# Documentation

- [HELIX 3 Documentation](/docs): The unified SDK and platform reference for building virtual worlds on HELIX — across web and Unreal Engine.

- **Concepts**
- Introduction
  - [What is HELIX 3?](/docs/introduction): HELIX is an AI-native, engine-agnostic virtual-world platform. Create, publish, host, and monetize multiplayer 3D worlds — playable instantly from a single link.
  - [How HELIX works](/docs/introduction/how-helix-works): The core nouns — World, Build, Instance, Player, Event — and the lifecycle of a player opening a world.
  - [Platform vs Engine SDK](/docs/introduction/platform-vs-engine): The single most important concept in HELIX — what belongs to the engine-agnostic Platform API versus an engine-specific runtime.
  - [Choose your runtime](/docs/introduction/choose-your-runtime): Web or Native (Unreal)? A quick decision guide and what each path gives you.
  - [Quickstart](/docs/introduction/quickstart): From zero to a published, playable multiplayer world in a few minutes using the HELIX CLI.
- Platform API
  - [Platform API overview](/docs/platform-api): The engine-agnostic surface of HELIX — identity, economy, storage, inventory, and social — exposed identically on every runtime.
  - [Authentication & Identity](/docs/platform-api/authentication): One HELIX account across every world. How launch tickets, sessions, guests, and profiles work — identically on web and Native.
  - [LIX & Economy](/docs/platform-api/lix-economy): LIX is HELIX's hard currency. The wallet, in-world purchases, the marketplace, and the rules that keep value stable — all server-authoritative.
  - [Cloud Save](/docs/platform-api/cloud-save): Durable, authoritative per-world key–value storage. The source of truth for anything that must survive a session.
  - [Memory Store](/docs/platform-api/memory-store): Volatile, low-latency storage shared across all instances of a world. TTL-required. Great for leaderboards, matchmaking, and hot counters — never the source of truth.
  - [Inventory & Items](/docs/platform-api/inventory): Universal items that work across worlds and runtimes, and the four-tier execution model that decides what the client may do and what the server must own.
  - [Social & Presence](/docs/platform-api/social): Friends, presence, and invites — the social graph that powers HELIX's virality loop, available identically on every runtime.

- **Runtimes**
- Web SDK
  - [Web SDK](/docs/web-sdk): The canonical TypeScript runtime for HELIX. Build instantly-playable multiplayer worlds for the web — and define the contract every other runtime mirrors.
  - [Multiplayer & Networking](/docs/web-sdk/multiplayer): HELIX's own real-time networking for web worlds — events, instances, and the hybrid server-authoritative model. Web-runtime only.
  - [Reference (Web SDK)](/docs/web-sdk/reference): The auto-generated API reference for @helix/sdk. This page is generated from the SDK's TypeScript source on every merge.
- Native (Unreal) SDK
  - [Native (Unreal) SDK](/docs/native-sdk): The HELIX Platform API inside Unreal Engine — the same functions in C++, Blueprint, PuerTS (TypeScript), and UnLua (Lua), plus engine-native networking.
  - [Languages](/docs/native-sdk/languages): Setup and the same HELIX call written in C++, Blueprint, PuerTS (TypeScript), and UnLua (Lua).
  - [Networking (Native)](/docs/native-sdk/networking): How multiplayer works in Native (Unreal) worlds — engine-native dedicated servers with HELIX replication wrappers. Distinct from the web multiplayer stack.
  - [Reference (Native SDK)](/docs/native-sdk/reference): The auto-generated C++ API reference for the HELIX Unreal plugin, with Blueprint, PuerTS, and UnLua mappings.

- **Build**
- Guides
  - [Guides](/docs/guides): Task-oriented recipes that combine the Platform API and a runtime to ship something real. Most show web and Native side by side.
  - [Build your first multiplayer world](/docs/guides/first-multiplayer-world): A complete walkthrough — join an instance, exchange events, and run one server-authoritative interaction.
  - [Persist player data](/docs/guides/persist-player-data): Save and load player progress correctly with Cloud Save — including the read-modify-write pattern for concurrent edits.
  - [Sell an item for LIX](/docs/guides/sell-item-for-lix): Add a server-authoritative, idempotent in-world purchase. The client requests; the server charges LIX and grants the item atomically.
- CLI & AI tools
  - [HELIX CLI](/docs/cli): Scaffold, run, validate, and deploy HELIX worlds from the terminal — the same commands humans and AI agents use.
  - [MCP server for AI agents](/docs/cli/mcp): HELIX ships a Model Context Protocol server so AI coding agents can discover, validate, and publish worlds as first-class tools.

- **Reference**
- [Feature matrix](/docs/feature-matrix): Exactly what's available in each runtime. The Platform API is identical; runtimes differ only in rendering, input, and networking.
- [Migrating from HELIX 2](/docs/migrating-from-helix2): What changed from HELIX 2 (QBCore/Lua, FiveM-style RP servers) to HELIX 3, what's deprecated, and how old concepts map to the new unified SDK.