A long brief does not reduce risk. It hides it. Every requirement somebody wrote down confidently is a decision that will be revisited when the budget moves, and usually by somebody who never read the document.
What we ask for instead
Three outcomes, the people who need to agree, and the constraint that is non-negotiable. Then we go and find the ten decisions nobody has made yet, in the open, early, while they are still cheap.
The uncomfortable one
Ask what will not be built. A project without an explicit exclusions list will discover one in month four, under pressure.