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ID Resolver is a mod designed to automate the way that block and item IDs used by mods are assigned within the client. It relies on GuiAPI as a dependency, but not when used on a Server (as no GUI loads for it to be necessary).
ID Resolver has developed a somewhat negative reputation amongst the modding community, with the mod being blamed for a wide variety of crashes, and errors in gameplay when items are set to zero (to remove them).
In reality, many of these issues are the fault of the mod developer, rather than the fault of ID Resolver; a mod has to be specifically coded to allow items to be removed with the =0 option, otherwise ID Resolver will exhibit some strange behaviour, like substituting other items in place of the removed item.
Another commonly reported fault is block and item IDs being referenced using hard-coded values; if these values are changed, then anything that references that specific value will either exhibit incorrect behaviour, or crash the game.
Mods using Minecraft Forge can ask it to assign a default ID for a block or item; if this is unavailable, Forge is able to assign a vacant ID for the block or item to use. Mods that use this method do not need ID Resolver to co-exist; however, mods that do not do this will most likely need it.