Fixed background instructions set by the app's developer that define an AI assistant's role, limits and tone in every conversation.
A system prompt is text sent to the model before the user's messages, usually hidden from the user. It defines who the assistant is, which language and tone it uses, which topics are allowed and what to do when it does not know the answer. Think of it as the assistant's job description inside your app.
When you build an assistant for customers, the system prompt is your main lever for behavior: reply in Arabic by default, never promise discounts that do not exist, and hand sensitive cases to a human. Without it, the model behaves generically and does not reflect your brand.
Practical tip: write the system prompt as a short list of clear rules and include examples of good replies. Then test it with tricky questions and manipulation attempts, because some users will try to push the assistant to ignore its instructions.