System prompt
Also called System message
The hidden instructions loaded before your message that tell the model who it is, what rules to follow, and how to behave.
Think of it like
The employee handbook a new hire reads before their first day — the user never sees it, but it shapes everything.
Example
A support bot's system prompt says "you are a helpful assistant for Acme Corp; never discuss competitors" — and it obeys.
How it actually works
It sits at the top of the context and carries the most weight in the model's attention. Everything from persona to output format to safety rules goes here. The model treats it as ground truth, which is why getting it right matters more than clever user prompts — and why attackers try to override it.
For product teams
The hidden instructions that set persona, rules, and safety — your main lever for controlling AI behavior.
For engineers
Prepended context with highest attention weight; defines persona, constraints, and output format.
Related
- Context window — The highest-priority part of the context window.
- Designing them well is context engineering.
- Prompt injection — Can be overridden by prompt injection if unprotected.
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