What is the difference between a system prompt and a user message, and why does that distinction matter ?
Answer
The system prompt sets the overall frame of the conversation: the model's role, the expected tone, the rules to follow, and it stays stable across the whole session. User messages carry the concrete request at each turn and can vary freely. Separating the two lets you fix a consistent behavior that's harder for the end user to override, rather than mixing everything into a single prompt where system instructions and the user's request carry the same weight.
Common trap
The trap is believing the system prompt makes the application completely immune to override attempts: it's a strong framing layer, not an absolute guarantee, which is why validating the model's output on the application side still matters for sensitive cases.