System Prompt Design

# System Prompt Design

## What it is

A system prompt is a set of instructions given to the model before the conversation starts. It defines the model's behavior, personality, boundaries, and context for the entire session — not just a single message.

If role-play prompting is putting on a costume, system prompt design is rewriting the character from scratch.

## When to use it

- When building an AI-powered product or tool
- When you need the model to behave consistently across an entire conversation
- When you want to restrict or expand what the model talks about
- When default model behavior doesn't match your use case

## When it doesn't work well

- For one-off tasks where a single well-written prompt is enough
- When instructions are vague or contradictory — the model will default to its own judgment

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## Example

**Use case:** A customer support assistant for a software company

**Poorly designed system prompt:**