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This is the “expert/technical” extension of the user FAQ.
There is some experimental functionality available in Loader's 0.5.0 development branch to run Fabric Loader on top of Paper specifically, but there are many caveats pertaining to Bukkit API's deficiencies (such as hardcoding the list of blocks/items in an enum, making it very difficult to support modded blocks or items). This will probably never be officially supported and should be treated more as a curiosity/special-case scenario.
The best approach here would be to port SpongeCommon and a modified version of SpongeVanilla as a Fabric mod implementing the Sponge API. As Fabric is using a fork of SpongePowered Mixin at its core, this is made somewhat easier - but the mappings differences make up for this, so to say.
The rendering patch Fabric API uses, Indigo, assumes (for performance and code simplicity reasons) that the vanilla vertex format is kept intact. Mods generally don't change it, however ShadersMod and similar mods are a popular exception. As such, Indigo doesn't play nicely with it as-is.
Solutions for this have been looked into, but not yet functional. In addition, some of the unofficial mod projects are trying to work on one themselves. The current workarounds will not work in the presence of content mods which actually make use of Fabric's rendering API.
In theory, there is nothing stopping you from running Fabric's mod loader on any version of Minecraft, under any obfuscation layer, all the way down to c0.0.11a. However, Yarn mappings don't exist for most of these versions - as such, making mods is made… a tad more complicated.
Yes! Generally, all JAR mods (such as old versions of OptiFine, or Better Than Wolves) should work fine, and allow the same degree of moddability you'd get with an unmodded version of Minecraft. However, there are some special notes:
For most pre-1.13 mod loaders, you must enable compatibility mode in Fabric Loader, by adding the JVM flag
With regards to running Minecraft Forge:
With regards to running ModLoader: No known problems.
With regards to the Mod Coder Pack, MCP:
* MCP's mappings have no clear licensing terms; between the old MCP license (as preserved on the Minecraft Gamepedia) and the new MCPConfig repository.
* MCP's mappings are not always updated to every non-“release” Minecraft version, nor are others generally allowed to do it themselves. This would forbid Fabric from updating to snapshots or other experimental releases.
* MCP's mappings have an updating process we consider insufficiently open to code review, in our opinion, utilizing IRC bot submissions over a merge request system. (However, this is largely a matter of preference.)
With regards to Spigot:
* Spigot's mappings only cover the server side and are very incomplete, * Spigot's mappings suffer from similar licensing issues as MCP.