I apologize if this has been asked before, but I've been scouring the internet for the past couple of days and haven't had much luck figuring it out myself.
I'm having issues getting mods to run in 1.2.5. A few years ago back around the beginning of 2017 I managed to get it to work by modifying the json file to stop it from replacing the client jar, but that method seemed to stop working after they updated the launcher again and now I'm at a complete loss.
I'd really appreciate any help or advice! Hoping there's a simple solution I'm just overlooking.
[Edit] MultiMC worked perfectly! Thank you so much, that was a great suggestion!
You can easily bypass the "self-repair feature" by removing the download URL for the Minecraft jar in the json file, which is very conveniently (probably not a coincidence) located right before the version "id" (which also has to be changed along with the name of the files and containing folder):
After (note the missing "downloads" section):
"assets": "legacy", "id": "TMCWv5",
I even provide a pre-modified json file with my own mods so you only need to replace the default file, otherwise, you install them as usual for a jar mod (I don't recommend using 3rd party launchers unless you already have one because I'd never want to download a separate launcher just to use a mod; somebody even posted installation instructions for MultiMC for my own mod but it seems just as complicated, I can easily install a mod on the official launcher within a minute and update it even faster). You can also easily reuse a modified json file as you only need to change the name and "id" (or just keep the same name, e.g. "1.2.5_modded", but having it reflect the mod makes it easier to know what the modded jar has, as well as maintain backups of the files, etc); notably, when they added the downloads (in early 2016) existing json files continued to work with no issues (I only found out about it after somebody else mentioned it since I was just reusing the same file).