Use of AGPL software on server with closed-source software

The only material difference between the GPL and the AGPL is that the AGPL extends the concept of distribution to include "providing the web interface of a program to an internet-facing customer."

If you really want a workaround, the way you do it is by wrapping the AGPL library in an executable, and calling it from the command line. The FSF calls this "arms-length communication;" it means that your AGPL library is not "linked" with your executable via early binding or late binding programming language mechanisms.

See here, where it says:

In many cases you can distribute the GPL-covered software alongside your proprietary system. To do this validly, you must make sure that the free and non-free programs communicate at arms length, that they are not combined in a way that would make them effectively a single program.

The difference between this and “incorporating” the GPL-covered software is partly a matter of substance and partly form. The substantive part is this: if the two programs are combined so that they become effectively two parts of one program, then you can't treat them as two separate programs. So the GPL has to cover the whole thing.

If the two programs remain well separated, like the compiler and the kernel, or like an editor and a shell, then you can treat them as two separate programs—but you have to do it properly. The issue is simply one of form: how you describe what you are doing.

In short, the AGPL library has to be a standalone program, and your application has to be able to function without it.

As always, if you have real legal concerns, you should consult a real lawyer who specializes in this sort of thing.

In general, as long as you are not "conveying" the work, copyleft does not apply. FSF doesn't consider "internal use" a form of conveyance.

If you later expose your service as an external API on the Internet, that is considered conveyance under the AGPL. To comply with AGPL terms, you would have to open-source any other software that you also intend to convey which depends on that API for its proper functioning.

The usual disclaimers apply; if you want tractable advice, seek the counsel of an attorney that specializes in software licensing.