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Instructions given to a model before the user conversation that shape its behavior.
The system prompt is the developer's primary lever for customizing a model's persona, capabilities, output format, and safety behaviors. It is processed before any user message and typically carries higher authority than user turns. Well-crafted system prompts reduce hallucinations, enforce formatting, and constrain agents to their intended scope.
Plainly
Think of System Prompt as a named building block in a big LEGO app. Instructions given to a model before the user conversation that shape its behavior.
In practice
Use it when you are mapping how the app is structured or explaining a feature to a teammate or agent. In practice, define the owner, input, output, and failure mode before you rely on it.