Caelus API Mod (1.21.5, 1.20.6) – Shared Library

Caelus API is a lightweight library mod that unlocks elytra flight mechanics for other mods by exposing a generic flight attribute, available for Minecraft Fabric, Forge, and NeoForge.

Elytra flight in vanilla Minecraft is hardcoded to a single item — the elytra itself. That means if a mod developer wants to let you glide with custom wings, a jetpack, or an enchanted cape, they have to fight against rigid vanilla code to make it work. Caelus API solves this by replacing that hardcoded check with a flexible flight attribute that any mod can hook into. With over 4.9 million downloads, it has become one of the most widely depended-upon utility mods in the Minecraft modding ecosystem, quietly powering popular mods like Curious Elytra, Colytra, and countless armor mods that grant flight capabilities.

Key Features of Caelus API

• Generic Elytra Flight Attribute — Instead of checking whether you are wearing the vanilla elytra item, Caelus introduces an entity attribute that controls flight permission. This means any mod can grant elytra-style gliding to any item, armor piece, or trinket without overwriting vanilla behavior. For players, this translates to far fewer mod conflicts when running multiple flight-related mods together.
• Seamless Vanilla Compatibility — The vanilla elytra item is automatically converted to use the new attribute system, so its behavior remains identical to what you expect. You will not notice any difference in normal gameplay. This design ensures zero disruption to your existing world and inventory.
• Simple Developer API — Mod developers only need a single method call to tap into elytra flight mechanics, making integration straightforward. This low barrier to entry is why so many mods list Caelus as a dependency. For you as a player, it means the mods you love get better flight support with fewer bugs.
• Attribute-Based Control System — Flight permission works on a numeric scale: a value of 0 disables flight, while a value of 1 or higher enables it. This granular control lets mod developers create conditional flight — for example, flight only while wearing a full set of custom armor, or flight that degrades as durability decreases.
• Multi-Loader Support — Caelus API is available on Fabric, Forge, and NeoForge, covering all three major mod loaders. Whether you run a lightweight Fabric setup or a heavy Forge modpack, Caelus fits right in without requiring you to switch platforms.
• Lightweight and Passive — As a library mod, Caelus API appears to be extremely lightweight. It adds no items, no blocks, no GUI elements, and no configuration screens. It simply sits in your mods folder and does its job when other mods call upon it, with no noticeable impact on load times or frame rates.
• Open Source Under LGPL-3.0 — The full source code is available on GitHub under the LGPL-3.0-or-later license. This transparency means the community can audit the code, contribute fixes, and trust that the mod does exactly what it claims — nothing more, nothing less.

Screenshots

How to Install Caelus API

1. Choose and install your preferred mod loader: Fabric, Minecraft Forge, or NeoForge. Make sure it matches your Minecraft version.
2. Download the correct Caelus API file for your Minecraft version and loader from the download section below. For Minecraft 1.21.5, only NeoForge is currently supported.
3. Place the downloaded .jar file into your .minecraft/mods folder. If the folder does not exist, launch the game once with your mod loader to generate it.
4. Install any mods that depend on Caelus API — popular options include Curious Elytra, Colytra, and various custom armor mods that grant elytra flight.
5. Launch Minecraft and verify the mod is loaded by checking the mod list in your loader's menu. Caelus works silently in the background, so there is no in-game configuration needed.

Requirements & Compatibility

What's New

• Fixed a bug where glider damage hooks caused helmets to incorrectly lose durability during elytra flight (v8.0.1 for 1.21.4)
• Added support for Minecraft 1.21.5 on NeoForge
• Continued multi-loader support across Fabric, Forge, and NeoForge for recent Minecraft versions
• The project follows Semantic Versioning and maintains a detailed changelog on GitHub for full transparency

Pros and Cons

Alternatives to Caelus API

• Elytra Slot — Adds a dedicated curio slot for the elytra so you can wear it alongside a chestplate, often used together with Caelus API.
• Colytra — Lets you attach an elytra directly to any chestplate as a combined item, and uses Caelus API under the hood for its flight mechanics.
• Curious Elytra — Moves the elytra to a Curios accessory slot, freeing up the chest armor slot while relying on Caelus for flight attribute handling.

Download Caelus API

Always download mods from official sources to stay safe.

Frequently Asked Questions

Does Caelus API change anything in my game by itself?

No — Caelus API is a behind-the-scenes library mod that makes no visible changes to your game on its own. It only provides the flight attribute system that other mods use to grant elytra-style gliding to custom items. You will not notice its presence unless you have dependent mods installed.

Is Caelus API required for Colytra and Curious Elytra?

Yes — both Colytra and Curious Elytra depend on Caelus API to handle their elytra flight mechanics. Without Caelus installed, these mods will either fail to load or lose their flight functionality entirely. Always check your dependent mods' requirements pages to confirm which version of Caelus you need.

Does Caelus API work with NeoForge on Minecraft 1.21.5?

Yes — Caelus API supports NeoForge for Minecraft 1.21.5 as of its latest update. However, Forge and Fabric builds are not currently available for 1.21.5. If you are running Forge, the most recent supported version is 1.21.1.

Will Caelus API cause performance issues in large modpacks?

No, it appears to be extremely lightweight. Caelus API adds no blocks, entities, or rendering overhead. It simply registers a single entity attribute and modifies how elytra flight checks are handled internally. Even in modpacks with 200+ mods, Caelus should contribute negligible load to your system.

Can I use Caelus API on a multiplayer server?

Yes — Caelus API works on both client and server. For multiplayer, both the server and all connecting clients need the mod installed along with any dependent mods. Since it is a library mod, it follows standard installation procedures and does not require any special server-side configuration.

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