Cursor Rules + Claude: stop breaking builds

I use Cursor Rules to make Claude follow my stack. I add a tiny “done = tests pass” contract. I lint AI output hard. Automatically. I keep prompts short. Diff-first. Context I build small SaaS projects. Usually solo. Usually fast. Cursor + Claude makes me fast. It also makes me sloppy. The failure mode is always the same. Claude writes code that looks right. It compiles. Then it breaks edge cases, ignores my existing patterns, or sneaks in a dependency I didn’t want. I spent 4 hours on this once. Most of it was wrong. Not because Claude was “bad”. Because my workflow was vibes. No guardrails. So I started treating AI like a junior dev with a very specific contract. Same stack. Same conventions. Same “definition of done”. And I automated the policing. 1) I write Cursor Rules first. Not prompts. Prompts rot. Fast. Rules don’t. They sit there. Every request gets them. In Cursor, I keep a project rule file that forces Claude into my existing choices: Next.js App Router, TypeScript strict,