Most agencies keep the blueprint. It is the quiet reason it is so hard to leave one.
What you actually buy
The spec sprint is paid, and what it produces is yours: a complete, buildable specification, downloadable whenever you want it, whether or not you build a line with us.
Why we give it up
Because a client who can leave is a client who stays for the right reasons. It also forces us to be honest at the only moment honesty is cheap — before anybody has spent a development budget.
What it means in practice
Build with us. Take it to another team. Put it in a drawer until next year. All three are fine, and none of them requires a conversation with us first.