Videos from the 2020 Copyleft Conference

The FSF takes the position that any sort of linking constitutes a derivative work, but this is a question of law for a court to resolve. The FSF does not have the power to override the plain language of either the local copyright statute or their own licenses. The {,A,L}GPL are all very explicit about this. Consider for example this line from GPLv3:

> To “modify” a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a “modified version” of the earlier work or a work “based on” the earlier work.

So, if whatever you did does not "require copyright permission" according to your local jurisdiction, then section 5 of GPLv3 ("Conveying Modified Source Versions") does not attach and copyleft is not applicable to your product. AGPLv3 has an identical definition, and LGPLv3 incorporates GPLv3's definition by reference. A prudent lawyer would probably advise you to assume that permission is required, but to the best of my knowledge, the claim that dynamic linking constitutes a derivative work has not been seriously tested in a court of law and might not actually be correct. Even if a court were to rule it correct, it might vary by jurisdiction or by circumstances.

(Whether it is wise to actually litigate this out is another question, of course. Lawsuits are expensive and may have uncertain outcomes. Most businesses will likely prefer to just avoid AGPL-licensed software altogether. This is probably why we don't have many clear court rulings on this point.)

Finally, I feel obligated to quote the actual part of the AGPL that gives most people pause (section 13, "Remote Network Interaction; Use with the GNU General Public License"):

> Notwithstanding any other provision of this License, if you modify the Program, your modified version must prominently offer all users interacting with it remotely through a computer network (if your version supports such interaction) an opportunity to receive the Corresponding Source of your version by providing access to the Corresponding Source from a network server at no charge, through some standard or customary means of facilitating copying of software. This Corresponding Source shall include the Corresponding Source for any work covered by version 3 of the GNU General Public License that is incorporated pursuant to the following paragraph.
>
> [snip GPL compatibility paragraph]

It is important to bear in mind *which program* is the "modified version" and how it is interacting with the end user. For example, if you made a plugin to a traditional email client rather than a webmail service, then the user would not be interacting with your plugin "remotely through a computer network" but locally on their own machine, so the AGPL would not impose any additional requirements on top of regular GPL formalities. You might still object to those formalities, but in this case, your beef is with copyleft in general, not AGPL in particular.

So going with your example: the tax code changes, so the program has to change with it: people pay you to write the new code.

Where is the problem ?