The short version: the Sastra engine is open source (GPL-3.0), so improvements to the engine stay open. Everything you create with it (your story, art, music, and characters) is entirely yours, even for a commercial release.
You are free to
- Make visual novels, free or commercial, and keep all the money.
- Keep your story, art, music, and characters fully proprietary.
- Use, copy, and share the engine for any purpose.
- Bundle the engine with your game and distribute it anywhere.
- Modify the engine's code for your own needs.
You must not
- Ship a modified engine without also offering its source under GPL-3.0.
- Claim you wrote the original engine, or strip out its licence notices.
- Re-license the engine code itself under a closed or proprietary licence.
- Expect any warranty. The engine is provided as is.
Two different things, two different rules
The key to the licence is that a Sastra game is made of two separate parts, and they're treated differently.
1. The engine (index.html)
This is the code Ken “Krysion” Nisaka wrote: the HTML, CSS, and JavaScript that reads your files and runs the game. It's licensed under the GNU GPL v3. In short, you can use and modify it freely, but if you distribute a changed version of the engine, you have to make your changed engine code available under the same GPL-3.0 licence so others benefit too. That's what "copyleft" means.
2. Your game data and assets (everything you make)
Your dialogue scripts, images, sprites, audio, video, and custom fonts are not covered by the GPL. Thanks to a special Game Data and Asset Exception written into the licence, your creative work can stay completely private, proprietary, and commercial. Loading or bundling your assets with the engine does not force you to open-source them.
Think of it like a film projector and a film. The projector's blueprints are open for anyone to improve. The movie you screen on it is still entirely your own.
What rights do you have?
Under this licence you can run the engine for any purpose, study and change how it works, share copies of it, and share your changes to it under GPL-3.0. You also keep full ownership and commercial rights over the game you build. You never have to pay, register, or ask permission.
Example scenarios
I want to sell my visual novel on itch.io or Steam.
Totally fine. Your story and art are yours, so charge whatever you like and keep the proceeds. You're bundling the unmodified engine, which the licence explicitly allows.
My game's art and writing are copyrighted to my studio. Is that allowed?
Yes. The asset exception keeps your creative work fully proprietary and confidential. The GPL only touches the engine code, not your content.
I tweaked index.html to add a feature and I'm distributing my game.
Then you've modified the engine, so you must make your modified engine source available under GPL-3.0 to the people you give the game to. Your story and assets still remain yours. Only the engine changes need to be shared.
I didn't change the engine at all, I just wrote chapters and added art.
Nothing extra is required of you. You're using the engine as is. Keep the LICENSE.txt with it and you're done.
Can I take the engine, rename it, and sell it as my own closed-source VN engine?
No. The engine itself is copyleft, so you can't relicense it as proprietary, remove its notices, or pass it off as original work. Improvements to the engine must stay open.
The fine print
The engine is provided as is, without any warranty: no guarantee of merchantability or fitness for a particular purpose, and the author isn't liable for what happens when you use it. This is standard for open-source software.
This page is a friendly summary to help you understand the terms. It is not the licence itself and isn't legal advice. The authoritative text is the GNU GPL v3 plus the Sastra Game Data and Asset Exception, included as LICENSE.txt in the template ZIP and in the header of index.html. You can read the full GPL-3.0 at gnu.org/licenses.