[andrewl-hn]

When you include an AGPL library your program becomes a derivative work, so the rest of your program becomes AGPL-licensed as well. The boundaries of what part of the system should or should not be included are vague, because *GPL licenses are written with C semantics in mind. So, while having a library directly in the same running process definitely makes you work a derivative, it's not clear if you the same is true if the library is a part of a different process and your program talks to it via an IPC, a filesystem, a database, an API, etc. Depending of the interpretation you may have toped-source just a tiny part of your program running on a server, or all server-side and client-side code and all supplemental scripts, tools, etc. needed for the system to run.

So, it's legally ambiguous, and since no-one has legally tested the waters the interpretation of the license can be very different, but most people prefer to stay away from AGPL code altogether.