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What is a system prompt?
A system prompt is a hidden set of instructions that tells an AI how to behave before you say anything.
- Category Building with AI
- Also called system instructions, custom instructions
- Updated 20 Oct 2026
System prompt, explained simply
A system prompt is background instruction for an AI. The app or developer writes it. It sets the AI's role, tone and rules.
It matters because it shapes every answer you get. It is why one chatbot sounds friendly and another sounds formal.
For example, a travel app might tell its AI, "You are a helpful trip planner. Keep answers short."
Many tools let you write your own, often called custom instructions.
On ITG Now, our free tool guides show where to put custom instructions so a tool follows your style.
Related terms
PromptPrompt engineeringChatbotGuardrailsPrompt injection
Questions people ask about System prompt
System prompt vs user prompt: what is the difference?
The system prompt is set up front by the app or you. The user prompt is what you type in each message.
Can I see a system prompt?
Often not. Many apps keep theirs private, but some let you set your own custom instructions.
Can a system prompt be bypassed?
Sometimes people trick the AI into ignoring it. That is why developers add other safety checks too.