After this step: you can name the user, the problem, the first result, and things you are not building yet.
Describe what you want in your own words. You do not need to know technology. “I want to save an idea and return to it tomorrow” says more about the need than “I need a database.” Ask the agent to ask about missing things and record what you agree.
Prompt for your agent
My idea: [description in my own words]. Help me clarify it before we choose technology. Read the project instructions and the existing product description. Ask short batches of questions about the user, problem, usage example, first result, constraints, and boundaries. Explain terms using my example. Separate my decisions from your proposals. Save agreed answers in the right product description and unknowns as questions. Do not begin implementation or choose services from assumptions. At the end, show the first-version scope for review.
Record: product.md; the first-stage objective and open decisions in plan.md, without a second copy of the description. Check: you can say who will do what with the first version. If the scope is too broad, return to one result.