Foundations
System prompt
Standing instructions given to a model on every request, defining its role, tone and limits.
The system prompt is where you tell the model what it is: which company it represents, what it may and may not discuss, what format to answer in.
It is genuinely useful for tone and behaviour. It is not a security boundary. A system prompt saying "never reveal customer data" is a request, not an enforcement — anyone determined enough can often talk a model past it. See prompt injection.
Anything that actually must not happen belongs in your backend code, not in the prompt.