AI Feature System Prompt Prompt

Write a system prompt for an AI feature.

What this AI does: [describe — e.g. writes email replies for support team]
User: [end user / internal employee / automated pipeline]
Input: [what it receives]
Expected output: [what it should produce]
Tone and style: [how should it write?]
Must always do: [requirements]
Must never do: [constraints]
Edge cases: [tricky situations]

Please write:
1. A complete system prompt ready to use
2. Explanation of each section and reasoning
3. 3 test prompts to evaluate if it's working
4. Things to watch for that indicate it needs tuning