Specification-Driven Development (SDD) methodology for building software where specifications are executable and drive code generation. Use when users want to: (1) Create feature specifications with /speckit.specify, (2) Generate implementation plans with /speckit.plan, (3) Create executable tasks with /speckit.tasks, (4) Follow test-first, library-first development, or (5) Implement constitutional architecture principles. Essential for structured AI-assisted development workflows.