How can I use GPL software like Stockfish Chess Engine in my Unity game?

How can I use GPL software like Stockfish Chess Engine in my Unity game?

I am working on a 3D Chess game and I want to implement an A.I bot to play with the player. I found a chess engine called Stockfish which serves this purpose, and I would like to implement that engine in my project.
Is it as simple as including the Stockfish code in my project or importing a binary as a plugin, or do I have to do something more complex because of the GPL licence?

Solutions/Answers:

Answer 1:

You can implement their code as a plugin in your game, as long as you plan for your game to also be GPL-licensed.

You cannot incorporate Stockfish into a proprietary product, in the sense of bundling them together so that the user perceives them as a single program – which arguably describes typical methods of using 3rd-party plugins within your Unity project.

You cannot incorporate GPL-covered software in a proprietary system.
The goal of the GPL is to grant everyone the freedom to copy,
redistribute, understand, and modify a program.


You still have other options if your game is proprietary:

However, 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.

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.

I am not an expert in this matter, so I will leave this as a community wiki to be updated by other users. Here is a sketch of how I think you could proceed:

  • Create an installer for your game that installs your Unity executable and assets in one place, and Stockfish separately.

  • Your game runs the Stockfish program, communicating with it using Universal Chess Interface commands.

  • You provide a configuration setting where a player can choose to redirect this behaviour to use a different UCI-compatible chess engine if they so choose, so the two programs can operate independently of each other.

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