Gates whether a new test should exist and forces it to be efficient, protecting CI from low-value test bloat. Use before any change to what a pytest, Jest, or Playwright test asserts or sets up, down to one fixture or one assertion added to an existing block.
Install
npx skills add PostHog/posthog --skill writing-testsnpx skills add PostHog/posthog --skill writing-tests -a claude-code -gnpx skills add PostHog/posthog --skill writing-tests -a codex -ggemini skills install https://github.com/PostHog/posthog.git --path .agents/skills/writing-testsgh skill install PostHog/posthog writing-testsgit clone --depth 1 https://github.com/PostHog/posthog /tmp/PostHog__posthog
mkdir -p ~/.claude/skills ~/.agents/skills
cp -R /tmp/PostHog__posthog/.agents/skills/writing-tests ~/.claude/skills/writing-tests # Claude Code
cp -R /tmp/PostHog__posthog/.agents/skills/writing-tests ~/.agents/skills/writing-tests # Codex, Cursor, Gemini CLI, Copilot, OpenCode