Custom GPT System Prompt Builder

Help me write a system prompt for a custom AI assistant.

Role: [e.g. SEO copywriter for SaaS]
Constraints: [tone, length, things to avoid]
Knowledge: [what it should know]
Output format: [markdown, JSON, bullets]

Produce a complete system prompt under 500 words with:
- Role & expertise
- Step-by-step behavior
- Output format rules
- 2 example user messages + ideal responses