...declare which versions of the dependent mod require which versions of the dependency and assume my users will download a compatible version of the dependency, or include the dependency with the correct version in my library? And if so, how?

 

Both mods do stuff on the client and server, so if including dependency libraries hides them from the client I may have to rely on my users being responsible.

32 minutes ago, TheThorneCorporation said:

declare which versions of the dependent mod require which versions of the dependency and assume my users will download a compatible version of the dependency

Remember you can deprecate methods in the core to be removed later to allow backwards compatibility with older versions if the changes affect a wide range of mods you are creating. If you are including the libraries as a standard utility, you are most likely not going to reuse the code in other projects.

May I direct your attention to the mods.toml file.

https://mcforge.readthedocs.io/en/latest/gettingstarted/structuring/#the-modstoml-file

 

With it you can declare mods that yours depends on and specify a version (i.e. a minimum or strict version). Doing so will show the user an error message telling them about the missing mods.

 

On 5/19/2020 at 4:00 PM, TheThorneCorporation said:

Both mods do stuff on the client and server, so if including dependency libraries hides them from the client I may have to rely on my users being responsible.

If I understand your meaning, according to this page in the docs (https://mcforge.readthedocs.io/en/latest/concepts/sides/#writing-one-sided-mods) mods are expected to work regardless of what side they are on, and if they don't that is a problem the the mod author needs to address.