Structure Gel API

Structure Gel API is a Minecraft Java library mod from Modding Legacy that gives other mods, and builders, colored gel blocks for shaping how generated structures load into the world. It has been downloaded more than 5 million times on Modrinth, mostly as a dependency other structure-generation mods build on rather than something players install for its own sake. It runs on Forge and NeoForge, needs both client and server, and adds no playable content beyond the gel blocks themselves.

How the gel blocks work

Place a gel block while sneaking to trigger its spread effect, and it fills connected space with that color; using gunpowder on a gel triggers a chain reaction that removes every connected block of the same type. Six colors each spread differently:

• Red: spreads along the cardinal directions, only replacing air.
• Blue: spreads like red, but won't spread into locations where the sky can be seen.
• Green: spreads like red plus diagonally, useful for reaching around corners made by vines or complex builds.
• Cyan: combines green and blue, spreading diagonally while still avoiding the sky.
• Orange: spreads a distance set by how many gel blocks are held when placed, handy for builds in open areas.
• Yellow: only spreads along the plane you're facing when placed, useful for sealing walls, windows, or other holes.

When the structure is saved as a template, whatever a gel color marked is replaced with air on load, while air the gel never reached is left alone. That's the actual worldgen use case: a structure can spawn without dragging in the open space a builder needed just to shape it while editing.

Loader and version support

NeoForge is the actively maintained line. Its newest build, 2.21.2 for Minecraft 26.1.2, published in August 2026. Forge trails a long way behind: its last build, 2.16.2 for 1.20.1, shipped in April 2024, and nothing has followed it since. There is no Fabric or Quilt build for this project.

Installing Structure Gel API

1. 1.Confirm whether the mod that needs Structure Gel API runs on Forge or NeoForge; see how to install Forge and NeoForge for Minecraft Java for the loader setup.
2. 2.Download the build matching that loader and your Minecraft version from Structure Gel API's Modrinth page.
3. 3.Drop the .jar into your mods folder alongside the dependent mod and your loader's own files. Unsure where that is? See how to install Minecraft mods on Java Edition.
4. 4.Playing multiplayer? Install the same build and version on the server too; Modrinth lists both sides as required.