Problem

The writing assistant should be triggerable without having to highlight text in the editor. There are many possible use cases for not wanting to pass in context to the writing assistant.

Solution

A good implementation I've found is Notion's writing assistant:

A nice to have feature is a button in the writing assistant for when no text has been highlighted: It'll toggle whether to pass the (full) contents of the current note to the writing assistant or nothing from the current note to the writing assistant.