How to Write an AI Agent Prompt That Actually Works (Not Just Once)

Most founders who complain that AI "isn't reliable" are writing prompts the same way they write text messages. Casual. Vague. Context-free. Then they wonder why the output is inconsistent. The problem isn't the model. It's the prompt architecture. When I started running Xero on AI agents, the single biggest leverage point wasn't picking the right model or buying better tools. It was learning how to write prompts that produce the same quality of output on the 50th run as they did on the first. This post covers exactly that.