> Imagine if Linux published only a binary without the codebase. Or published the codebase without the compiler used to make the binary. This is where we are today. This was such a helpful way to frame the problem! Something felt off about the "open source models" out there; this highlights the problem incredibly well.
The GPL describes the source as the "preferred form for modification".
> The “Corresponding Source” for a work in object code form means all the source code needed to generate, install, and (for an executable work) run the object code and to modify the work, including scripts to control those activities. However, it does not include the work's System Libraries, or general-purpose tools or generally available free programs which are used unmodified in performing those activities but which are not part of the work. For example, Corresponding Source includes interface definition files associated with source files for the work, and the source code for shared libraries and dynamically linked subprograms that the work is specifically designed to require, such as by intimate data communication or control flow between those subprograms and other parts of the work.
...
> You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License
Model weights alone are not Corresponding Source. In order to distribute a model you made under the GPLv3, you would have to give users the model weights and the scripts needed to turn the model weights into a runnable model. That's assuming that you only work with the model weights when modifying the model. If you in particular retrain the model as part of modifying the model, then you would have to provide the training data and initial training scripts as well.
Even though I wrote about a particular free software license which happens to be an open source license, the open source definition from the Open Source Initiative also refers to the preferred form of changing the work [2]:
> The source code must be the preferred form in which a programmer would modify the program. Deliberately obfuscated source code is not allowed. Intermediate forms such as the output of a preprocessor or translator are not allowed.
For good measure, here is the relevant excerpt from the free software definition from the Free Software Foundation [3]:
> Obfuscated “source code” is not real source code and does not count as source code.
> Source code is defined as the preferred form of the program for making changes in. Thus, whatever form a developer changes to develop the program is the source code of that developer's version.
> Freedom 1 includes the freedom to use your changed version in place of the original. If the program is delivered in a product designed to run someone else's modified versions but refuse to run yours—a practice known as “tivoization” or “lockdown,” or (in its practitioners' perverse terminology) as “secure boot”—freedom 1 becomes an empty pretense rather than a practical reality. These binaries are not free software even if the source code they are compiled from is free.
The FSF's free software definition requires that the user be practically - not merely theoretically - allowed to modify the source code and turn the source code into a running program. Because of that, the free software definition considers build scripts to be part of the source code. I can't find an explicit analogue of the practically-modifiable requirement in the open source definition, but I think providing the model weights without providing the scripts needed to turn the weights into a functioning copy of the existing model would be obfuscation i.e. a violation of the open source definition.
[1] https://www.gnu.org/licenses/gpl-3.0.en.html