Can I write long, detailed custom instructions for the AI?

Yes. A custom instruction can be up to 3000 characters, enough for a structured brief: tone of voice, what to always mention or avoid, how to handle recurring cases, which policies to refer to. The counter under the text box shows how much of the 3000 you have used. Instructions can be limited to groups and tags, so several focused instructions often work better than one that covers everything. Instructions are managed under Configurations > Ticket settings by owners and admins.