead of any double. Do NOT use for choosing test levels or what to test (use testing-strategy), the design discipline of writing tests first (use test-driven-development), specific mocking-library API choice (library docs), or general production stubs and feature flags (use feature-gating or domain-specific skills).
The Test Doubles Design Claude Code skill is built for software engineers, backend developers, full-stack teams, and technical leads building and maintaining production systems. It's part of ClaudSkills (also referred to as Claude Skills or Claude Code Skills) — the open community-curated registry of 69,000+ SKILL.md files for Anthropic's Claude Code agent and the wider Claude ecosystem (Claude API, Claude Agent SDK).
mkdir -p ~/.claude/skills/test-doubles-design curl -L https://claudskills.com/skills/test-doubles-design/SKILL.md \ -o ~/.claude/skills/test-doubles-design/SKILL.md
Or just download SKILL.md directly and drop it into ~/.claude/skills/test-doubles-design/. Claude Code auto-discovers it on next session.
Skills live at ~/.claude/skills/test-doubles-design/SKILL.md on macOS/Linux, or %USERPROFILE%\.claude\skills\test-doubles-design\SKILL.md on Windows. See the full install guide for step-by-step instructions.
The ClaudSkills desktop app installs any skill directly into ~/.claude/skills/ with one click — no terminal required. Pro starts at $9/mo or $149 lifetime.
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SKILL.md files, not affiliated with, endorsed by, or sponsored by Anthropic.