Quilted Fabric API (QFAPI) and Quilt Standard Libraries (QSL) is the combined name for Quilt Standard Libraries (QSL) and Quilted Fabric API (QFAPI).

Note: For Players, we recommend the QuiltLoader QSL+QFAPI because it is backwards compatible with most traditional Fabric Mods plus a few extra, but for Developers, we suggest sticking to Fabric API and Fabric Loader unless you need Quilt Specific Features, though do note that both, especially Quilt are very democratic so you can submit to both of them your ideas. You can also use FeatureCreep if you want to target multiple Platforms.

Quilt Standard Libraries (QSL) are a set of libraries designed for making QuiltMC mods. It is a little like Fabric API but for QuiltMC, which adds a lot of new stuff not found in Fabric API, including:

Auto Test Server Arguments
Event Entry points
Crash Context API
Ability to exclude entries from registry syncing
A helper for additional events
Resource Loader Events
Resource Pack Grouping API
Many additions to the Block API, mainly including registries and entities
Recipe API and Attachments
Better client fallback and tags with vanilla integration
Multipart Entities
Entity selector Options
Surface Rules
Modded DFU

There are also some things which are not in the Quilt Standard Libraries that are in the Fabric API; these can be found in the Quilted Fabric API and include some of the following:

Rendering Libraries (Some are WIP)
Game Rules
Message API
Screen Handling
Data generation (Some are WIP)
Resource Conditions
Loot API
Convention Tags
Tileables for Blocks
Some events
Gametest API

Quilted Fabric API is a port of the Fabric API Built on top of Quilt Standard Libraries; this is part of what allows Quilt to run Fabric Mods and contains lots of the stuff missing from the Quilt Standard Libraries.

It is important to note that Quilt Standard Libraries and Quilted Fabric API are separate yet packaged the same in some cases. You can download Quilt Standard Libraries independently on the Quilt Standard Libraries GitHub Page without QFAPI. The separation of the two led to lots of miscommunication at the beginning of Quilt. Luckily to fix this, the version of QSL on Modrinth (and Later CurseForge and now MinecraftStorage) came with QFAPI by default. Though there are some instances where you may not want QFAPI, an example would be if you wished to bloat less or just needed to test something with only QSL or only needed the QSL entry point and were doing non api modding. Another reason one may not want QFAPI is if you were running some extraction layer like Felt’s Quilt to Fabric and were using a traditional Fabric API (which is not compatible with QFAPI) (though if you are using felt, you would likely want to use their port and FeltMC, in general, should be avoided because of their attitude but can help run on CurseForge packs or when quilt-loader does not support a mod that does work on Fabric).

Quilt Standard Libraries require QuiltLoader, which is a standalone mod loader. Though it can also be launched through MC Forge with Quilt Jumpstart or through Fabric with FeltMC’s Quilt to Fabric (Though we would avoid this one), these two options are primarily unofficial and may not work as well as the standalone but have still been used in many events.

QSL is more critical for Quilt than Fabric API is for Fabric (still crucial for some stuff) because QuiltLoader has more limited entry points and is supposed to load pre-existing fabric mods or SpongeMixins mainly.

To develop mods for Quilt, one can download QuiltLoom (Based on Fabric Loom)(It can also use Yarn mappings but use Quilt ones by default) or update their existing Architectury Templates or Multiloader projects to have Quilt support. Most Fabric mods should work on Quilt, so unless there is a specifically mentioned feature you need, it is likely best you continue using Fabric or use a different API (like Rewoven or a potential future QSL port to Fabric).

QuiltMC has seen slow and limited adoption outside Modrinth, which they generally rely on for the mods. Quilt initially had great support in its long development process. Still, some people who were interested in the community did not port or even test and mark compatibility on their existing fabric mods, and even though it has been on CurseForge for a while is still not officially supported by the CurseForge Launcher (though by others) and has not seen too many exclusive mods (and most of the mods in general marked as Quilt are just Fabric mods marked as Quilt) and to an extent has been on the decline. We eventually think SpruceLoader, if they make any progress, will ultimately supersede Quilt as it plans to run Quilt Mods. The Fabric atmosphere, in general, is much more splintered than most other loaders. Despite all this, Quilt still has a big community behind it and is quite popular.

Some trendy mods have moved to Quilt, such as: Ok Zoomer (This was a controversial case because of the blocking of Fabric).

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Quilted Fabric API (QFAPI) and Quilt Standard Libraries (QSL) Mod