Last updated: April 5, 2026 · Prompting & Usage · by Daniel Ashford
What is System Prompt?
Persistent instructions that define how the model should behave.
Definition
A system prompt is a set of instructions that persist across the entire conversation. It defines the model persona, behavior constraints, output format, and rules. The system prompt is invisible to end users in most implementations.
How It Works
System prompts are the primary mechanism for controlling model behavior in production. They include the role, constraints, output format, and knowledge boundaries. They consume input tokens on every request, so keeping them concise matters for cost.
Example
System prompt for a triage chatbot: "You are a symptom assessment tool. Ask about symptoms one at a time. Never diagnose. Never recommend medications. Always recommend emergency care for chest pain or difficulty breathing."
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