Definition
A system prompt is an instruction injected by the developer or product before a conversation begins, setting the model’s role, rules and output constraints; it is typically invisible to end users.
How it works
Most APIs expose it as a message in a dedicated system role placed at the front of the conversation, where it joins the user messages in the model’s context. It is a convention rather than a hard guarantee: attacks such as prompt injection can still bypass its rules.
Usage
Defining a chatbot’s persona and boundaries, fixing the output format, and embedding product rules or safety policy are all typically written into the system prompt.
Related and confusable terms
Contrast with the ordinary prompt: the system prompt is a standing instruction, the user prompt a one-off input; both are part of the model’s context.