What is System Prompt?
Instructions provided to an LLM before the conversation begins, setting its role, behavior, and constraints.
Definition
A system prompt is a set of instructions provided to an LLM at the start of a conversation (before any user messages) that establishes the model's persona, behavior rules, knowledge context, output format requirements, and constraints. System prompts are invisible to end users but shape all of the model's responses. In AI agents, the system prompt contains the agent's core instructions, available tools, and operational rules.
Example
A support agent's system prompt might say: 'You are a customer support agent for Acme Corp. You have access to the knowledge base, ticket system, and CRM. Always cite the source of your answer. Never make commitments about refunds without escalating. Respond in under 100 words.'
System Prompt vs prompt-engineering: What's the difference?
Instructions provided to an LLM before the conversation begins, setting its role, behavior, and constraints.
The system prompt is the specific artifact (the text provided before the conversation). Prompt engineering is the broader practice of designing that text effectively.