What is spec-driven development?
Spec-driven development (SDD) is a software planning methodology where the complete architecture specification is written before any code is generated by an AI coding agent. The spec defines what the system does and explicitly states what the agent is not allowed to touch. It is usually written before beginning development with Cursor, Claude Code, or Lovable.
How it works
Traditional development has always had some form of upfront design: PRDs, architecture diagrams, technical specs. Spec-driven development adapts that discipline for AI-native workflows, where the "developer" is often an autonomous agent that makes architectural decisions on your behalf if you don't constrain it first.
Without a spec, an AI coding agent infers your architecture from context: the prompt you wrote, the files it can see, and the patterns it learned during training. This works well for small, isolated tasks. As the codebase grows, it begins to breaks down. The agent starts making decisions that contradict earlier ones, overwrites files it shouldn't touch, and hallucinates dependencies that don't exist.
A spec solves this by giving the agent a single source of truth to anchor against every session. It typically covers:
- Data schema: What your tables, models, and relationships look like
- Tech stack: The specific libraries, frameworks, and services in use
- Routing logic: How the app is structured and how pieces connect
- Boundaries: What the agent is explicitly not allowed to change
How it differs from a PRD
Product requirements documents (PRDs) are for product teams. Specs are written for AI agents. A PRD defines the user experience, while a spec dictates the system's reality. Think of it as the exact technical brief a senior engineer hands to a junior developer before a sprint.
Why it matters for vibe coding and agentic building
Vibe coding tools like Lovable, Bolt, and Replit work well for early prototypes. Agentic tools like Cursor and Claude Code work well for more complex builds. Both hit the same wall. As the session gets longer and the codebase grows, outputs start to drift. Spec-driven development is the practice that prevents the drift, not by making the AI smarter, but by giving it less to guess.
Zalcro is built specifically for this workflow. You describe your idea, it asks the clarifying questions a senior engineer would ask, and outputs the architecture document and execution prompts your agent needs to stay on track.