I'm looking for a Minecraft client with all versions at the same time?

I'm looking for a Minecraft client with which you can play 1.12.2 at the same time but also restart 1.12 without restarting the game (please no hack client) cool weir even if the client has published the source code

I don't know anyone, but I'm currently developing a client with my development team. I'll put that in. Then there's such a client.

Your client then loads 40 different versions at the same time? Or just the major versions? Even then there would still be 17 versions if you go from 1.0. And what about the other version numbers 1.15.2 e.g. They also extra or do you leave them out? Or only the last of the respective ones, then there would be 17. And what about Alpha and Beta versions? And how do you solve all the differences in the versions with the options alone? And what about snapshots? What about mods? In the end, should your client load 300 different versions at the same time?

You can download the version. We know what we do, my programmers all have more than 5 years of experience.

Interesting can you tell me the name of the client? So that I can find him when he's done

It is not yet certain. But you can send me an FA and ask again in a month. I already have a network where we offer things and a website, but we will change our name because there's no .com domain anymore. If our name is fixed then I can send you the website where you can download it.