Java API

Java extensions should reuse the existing data definitions, actions, conditions, costs and runtime services first. When adding gameplay, decide on the server-side settlement entry point first, and only then add the client display and the network payload.

• Public API
• Interfaces
• Registries and Data Tables
• Network Protocol and Server Authority
• Hotbar Entries
• Client Screens

Extension Surfaces

Surface	What an addon can do
Datapack registries	Add or override data table entries under data/<namespace>/mxt/; see JSON Data Formats.
Attachments	Store per-entity, per-level and per-chunk state in NeoForge attachment types; the mod registers its own in MxtAttachments, for example CULTIVATION, SPIRIT_IDENTITY and RESOURCE_HOLDER, and they are read with entity.getData(...).
Actions and conditions	Reuse the built-in entity, bi-entity, block, item and damage action and condition types, or register new built-in types; see Types Reference.
Number providers	Supply any numeric field from a constant, an expression or a registered provider type; see Number Provider Types.
Formation modules	Add a formation module by writing one FormationActionType record and registering it; see Public API.
Information panel	Add your own lines to the character information panel with InformationManager; see Information Panel.
Hotbar entries	Add a client-side hotbar entry by implementing HotbarEntry; see Hotbar Entries.

Next Steps

• Public API — the runtime services an addon calls into, from AuraService to DefinitionText.
• Interfaces — AuraAccess, ItemAuraAccess, UseItemAuraAccess, TooltipAppender, Cost and HotbarEntry.
• Registries and Data Tables — built-in type registries, the datapack registry list and the codec naming convention.
• Network Protocol and Server Authority — every C2S and S2C payload, and the one channel that carries item contents.
• Information Panel — register your own lines in the character information panel.
• Hotbar Entries — how to add an entry to the client hotbar.
• Client Screens — where the screens live, and how the item picker clones the vanilla creative search tab.
• Types Reference — every built-in action and condition type.
• JSON Data Formats — every field of every data table.
• Datapack Overview — the datapack directory layout and when changes take effect.
• KubeJS API — the scripted alternative for content that does not need Java.