Chat models take messages in roles. The system prompt sets standing rules for the whole conversation: who the assistant is, what it may and may not do, and how it should answer. User messages carry the request of the moment. Models generally give the system prompt more weight, but that is a tendency, not a guarantee, and mixing untrusted text or secrets into it causes problems.
These guides explain what goes in the system prompt and what goes in the user turn, how to set a role or a style guide there, how to keep untrusted input separate from instructions, and best practices for the system prompt of an AI agent that calls tools.
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