How to Write a SOUL.md That Makes Your AI Agent Actually Useful

If you are building AI agents - whether with Claude, GPT, LangChain, or any framework - the most important file is not your code. It is your agent's identity document. I call it SOUL.md. It is the system prompt that defines who your agent is, what it can do, and what it must never do. A vague SOUL.md produces a vague agent. A specific one produces an agent that actually gets work done. After studying agents that generate real revenue (including autonomous agents running entire businesses), here is what separates agents that work from agents that do not.