Custom Block Outlines (CBO)
Have you ever looked at a brewing stand, a redstone wire, or a flower and thought the hitbox just feels wrong? Custom Block Outlines (CBO) is a Fabric mod that hands you full control over the outline and collision shape of nearly any block in the game. Instead of accepting vanilla geometry, you can copy the shape of another block or build your own from scratch using simple cuboids. It is a niche tool, but for map makers, datapack tinkerers, and players who care about precision, it solves a very specific problem.
What Custom Block Outlines (CBO) Actually Does
At its core, this mod changes how blocks are outlined and how they collide. You can point one block at another and inherit its shape, or you can define a custom set of cubes that make up the outline. That means a poppy could suddenly have the outline of spruce stairs, or a brewing stand could be split into two separate collision boxes. The mod is deprecated and no longer actively maintained, so treat it as a stable snapshot rather than a growing project. Even so, it remains one of the few tools that lets you edit block outlines without touching game code.
Supported Versions and Loader
Custom Block Outlines (CBO) is built for Fabric and requires the Fabric API to run. It targets older Minecraft releases from the 1.16 to 1.18 era, so check that your game version matches before you download Custom Block Outlines (CBO). If you are on a newer release, expect compatibility issues, since the mod has not been updated to follow modern block state changes. For players who still run legacy worlds or modpacks, it works exactly as intended.
How to Install Custom Block Outlines (CBO)
Installation follows the standard Fabric workflow, and knowing how to install it correctly saves a lot of frustration later.
- Install the Fabric loader for your Minecraft version.
- Drop the Fabric API jar into your mods folder.
- Place the Custom Block Outlines (CBO) jar alongside it.
- Launch the game once so the mod can generate its folders.
On first launch, the mod creates an "outlinepacks" folder inside your run directory, which is usually the .minecraft folder. That folder is where all your outline packs live. Each pack is just a folder of JSON files, one per block you want to change. The pack folder name does not matter, but each JSON file must be named after the block ID, such as brewing_stand.json or redstone_wire.json.
If you would rather skip the manual file shuffling, the foxygame.net launcher can handle the setup for you, placing Custom Block Outlines (CBO) into the right profile and keeping your Fabric API version in sync automatically.
Writing Your First Outline Pack
Copying Another Block
The simplest approach is to borrow an existing shape. In poppy.json you would write an override block that points to minecraft:spruce_stairs with a blockstate of facing=north,half=bottom,shape=straight. If the block you are copying shifts position depending on where it sits, like flowers, bamboo, or pointed dripstone, add a pos tag with the exact coordinates. A dandelion at 24, 65, -121 would use an override with that position, and the blockstate tag becomes optional.
Building a Custom Shape
For full control, define your own cubes. In brewing_stand.json you can list two boxes, one from 0,0,0 to 8,8,8 and another from 8,8,8 to 16,16,16. The format for each cube is x1, y1, z1, x2, y2, z2, where 0,0,0 is the bottom north-west corner and 16,16,16 is the top south-east corner. Cubes can extend past the normal block boundary, but the results get unpredictable. Stacking too many cubes will also cause lag, so keep shapes lean. If a file contains both an override and cubes, the override wins.
Server and Client Notes
If you play on a server, the same packs must exist on both the server and the client. Nothing will crash if they are missing, but the game will stutter and behave glitchily as the two sides disagree about block shapes. For singleplayer, this is a non-issue.
Final Thoughts
Custom Block Outlines (CBO) for Minecraft is a specialist mod for a specialist audience. It is deprecated, Fabric-only, and limited to older versions, but nothing else offers this level of control over block outlines and collision boxes with such a light touch. If precise geometry matters to your build, it is worth the setup.