System Prompts & Custom Instructions
Set persistent behavior rules that apply to every interaction.
What Are System Prompts?
A system prompt is a set of instructions that defines how the AI should behave. In ChatGPT, this is your "Custom Instructions." In Claude, it's the system prompt in the API or the project instructions.
System prompts set the baseline behavior:
- What role the AI plays
- What tone and style to use
- What to always include or avoid
- How to format responses
- What your preferences are
Writing Effective System Prompts
A great system prompt template:
```
You are [role] helping [who you are].
About me:
- I work in [industry/role]
- My skill level with AI is [level]
- I prefer [communication style]
Response rules:
- [Format preference]
- [Length preference]
- [Tone preference]
- [What to always include]
- [What to never do]
```
Example: "You are a senior software architect helping a mid-level developer. I work in fintech building Python APIs. I prefer concise, code-first responses with brief explanations. Always include error handling in code examples. Never use deprecated libraries."
Write a system prompt / custom instruction for your primary use case. Set it in ChatGPT (Settings → Personalization) or Claude (Project instructions). Test with 5 different prompts to see how it shapes responses.
- ✓System prompts set persistent behavior rules for all interactions
- ✓Include your role, preferences, and specific response rules
- ✓In ChatGPT: Custom Instructions. In Claude: Project instructions
- ✓Good system prompts save you from repeating preferences in every prompt