AGENTS.md, SKILL.md, DESIGN.md: How AI Instructions Split into Three Layers

In April 2026, Google Labs released a spec called DESIGN.md. It's a design system specification readable by AI agents, packaged with a CLI validator: npx @google/design.md lint. With DESIGN.md in the picture, we now have three different file types for instructing AI agents. AGENTS.md has been spreading as an industry standard since 2025 (jointly developed by OpenAI, Google, Sourcegraph, Cursor, and Factory; donated to the Linux Foundation in December 2025). SKILL.md sits at the core of Anthropi...