Structure beats length
A good instruction is a short contract: role, scope, prohibitions, data to collect, escalation triggers, tone. Long free-form prose produces inconsistent behaviour because nothing in it is checkable.
- Name the role in one line
- List what it may answer — an allowlist, not a denylist
- State prohibitions explicitly, especially advice, pricing and legal claims
- Define the fields to collect and the order to ask for them
- Define escalation triggers in observable terms
Tone and turn length
One question per turn. Messaging is a conversation, not a form. Assistants that ask three things at once get one answer back and lose the other two.
Test with hostile input
Try nonsense, aggression, and off-topic questions. The way your assistant fails is the part customers remember.