Use when writing or refactoring unit tests - a new test file, added cases, a flaky test, an unreadable one. Encodes behavior-first testing: one behavior per test, names that state the claim, deterministic setup, mocks only at boundaries you do not own.
Install
npx skills add riekelt/principal-engineer --skill writing-unit-testsnpx skills add riekelt/principal-engineer --skill writing-unit-tests -a claude-code -gnpx skills add riekelt/principal-engineer --skill writing-unit-tests -a codex -ggemini skills install https://github.com/riekelt/principal-engineer.git --path plugins/principal-engineer/skills/writing-unit-testsgh skill install riekelt/principal-engineer writing-unit-testsgit clone --depth 1 https://github.com/riekelt/principal-engineer /tmp/riekelt__principal-engineer
mkdir -p ~/.claude/skills ~/.agents/skills
cp -R /tmp/riekelt__principal-engineer/plugins/principal-engineer/skills/writing-unit-tests ~/.claude/skills/writing-unit-tests # Claude Code
cp -R /tmp/riekelt__principal-engineer/plugins/principal-engineer/skills/writing-unit-tests ~/.agents/skills/writing-unit-tests # Codex, Cursor, Gemini CLI, Copilot, OpenCode