System Prompt vs. User Prompt

Part of the Fundamentals section of Coddy's AI Prompts journey — lesson 20 of 23.

You've now seen both the system prompt and the user prompt in action. But when should you use which? Understanding the difference helps you structure your prompts more effectively.

The system prompt is for things that should persist across the entire conversation—identity, rules, and baseline behavior. The user prompt is for specific requests, questions, and task-by-task instructions.

System Prompt	User Prompt
Who the AI is	What you're asking right now
Rules that always apply	One-time instructions
Set once, affects everything	Changes with each message

Here's a practical example. Imagine you want a friendly tutor who explains things simply. You could put this in your user prompt every single time:

You're a friendly tutor. Explain photosynthesis simply.

Or you could put "You are a friendly tutor who explains concepts simply" in the system prompt once, then just ask:

Explain photosynthesis.

The result is the same, but the second approach is cleaner. You don't repeat yourself, and every future question automatically gets the friendly tutor treatment.

Use the system prompt for the "always" stuff. Use the user prompt for the "right now" stuff. When you separate persistent context from immediate requests, your prompts become shorter and your conversations stay consistent.

Challenge

Use the system prompt to establish that the AI always speaks like a cheerful robot (using words like "beep" or "boop"). Then, in your user prompt, ask it to list exactly 2 colors — keeping your user prompt short and simple since the personality is already set.

The AI's response must:

Contain beep or boop (robot personality from system prompt)
Contain exactly 2 color words (choose from here: red, blue, green, yellow, black, pink, green, purple)
Your user prompt must be under 10 words (proving the system prompt handles the personality)

Cheat sheet

The system prompt defines persistent behavior across the entire conversation—identity, rules, and baseline behavior. The user prompt contains specific requests and task-by-task instructions.

System Prompt	User Prompt
Who the AI is	What you're asking right now
Rules that always apply	One-time instructions
Set once, affects everything	Changes with each message

Example: Instead of repeating "You're a friendly tutor" in every user prompt, set it once in the system prompt. Then your user prompts can be shorter and more focused on the actual question.

System prompt: You are a friendly tutor who explains concepts simply

User prompt: Explain photosynthesis.

This approach keeps prompts cleaner and conversations consistent by separating persistent context from immediate requests.