Claude Certification
← Prompt Engineering & Structured Output
Lesson 1 · 6 min

System Prompts

Where to put role, constraints, and output contract.

Put role and persistent behavior in the system prompt. Put per-call data in the user message. Prompts that move volatile content into the system prompt blow the prompt cache. Order matters: static first, dynamic last.

Production scenario

Real-world example: Multi-tenant support assistant

A SaaS company runs a support assistant across thousands of tenants. The dumb design: pack tenant config into the user message every call. Cache hit rate: ~3%.

The fix: split the prompt into stable and volatile sections.

[system, cached]
  You are the support assistant for tenant: Acme Co.
  Tenant plan: enterprise.
  Tenant region: us-east-1.

[user, per call]
  Today: 2026-05-12.
  Question: how do I export users?

Cache hit rate climbed to ~78% within a day. Input cost dropped 60%.

Why this matters: stable role + tenant config in the system prompt unlocks caching. Anything that changes per call belongs in the user message.

Knowledge points in this lesson
  • Stable role/constraints belong in system prompt
  • Volatile per-call data goes in user message
  • Reusing system prompts unlocks caching
  • Static first, dynamic last for cache hits
  • Avoid putting dates in the system prompt
Quick check
Prompt EngineeringSelect one
Which content belongs in the system prompt vs. the user message?