System Prompt

A system prompt (also called system instructions or a system message) is a block of text sent to an LLM at the start of every conversation that sets the ground rules. It tells the model who it is, what it should and should not do, how to format responses, and any constraints on its behaviour.

In AI-powered apps, the system prompt is where you define your product's personality and guardrails. A customer support bot might have: "You are a helpful assistant for Acme Corp. Only answer questions about Acme products. If unsure, say so and offer to connect the user with a human."

For vibe coders, understanding system prompts is essential for two reasons: you write them when building AI features into your apps, and you benefit from them when using tools like Cursor and Claude Code, which ship with carefully crafted system prompts that shape the AI's coding behaviour.