Underlay

Evidence

Ran on Kiln's test stand (2026-11-04): Underlay did not start: java.lang.NoClassDefFoundError: net/minecraft/tags/TagKey; UnderlayClient did not start: java.lang.NoClassDefFoundError: net/minecraft/network/protocol/common/custom/CustomPacketPayload.

What it needs from Kiln

• Mixin
• com.mojang.blaze3d.vertex.PoseStack
• net.fabricmc.fabric.api.client.networking.v1.ClientPlayConnectionEvents
• net.fabricmc.fabric.api.client.networking.v1.ClientPlayConnectionEvents.Disconnect
• net.fabricmc.fabric.api.client.networking.v1.ClientPlayNetworking
• net.fabricmc.fabric.api.client.networking.v1.ClientPlayNetworking.Context
• net.fabricmc.fabric.api.client.networking.v1.ClientPlayNetworking.PlayPayloadHandler
• net.fabricmc.fabric.api.client.renderer.v1.mesh.MutableQuadView
• net.fabricmc.fabric.api.client.renderer.v1.mesh.QuadEmitter
• net.fabricmc.fabric.api.client.renderer.v1.render.AltModelBlockRenderer
• net.fabricmc.fabric.api.client.rendering.v1.level.LevelRenderContext
• net.fabricmc.fabric.api.client.rendering.v1.level.LevelRenderEvents
• net.fabricmc.fabric.api.client.rendering.v1.level.LevelRenderEvents.CollectSubmits
• net.fabricmc.fabric.api.command.v2.CommandRegistrationCallback
• net.fabricmc.fabric.api.entity.event.v1.ServerEntityLevelChangeEvents
• net.fabricmc.fabric.api.entity.event.v1.ServerEntityLevelChangeEvents.AfterPlayerChange
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerLevelEvents
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerLevelEvents.Load
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerLevelEvents.Unload
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerLifecycleEvents
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerLifecycleEvents.EndDataPackReload
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerLifecycleEvents.ServerStopping
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerTickEvents
• net.fabricmc.fabric.api.event.lifecycle.v1.ServerTickEvents.EndTick
• net.fabricmc.fabric.api.networking.v1.PayloadTypeRegistry
• net.fabricmc.fabric.api.networking.v1.ServerPlayConnectionEvents
• net.fabricmc.fabric.api.networking.v1.ServerPlayConnectionEvents.Join
• net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking
• net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking.Context
• net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking.PlayPayloadHandler
• net.minecraft.client.OptionInstance
• net.minecraft.client.Options
• net.minecraft.client.renderer.SubmitNodeCollector
• net.minecraft.client.renderer.block.BlockStateModelSet
• net.minecraft.client.renderer.blockentity.BlockEntityRenderDispatcher
• net.minecraft.client.renderer.blockentity.BlockEntityRenderer
• net.minecraft.client.renderer.blockentity.state.BlockEntityRenderState
• net.minecraft.client.renderer.chunk.RenderSectionRegion
• net.minecraft.client.renderer.feature.ModelFeatureRenderer
• net.minecraft.client.renderer.feature.ModelFeatureRenderer.CrumblingOverlay
• net.minecraft.client.renderer.state.level.CameraRenderState
• and 61 more

On the roadmap: Minecraft's libraries and client API (0.2), Code next to data (0.3), Mixin (0.5)

In the author's words

Place any block (such as carpets) inside anything!

Scan

Scanned on 4 November 2026 against Kiln 0.2.0, file underlay-1.0.5-hotfix.1-fabric-mc26.3.jar. The scan reads the mod's classes; it does not run them.