Adding mods to the Official Vanilla Pack

[Scipio Xaos]

So...  I've been trying to do this for a while now, but I haven't had much luck.  Anyways, here's the rundown.

Whenever I play vanilla there's a few mods I like to use.  Notably, I really can't play the game without optifine since my computer is crud.   In the past I've modded the .technic/modpacks/vanilla/bin/minecraft.jar directly by applying the appropriate optifine to it and this works, but I'd like to add more mods like Inventory Tweaks (cause sorting is a pain sometimes ).  Well..  I've done some examining of the files in the bin/ directory as well as having read the "How to make a modpack." guide so I thought I'd try and twist the vanilla pack into a forge modpack.  Afterall, when I examine other modpacks I have installed I can see that the bin/modpack.jar is essentially the forge universal for the appropriate version of minecraft.

Well..  I replace the vanilla/bin/modpack.jar (which just contains an empty directory) with the appropriate version of forge and that's about as far as I get.  When I attempt to launch the vanilla pack it crashes.  Every time.  I did some digging into the logs and found this:

[21:10:46] [main/ERROR] [FML/]: There is a binary discrepency between the expected input class ty (ty) and the actual class. Checksum on disk is 8c2cd3f1, in patch c7c760cb. Things are probably about to go very wrong. Did you put something into the jar file?
[21:10:46] [main/ERROR] [FML/]: The game is going to exit, because this is a critical error, and it is very improbable that the modded game will work, please obtain clean jar files.

Now, I've searched around and from what I can tell it indicates that the minecraft.jar is not correct.  This doesn't make sense since, supposedly, the minecraft.jar is an unmodded version of 1.8.7.  So I did more digging...

Using the official minecraft launcher I installed 1.8.7 and did an md5sum on .minecraft/versions/1.8.7/1.8.7.jar and .technic/modpacks/vanilla/bin/minecraft.jar  Lo and behold, the sums are different.  So.. as far as I can tell the official vanilla modpack is downloading and installing a modded version of the 1.8.7 jar.  (I have a theory that it's different because the pack is "Solder enabled" though I don't really know what that entails..)

In conclusion, can anyone give me a hand?  All I want is to be able to add mods to the official Vanilla pack (by dropping them in the autogenerated ".technic/modpacks/vanilla/mods/" folder).  I could install a "Forge 1.8" modpack, but then what do I do when the official vanilla pack is updated to 1.9 in a few months?  Wait for a Forge 1.9?  Seems rather counter-intuitive when, supposedly, the official modpack should support modding.   Any suggestions?

Edited August 3, 2015 by Scipio Xaos
Fixed some typos.

[planetguy]

IIRC Forge is still on 1.8.0, not 1.8.7, so you will have to set Vanilla to 1.8.0.