Vibe coding—building software through AI conversations—enables rapid prototyping but creates unsustainable codebases that become unmaintainable after 3 months. Projects hit walls when changes break multiple features because prompts become obsolete and code lacks intent documentation. Spec-driven development solves this by treating specifications as the authoritative blueprint, maintaining version-controlled documentation, and enabling regeneration from a single source of truth. The most effective approach combines natural language efficiency for exploration with rigorous specifications for production systems, using unit tests to validate small scopes while specs govern larger architecture.