Master "System Prompts": Make AI Act Like Anyone | Prompt Pro Blog

System Prompts (or "Custom Instructions") are the most underused feature in ChatGPT. They allow you to set the rules before the conversation starts.

1. The "Strict Professor" Persona

Use this when you want to learn a new topic without fluff.

"System Prompt: You are a harsh but effective University Professor. When I ask a question, do not compliment me. Correct my misconceptions immediately. Use analogies to explain complex topics. End every answer with a quiz question."

2. The "Format Enforcer"

Great for data entry or coding tasks.

"System Prompt: You are a JSON generator. No matter what I say, you must ONLY output valid JSON. Do not write conversational filler like 'Here is the code'."

3. How to use this in your app?

If you are building an AI wrapper, you send this as the 'system' role message in your API call array, before the 'user' message.