Spec-Driven Development (SDD)
Definition
A software engineering methodology where well-crafted specifications precede code generation, serving as structured prompts for AI coding agents. The 2026 industry consensus response to 'vibe coding' — the practice of generating code from informal natural language without specifications. SDD principles: collaborate with AI to create clear specifications BEFORE coding, break work into small testable increments, AI generates unit tests for its own code, design documentation is enforced even for AI-generated components. GitHub's Spec Kit is the primary open-source toolkit. Promoted by Thoughtworks and InfoQ as the 2026 standard. Distinct from but complementary to context engineering (SDD = the WHAT, context engineering = the HOW).
Builder Context
SDD is the methodology that makes design consistency achievable in agentic builds. Without it, each session makes locally reasonable decisions that accumulate into global inconsistency (design drift). The practical workflow: (1) write a spec for the component/feature including which design tokens, which page type rules, which card patterns to use; (2) feed the spec to your agent as the primary prompt; (3) agent generates code + tests from the spec; (4) visual regression confirms the output matches the spec. The spec IS the design contract — it's how you transfer design intent from your design system to the agent's context. Teams that adopt SDD report 60-80% reduction in design-related review comments.