Context Engineering: Why Your Prompt Is the Smallest Problem

I was at a client project recently, facing a situation I know well by now. New codebase, no AI tooling set up, and a developer on the team asks me: "How do you get the AI to produce such good results?" I looked at his screen. He had Claude open - in the browser, no project context, a fresh chat for every single task. That's the problem. Not the tool, not the model, not the prompt. The missing structure. After a year of using AI daily in real projects, I'm convinced: the difference between "AI is pretty decent" and "AI has fundamentally changed how I work" isn't the perfect prompt. It's the context you give the AI - before it answers the first question . CLAUDE.md Is Where Everything Starts When I start working on a new project with Claude Code, the first thing I create is the CLAUDE.md . Not the first component, not the first feature - the context file. What goes in it? Everything a new developer would need to know on day one: Tech stack and versions Project structure and why it's set