Spec-Driven Development guides an agent through writing a structured specification, technical plan, and task list before writing any code. It establishes a shared source of truth between the human engineer and the agent by covering objectives, tech stacks, commands, project structures, code styles, testing strategies, and boundaries. Use it when starting a new project or feature, when requirements are ambiguous or touch multiple files, or when a requirement needs to be decomposed into a capability map before implementation.