This article explores how to write effective DESIGN.md files for AI tools like Claude. The author found that simply listing design tokens is insufficient; files need to include context, intent, and constraints. Building a DESIGN.md from scratch revealed the importance of a product brief and explicit "don'ts" for the system. Claude can extract design tokens accurately but struggles with the reasoning and strategic "why" behind design decisions. Ultimately, a well-crafted DESIGN.md requires designer input for its rationale, not just token extraction.