A good brief does not need to be a specification. It needs to make the underlying decision visible, because that is what lets an agency propose something better than what you asked for.
Say what changed. Projects start for a reason — a competitor moved, a system reached its limit, a campaign underperformed. That context shapes the recommendation more than any feature list.
Name the constraint you cannot move. A launch date, a budget ceiling, a platform you are committed to. Constraints stated early produce realistic proposals; constraints revealed late produce rework.
Describe who this is for, specifically. 'Everyone' is the least useful answer. One accurate sentence about the person on the other end will change the design more than three pages of requirements.
Finally, say how you will judge it. If success means more enquiries, say so. If it means your team stops copying data between systems, say that instead. We will design toward whichever you name.
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