Most agencies will happily quote for a build off the back of a briefing document. We will not, and the reason is uncomfortable: briefing documents describe the solution the client has already chosen, not the problem they have.
What a diagnostic sprint is
Two to three weeks, a fixed fee, and a written output: what is actually holding growth back, with evidence; what we would do about it; and what that would cost. Deducted from the project fee if you go ahead. Yours to take elsewhere if you do not.
What it protects against
Chiefly, building the wrong thing well. The second most common outcome is discovering the problem is not a build at all — it is pricing, or a sales process, or a measurement gap that made a healthy funnel look broken.
We have ended engagements here
Twice we have finished a diagnostic sprint by recommending the client not proceed. In both cases the honest answer was that the money was better spent elsewhere. Both clients came back within eighteen months with a better-formed problem, and both of those became substantial projects.
The commercial logic
A fixed-fee diagnosis is a small commitment that de-risks a large one, for both sides. It also filters. Clients unwilling to spend three weeks understanding their own problem before committing six figures to solving it are, in our experience, not clients who will enjoy working with us.