What Is a System Prompt and How Does It Work?

It is an implicit command provided to the model before the conversation takes place. It defines the role, guidelines, and style in which the model operates at each step, without requiring the user to give the same command repeatedly.

The system prompt is provided by the developer and defines the behavior of the entire conversation. The user prompt is the individual input from a person each turn. The system prompt is given more importance than the user prompt by the model.

At times. A well-crafted system prompt takes precedence, but a smart or malicious user message may defeat a poor prompt. Testing against such attempts and adding safeguards minimizes the danger.

Yes. It is part of the context on every call, thus contributing to the cost and the context window size. Its stability and leading position in the input allow prompt caching to reuse it instead of reprocessing it each time.

Typically the model's role, the guidelines it needs to abide by, the tone and style, any always-needed context, a default output format, and safe limits on inappropriate or out-of-scope queries.