Learning path · Foundations · 05
System Prompts
Persistent instructions that define role, rules, and tone across turns, separate from the user's immediate request.
Why it matters
- Encodes product behaviour once instead of repeating it every message.
- Primary place for safety, brand, and domain constraints.
- Mutable system prompts are a common prompt-cache miss pattern in production.
Key ideas
- Standing rules
- Role definition
- Versioning
Top resources
- 01DocsAnthropic
Prompt engineering overview
Why this resource. Treats the system turn as durable policy, which is the point of this concept.
Covers in this concept
- system role
- constraints
- policy
- 02DocsOpenAI
Prompt engineering
Why this resource. Shows how instructions sit above user messages in the request.
Covers in this concept
- roles
- format control
System prompts set the constitution of an assistant: what it may do, how it cites sources, when it escalates. " Version system prompts like code. If you mutate the system string per tenant or per request, you may forfeit prefix-cache savings on providers that cache stable prompt prefixes. Separate stable global rules from dynamic per-user inserts when optimizing latency and cost. Review system prompts in the same release process as API schema changes, because they are behaviour contracts customers experience directly.
Updated 2026-08-09 · Full learning path