System Prompt

A hidden instruction block prepended to every conversation that tells the model its role, constraints, available tools, and behavioral rules. It's the developer's primary way of shaping agent behavior.

Example: A system prompt might say: 'You are a customer support agent. Only answer questions about our product. Never discuss pricing.'

In practice: In agentic systems, the system prompt also typically lists available tools and explains when to use each one. Badly written system prompts are a common source of unreliable agent behavior.