A bad brief costs two weeks of guessing. A good brief costs thirty minutes to write and saves multiple revision rounds.
Start with the user and the job. Who is this for, what problem are they paying to solve, and what does success look like in one sentence? Skip the vision paragraph until the basics are clear.
List what is in scope and explicitly out of scope. "MVP with login, dashboard, and settings" is useful. "Something like Notion but simpler" is not.
Share three reference products and one anti-reference. "I like the onboarding from X" and "I hate the density of Y" gives me direction without copying.
Name the decision-maker and the approval process. If three founders need to sign off, say so upfront. Hidden stakeholders are the main cause of timeline slips.
Attach whatever exists: pitch deck, competitor links, user interview notes, old wireframes. Imperfect context beats a blank slate.
Budget and timeline should be honest ranges, not anchors for negotiation. I can tell you what fits in four weeks versus eight. I cannot read your runway from silence.