Prompt Engineering for AI Agents: The Practical Guide

Agent prompts are fundamentally different from chatbot prompts. A chatbot prompt shapes a single response. An agent prompt shapes a planning loop that may run 10-50 steps — errors compound. The most important principle is specificity over brevity: vague instructions produce inconsistent behavior at scale. Define the agent's role, scope, and constraints in the first 200 words. Use numbered steps for procedures — agents follow structured formats more reliably than prose. Define what the agent should NOT do explicitly — most failures come from agents attempting tasks outside their intended scope. Tool descriptions matter as much as the main prompt: write each tool description as a clear function signature with examples of when to use it and when not to. Test prompts with adversarial inputs: edge cases, off-topic requests, and ambiguous queries. Regression test every prompt change against a fixed evaluation set before deploying.