You can find a prototype on the game page as a primary package.

The prototype is a testing build of the game, containing 2 rooms where you can do almost everything you can do in the original Undertale. Battles, dialogues with NPCs, interactive items, and so on. Narratively, it's not connected to the main game. Also, I used graphical materials from Undertale as placeholders because I currently lack most of my own sprites. The main purpose of the prototype is to test the game mechanics I've implemented, find bugs that I may have missed, and then fix them. Since I'm a programmer and aspire to be a game developer in the future, I decided not to use other engines or developments. While I had Undertale as a reference for the end result, I wrote the entire code from scratch, so it's important for me to ensure its correctness.

But so that you also have an interest in it, in addition to simple testing, I added something interesting there. The concept of the prototype is that the more you know about the possibilities and secrets of the original Undertale, the more opportunities you'll find in my prototype. But even if you know nothing and just run through these two rooms once without encountering any bugs, it's still useful testing. Although, of course, I'm interested in you trying to find as many as possible!

If the prototype works well, I'll use it as a basis to add all the further rooms and expand it into a full demo version, which will represent Chapter 1 of the future full game.