System prompts that survive contact

The system prompt is a contract: who the model is in this product, what it must refuse, what format it must use. Keep it under a page. Put examples in the user turn or in a small library of few-shots, not in an essay the model will half-obey.

• Write the must-nots as concrete bans, not vibes.
• Version the prompt. Diff it. Evaluate it on a tiny fixture set when you change it.
• Do not hide secrets in the system prompt. Users will extract them.