AI Vocabulary · 03 Working with AI
What is System Prompt?
Standing instructions that shape every answer, set by the app or by you, before your message arrives.
A system prompt is a set of instructions placed before every conversation. It defines the assistant's role, tone, rules and available tools. Every AI app has one written by its maker, and many let you add your own, for example as custom instructions, project instructions or a custom assistant.
Models are trained to give system prompts high priority. That is why two apps using the same model can feel completely different, and why an assistant sometimes follows a rule you never asked for.
Why it matters for you
A good custom system prompt saves your team from repeating the same instructions: company tone, target audience, formats, what never to do. Set it up once and every answer starts better.
Own it
Write your own instructions and keep them in a document you control. They are part of your know-how and should move with you when you change tools.
Where it comes from
The idea became standard with OpenAI's ChatGPT API in March 2023, which treated a conversation as a sequence of messages with roles, in a format called ChatML (OpenAI announcement). The "system" message became the place for standing instructions, and the term "system prompt" spread across providers from there.
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