A brief that arrives as a forwarded chat and two screenshots makes a designer guess. Guessing costs a round of revisions. I would rather get the same set of answers from every client, before any work begins.
Why and who. The goal, the audience and the one message. If these are weak, no style will rescue the work.
What and how. Deliverables, tone, things to include and avoid, and links to work the client likes, with the reason.
When and who approves. A delivery date, the person who signs off, and how many rounds of changes to expect. It stops surprises late in the project.
Who fills it and who reads it
A client or marketing lead fills it in. The creative team reads the rows, one brief for each project. A client's AI would ask "What should this project achieve?" Then "Who is it for?" Then "What do you need made?" It shows the answers and sends only after a yes. See agent-fillable forms.
A tip
Ask for the reason behind each inspiration link. A link alone says little. If the key_message answer is a list, push back before you open any design file.
Find more in forms for agencies and forms for creators. A shorter project intake form triages requests first. A web design questionnaire goes deeper when the work is a site.
Questions
Who should fill in the brief?
Whoever owns the goal, usually the client lead or the marketing manager. Send it to one person, so you do not get three competing briefs for one project.
Can the brief include files?
Not as uploads. Ask for links to a shared folder or to inspiration pages, and tell the client to open the folder to the people on the project.
What do I do when answers are vague?
Reply with the one question that matters, usually the key message. A visitor's AI also asks for missing details before it sends, which helps with thin answers.