System prompt
Instructions the app's maker places before your conversation to shape how the model behaves. You usually can't see them.
Before your first message reaches the model, the app puts its own instructions in front of it: the system prompt. It can set the tone, give today's date, describe the tools available, and list what to do and what to avoid. Your messages are read after it.
That's a large part of why the same model behaves differently in different apps: a different system prompt, a different personality. It also explains some surprises, like an assistant that declines a topic in one app and discusses it in another, or one that knows today's date without searching the web. Your own standing instructions are added to the conversation in a similar way.
For example
Anthropic publishes the system prompts its Claude apps use. They give Claude the current date and encourage certain behaviours, and don't apply when developers use the model directly.
Put it to use
Where you'll meet system prompt in ChatGPT, Gemini and Claude, how to use it there, and the mix-up most people make.
Daily Bytes Plus is in the Android app.