The short answer
Tell AI what decision or problem you are working through, what you know, and what remains uncertain. Ask it to compare a small number of possible deliverables and explain what each would help you do. Choose the direction yourself before asking for a finished draft or build. That keeps AI useful during discovery without letting it decide the goal for you.
A closer look
You do not need to pretend the brief is complete before you can use AI. Start with the business question, the audience, the evidence you have, and the decisions that are still open. Ask AI to outline two or three possible outputs—for example, a short FAQ, a full service page, or a sales handout—and show the tradeoffs for each.
Then decide which output fits the need and write the task brief around that choice. If the choice depends on a customer, client, or expert, take the options to that person instead of treating AI's preference as approval. Exploring the work and assigning the work are different steps.
Compare the writing and development process examples when deciding whether you need discovery, a draft or an implementation task next.