AI glossary
System prompt
Instructions set by the developer or app that frame the whole conversation: the model’s role, rules, tone and tools.
In chat apps you usually write the user messages. Behind the scenes there is often a system prompt too, which applies to every turn. Developers use it to define a role (“You are a support assistant for…”), set rules, describe available tools and give background the user should not have to repeat.
Many assistants let you write your own persistent instructions, which work in a similar way.
Example: A bookshop gives its assistant this: “You are the assistant for an online bookshop. Only answer questions about orders, shipping and books in the catalogue. If asked about anything else, say so politely.” That text goes with every customer message.
In practice
- Put in it what does not change: role, rules, tone and format.
- Do not keep secrets in it: with some ingenuity, a user can get the model to reveal it.
- Keep it stable to benefit from prompt caching.
How to write good instructions: prompting techniques.
