Definition
A system prompt is the set of instructions given to a large language model before any user says anything. It defines the assistant's role, tone, rules, knowledge about the business and boundaries. Users normally never see it. When you chat with a company's support bot and it refuses to discuss competitors or always answers in your language, that behaviour was written in the system prompt.
In a company, the system prompt is where policy meets product. It carries the brand voice, the escalation rules, the facts the assistant may state, the data it may request, and the topics it must decline. Teams keep system prompts in version control, review changes like code, and test them against a suite of tricky cases. A good system prompt reads like an onboarding document for a very literal new employee.
System prompts have grown from a paragraph to several pages that include tool descriptions, formatting rules and safety instructions, and prompt caching makes that long context cheap to reuse. The main misconception is that a system prompt is a security boundary. Determined users can sometimes extract or override it through prompt injection, so sensitive rules must also be enforced in code, not only in the instructions.
In practice
An insurer's claims assistant has a system prompt that tells it to speak formally, confirm the policy number before any detail, never estimate payout amounts, and hand over to a human when a customer mentions a lawyer.
Why it matters
The system prompt is your company's policy expressed to the AI. If you own the customer experience, you should read it, because every rule in it is a decision about your brand and your risk.
Frequently asked questions
- What is the difference between a system prompt and a user prompt?
- The system prompt is set by the company that builds the assistant and applies to every conversation; it defines role, rules and context. The user prompt is what the person types in each message. The model reads both, but the system prompt is designed to take priority.
- Can users see or change the system prompt?
- They should not be able to, but models can sometimes be tricked into revealing or ignoring parts of it. That is why anything that matters for security or compliance must also be enforced outside the prompt, in permissions, filters and application code.