System Prompt
A system prompt is a hidden set of instructions given to an AI model that defines its behavior, personality, capabilities, and constraints. Unlike user prompts visible in conversation, system prompts shape how the AI responds to all requests and persist throughout a session.
Example
Cursor's system prompt might include: 'You are an expert software developer. Always consider edge cases. Prefer TypeScript over JavaScript. When showing code changes, explain your reasoning.'
System prompts are the invisible rules that govern AI behavior. Understanding them helps you work effectively with AI tools and customize their behavior when possible.
How System Prompts Work
[System Prompt: Hidden instructions defining AI behavior]
[User Prompt: Your visible message]
↓
[AI Response: Shaped by both system and user prompts]
What Goes in System Prompts
- Role definition — "You are an expert software engineer"
- Behavior rules — "Always explain your reasoning"
- Constraints — "Never suggest deprecated patterns"
- Knowledge — "The current date is..."
- Formatting — "Use markdown for code blocks"
System Prompts vs Cursor Rules
| System Prompt | Cursor Rules |
|---|---|
| Set by the tool provider | Set by you |
| Hidden from users | Visible in your project |
| General tool behavior | Project-specific guidance |
| Can't modify directly | Fully customizable |
Writing Effective Instructions
Whether in system prompts or Cursor Rules:
- Be specific — Vague instructions get vague compliance
- Prioritize — Most important rules first
- Example — Show desired behavior, not just describe
- Test — Verify the AI actually follows your rules
The Hidden Layer
Most AI tools have system prompts you never see. When a tool "just works" with your coding style, that's often a well-crafted system prompt. Cursor Rules let you add your own layer on top.
Cursor
Emergent — An agentic vibe-coding platform that builds production-ready apps