Agent Server is open source Java on GitHub under Apache License Version 2.0
Stage 0 of the Base Technology Agent Server is 100% Java developed using Eclipse and ant, and is licensed as open source under the Apache License Version 2.0. The full source code (and downloadable zip file) is available on GitHub.
You may read, download, modify, and even redistribute the agent server and its source code according to the terms of the Apache License Version 2.0, without needing any payments or agreements to be signed.
That said, the agent server is not quite ready for prime time, so be prepared for bugs and other issues if you do take an advanced preview. Everything is "as-is."