Choose based on how defined the problem is and how much ongoing capacity you need. A Project suits a defined scope with a fixed timeline, like a GTM build. A Retainer embeds Kando monthly for ongoing execution across channels. A Sprint is a focused 4-6 week intensive for a specific, urgent question.
In practice, many clients start with a Sprint or Project to prove the model works, then move to a Retainer if there's ongoing scope — always still working toward a transfer date.