A SPEC.md file is the difference between vibe coding and software engineering.

For a complex feature I write a spec and hand it to the agent to turn into an implementation plan. What goes in it:

• Introduction. “You are building a dashboard to change settings on a security platform.”
• Problem. If it knows why, it finds solutions I didn’t think of.
• Scope. Keeps it focused on what I want.
• Out of scope. Paradoxically, this sharpens the scope.
• Functional requirements. The list of tasks to complete.
• Non-functional requirements. Respects my architectural decisions.
• References and glossary. Points it at the knowledge it should read.

The spec is where the thinking happens. The code is the easy part.