noun · AI concept
System prompt
also called system messageinstructions
The note pinned inside the lid. Instructions from the app builder that the AI reads before anything you type.
Developer-written instructions placed ahead of the conversation in each request to set behavior, scope and format.
A block of text the application puts at the start of every request to set role, rules, tone and tool descriptions. It sits in the same token stream as everything else and uses part of the context window.
Real-life example
You use two apps built on the same model. One answers in short bullet points and refuses to discuss anything but cooking, while the other chats freely. The model did not change; each app slipped a different note in front of your message before sending it.
Where the analogy breaks
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It is not a separate, protected channel. It is text in the same input as your message, so a model can lean on it less than you hope.
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It is not secret or secure. Assume a determined user can get a model to repeat it, so never put passwords or private keys in it.
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It is not free. It is sent with every request, so a very long one takes space from the context window and adds to the bill each time unless it is cached.