system prompt
The instruction text given to an LLM that sets its persona, rules, and capabilities before any user message.
Definition
A system prompt is the high-priority instruction block prepended to an LLM conversation, defining the model's role, allowed actions, output format, and any policy constraints. It is the first thing the model reads. Long stable system prompts are the prime candidate for prompt caching. Agents typically have layered system prompts: framework, skill, and per-session context.
When to use
See also
- prompt caching — Provider-side cache of repeated prompt prefixes — dramatically lowers cost and latency for long, stable system prompts.
- LLM — Large Language Model — a transformer-based model trained on internet-scale text to generate and reason.