Spec-Driven Development: Why Writing a Spec Before AI Prompts Matters

Developers using AI agents to generate code often skip critical design decisions, allowing the AI to silently encode assumptions about logic, performance, and user behavior. A spec-first approach, inspired by Behavior-Driven Development (BDD), involves writing a plain-language markdown specification before issuing any AI prompt or writing any tests. The author illustrates this with a search-and-filter UI component, showing how a simple SPEC.md file forced explicit decisions on debouncing, query logic, error handling, and URL state. Once the spec is complete, the AI is instructed to derive contract tests from it—without writing implementation—ensuring tests reflect intentional behavior rather than AI assumptions. The key caution is to never let the AI write both the spec and the tests simultaneously, as they will agree with each other while potentially encoding the same underlying errors.