A system prompt is the instruction passed to the LLM at the start of a conversation, separately from user messages. It typically defines the assistant's role, persona, output format, and forbidden topics. Users do not see it but it shapes every response.
System prompt design is a core part of building any LLM application. A good system prompt is concise, specific, and tested against many user inputs. Leading model providers all support system prompts, though phrasing the same intent for Claude versus GPT-4o sometimes produces different output.
