What Fabric API is
Fabric API is a modular library mod from modmuss50. It bundles the common hooks and intercompatibility measures that mods using the Fabric toolchain rely on, so individual mods do not each have to write the same low level code. The official description calls it lightweight and modular. On Modrinth it sits in the Libraries and APIs category, and the license is Apache License 2.0.
What it provides
The evidence gives four examples of what the API covers. It exposes functionality that's useful but hard for mods to reach on their own, with particles, biomes, and dimensions named as examples. But that adds events, hooks, and APIs meant to improve interoperability between mods. The result handles features the project treats as essential, including registry synchronization and adding information to crash reports. What you get also ships an advanced rendering API that's described as designed for compatibility with optimization mods and graphics overhaul mods. That is a stated design goal, not a tested result for any specific mod pairing.
Modules
Fabric API is split into modules so updates are easier and the codebase stays in smaller pieces. Real talk, module names in the repository include fabric-api-base, fabric-command-api-v1, fabric-lifecycle-events-v1, and fabric-networking-api-v1. Each module is meant to carry its own README explaining its purpose, but the repository notes that this documentation is still being worked on and not every module has one yet.
Requirements
Fabric API is a Fabric mod like any other and needs Fabric Loader installed first. Fabric is the only mod loader listed. On Modrinth both client side and server side are marked optional, so you may need the file on either end depending on the mods you run.
Minecraft versions
The Modrinth record lists these discrete versions: 26.3-pre-2, 26.3-pre-1, 26.2, 26.1.2, 26.1.1, 26.1, and 1.21.1. These are separate entries rather than a continuous range, so match the file you download to your exact game version.
Getting it and getting help
The repository points to CurseForge, GitHub Releases, and Modrinth as download sources. Place the downloaded jar file in your mods folder after Fabric Loader is set up. Support and discussion for both players and developers happen on the Fabric Discord server.
