System Prompts

Introduction

The system prompt is a special message at the top of the conversation that defines the assistant's persona, capabilities and rules. It's the highest-leverage place to shape behaviour because the model treats it with extra weight throughout the chat.

Beginner analogy: Like a job description plus dress code given to a new hire on day one — it shapes everything they do for the rest of their employment.

Understanding the topic

Core concepts to understand:

• Sent once at the start of the conversation.
• Defines role, tone, response format, refusal rules.
• Higher priority than user messages in most APIs.
• Often versioned and stored separately from app code.

Syntax reference

Visual workflow / architecture:

┌──────────── system ───────────────────────┐
│ You are an empathetic mental health       │
│ assistant. Never give medical diagnosis.  │
│ Always recommend a professional in        │
│ crises. Respond in the user's language.   │
└─────────────┬─────────────────────────────┘
              │
   ┌──────────▼──────────┐
   │   user → assistant  │  (multi-turn)
   └─────────────────────┘

Real-world use

ChatGPT, Claude.ai, Cursor and Perplexity all rely on detailed system prompts (some leaked online have 1500+ words). System prompts also enforce safety policies (no medical advice, no copyrighted text).

Best practices

• Keep the system prompt under 500 words if possible.
• Always include refusal rules and tone guidance.
• Version-control system prompts like code.

Hands-on exercise

Interview preparation — practice these questions:

• Q1. What is a system prompt?
• Q2. How does it differ from a user message?
• Q3. Why version-control system prompts?