Unity Modding Architecture

Overview

Guide for designing Unity games that support community modding. Covers asset loading systems, scripted mod APIs, mod manager patterns, distribution platforms, and techniques for modding existing games.

Modding Architecture Tiers

Tier	What Modders Can Do	Complexity
Data Mods	Replace textures, sounds, configs (JSON/XML)	Low
Asset Mods	Add new models, maps, items via Asset Bundles	Medium
Script Mods	Custom game logic via Lua/scripting API	High
Code Mods	Patch game assemblies via Harmony/BepInEx	Expert

Design your game to support at least Tier 1-2 from the start. Tiers 3-4 require deliberate API design.

Asset Loading for Mods

Addressables (Recommended)

Addressables provide a unified API for loading assets from local bundles, remote servers, or mod directories.

// Load a mod's asset catalog at runtime
public async Awaitable LoadModCatalog(string modPath)
{
    string catalogPath = Path.Combine(modPath, "catalog.json");
    var locator = await Addressables.LoadContentCatalogAsync(catalogPath);
    Debug.Log($"Loaded mod catalog with {locator.Keys.Count()} assets");
}

// Load an asset by address (works for both base game and mods)
var prefab = await Addressables.LoadAssetAsync<GameObject>("enemies/goblin");
Instantiate(prefab);

Mod workflow:

1. Modders use a Unity project with Addressables configured
2. Build Addressable groups into Asset Bundles
3. Ship the catalog.json + .bundle files
4. Game loads catalogs at runtime from the mod directory

Asset Bundles (Legacy but Simpler)

public async Awaitable<AssetBundle> LoadModBundle(string path)
{
    var request = AssetBundle.LoadFromFileAsync(path);
    await request;
    return request.assetBundle;
}

// Load specific asset from bundle
var bundle = await LoadModBundle("mods/weapons/swords.bundle");
var swordPrefab = bundle.LoadAsset<GameObject>("BroadSword");

Key rules:

• Never load the same bundle twice (track loaded bundles)
• Unload bundles when mods are disabled: bundle.Unload(true)
• Use AssetBundle.LoadFromFileAsync for local files (not LoadFromMemory)

Hot-Reloadable Data Mods

For simple data mods (JSON configs, CSV tables):

public T LoadModConfig<T>(string modPath, string fileName)
{
    string filePath = Path.Combine(modPath, fileName);
    if (!File.Exists(filePath)) return default;
    string json = File.ReadAllText(filePath);
    return JsonUtility.FromJson<T>(json);
}

Use FileSystemWatcher to detect changes and hot-reload during development.

Lua Scripting with MoonSharp

MoonSharp is a Lua interpreter written in C# that runs on all Unity platforms.

Setup

Install via NuGet or download the DLL. Register safe API surfaces for modders:

public class ModScriptEngine
{
    Script _lua;

    public void Initialize()
    {
        _lua = new Script();

        // Register safe game API
        _lua.Globals["SpawnEntity"] = (Func<string, float, float, float, bool>)SpawnEntity;
        _lua.Globals["GetPlayerHealth"] = (Func<float>)(() => player.Health);
        _lua.Globals["ShowMessage"] = (Action<string>)ShowMessage;
        _lua.Globals["RegisterEvent"] = (Action<string, DynValue>)RegisterEvent;
    }

    public void LoadMod(string luaFilePath)
    {
        string code = File.ReadAllText(luaFilePath);
        _lua.DoString(code);
    }

    bool SpawnEntity(string id, float x, float y, float z)
    {
        // Validate and spawn - never trust mod input
        if (!entityRegistry.Contains(id)) return false;
        var pos = new Vector3(x, y, z);
        if (!IsValidSpawnPosition(pos)) return false;
        entityRegistry.Spawn(id, pos);
        return true;
    }
}

Modder-Side Lua

-- my_mod/init.lua
RegisterEvent("OnPlayerEnterZone", function(zoneName)
    if zoneName == "boss_arena" then
        SpawnEntity("dragon_boss", 0, 5, 0)
        ShowMessage("A dragon appears!")
    end
end)

Security Considerations

Risk	Mitigation
File system access	Do NOT expose System.IO to Lua
Infinite loops	Set instruction count limits: script.Options.InstructionLimit = 100000
Memory abuse	Limit table sizes, monitor allocation
Network access	Never expose HTTP/socket APIs
Reflection abuse	MoonSharp sandboxes by default, but validate registered types

Mod Manager Pattern

Architecture

Game
├── ModManager (singleton)
│   ├── DiscoverMods(modsFolder)     // Scan for mod manifests
│   ├── ValidateMod(manifest)        // Version compat, dependencies
│   ├── LoadMod(modId)               // Load assets + scripts
│   ├── UnloadMod(modId)             // Clean teardown
│   └── GetLoadOrder()               // Dependency-sorted order
├── ModManifest (ScriptableObject / JSON)
│   ├── id, name, version, author
│   ├── dependencies[]
│   ├── gameVersionMin / gameVersionMax
│   └── entryPoint (script path)
└── ModSandbox
    ├── Lua scripting environment
    └── Restricted API surface

Mod Manifest Format

{
    "id": "com.author.mymod",
    "name": "My Awesome Mod",
    "version": "1.2.0",
    "author": "ModAuthor",
    "description": "Adds new enemies and weapons",
    "gameVersionMin": "1.0.0",
    "gameVersionMax": "2.0.0",
    "dependencies": ["com.author.corelib@1.0.0"],
    "entryPoint": "scripts/init.lua",
    "assets": "bundles/"
}

Steam Workshop Integration

Use Steamworks.NET or Facepunch.Steamworks for Steam Workshop integration:

1. Upload: Modder packages mod folder, calls SteamUGC.CreateItem + SteamUGC.SubmitItemUpdate
2. Subscribe: Players subscribe in Steam Workshop UI
3. Download: Game queries SteamUGC.GetSubscribedItems() at startup
4. Load: Read from SteamUGC.GetItemInstallInfo path into ModManager

Harmony Patching (Modding Existing Games)

Harmony is used by modders (via BepInEx/MelonLoader) to patch compiled game code at runtime.

[HarmonyPatch(typeof(PlayerHealth), "TakeDamage")]
class DamageModifier
{
    // Prefix: runs before original method
    static bool Prefix(ref float damage)
    {
        damage *= 0.5f; // Halve all damage
        return true;    // true = continue to original
    }

    // Postfix: runs after original method
    static void Postfix(PlayerHealth __instance)
    {
        Debug.Log($"Health after damage: {__instance.CurrentHealth}");
    }
}

If designing a game intended to be modded via Harmony, avoid aggressive code obfuscation and keep method signatures stable across updates.

Additional Resources

Reference Files

• references/mod-framework-detail.md -- Complete mod manager implementation, dependency resolution algorithm, mod load ordering, versioned API patterns, hot-reload systems, mod testing frameworks, BepInEx/MelonLoader setup guide