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The PolymodHandler class is the central hub for Friday Night Funkin’s modding system, built on the Polymod framework. It handles mod discovery, loading, script initialization, and asset management.

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Overview

PolymodHandler manages the complete mod lifecycle:
  • Scans and validates mods against API version requirements
  • Loads mod assets and scripts in priority order
  • Configures sandboxing and security restrictions
  • Hot-reloads mods during development
  • Registers scripted classes and modules

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API Version

public static var API_VERSION(get, never):String;
public static final API_VERSION_RULE:String = ">=0.8.0 <0.9.0";
The API version system ensures mod compatibility:
  • API_VERSION: Returns the current game version (e.g., “0.8.0”)
  • API_VERSION_RULE: Semantic versioning rule that defines compatible mod versions
  • Mods must specify their api_version in _polymod_meta.json

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Loading Mods

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Load All Mods

PolymodHandler.loadAllMods();
Loads all discovered mods from the mods folder, useful for development.

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Load Enabled Mods

PolymodHandler.loadEnabledMods();
Loads only the mods enabled in the player’s save data. This is the default behavior when starting the game.

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Load Specific Mods

PolymodHandler.loadModsById(["my-mod", "another-mod"]);
Loads an ordered list of mods by their IDs. Order matters - later mods can override earlier ones.

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Load No Mods

PolymodHandler.loadNoMods();
Initializes Polymod without loading any mods. Still configures the framework.

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Mod Discovery

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Get All Mods

var allMods:Array<ModMetadata> = PolymodHandler.getAllMods();

for (mod in allMods) {
  trace('Found mod: ${mod.title} v${mod.modVersion}');
  trace('  ID: ${mod.id}');
  trace('  Description: ${mod.description}');
}
Returns metadata for all mods in the mods folder, including disabled ones.

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Get Enabled Mods

var enabledMods:Array<ModMetadata> = PolymodHandler.getEnabledMods();
Returns metadata only for mods that are currently enabled in save data.

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Get Loaded Mod IDs

var loadedIds:Array<String> = PolymodHandler.loadedModIds;
List of mod IDs that were successfully loaded in the current session.

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Hot Reloading

PolymodHandler.forceReloadAssets();
Force-reloads all mod assets and scripts from disk. Useful for rapid iteration during development:
  • Clears all module and script caches
  • Reloads all mods (uses loadAllMods by default)
  • Reloads all game registries (songs, characters, stages, etc.)
  • Calls onCreate on all modules
Hot reloading freezes the game briefly while assets are reloaded. It’s designed for development, not production.

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Mod Lifecycle

When mods are loaded, PolymodHandler follows this sequence:
  1. Scan: Discovers all mods in the mods folder
  2. Validate: Checks API version compatibility
  3. Initialize: Configures Polymod with security rules
  4. Load Assets: Loads and merges mod assets (images, audio, data)
  5. Parse Scripts: Parses .hxc scripted class files
  6. Register Classes: Registers scripted classes with the engine
  7. Load Modules: Instantiates module scripts
  8. Call onCreate: Dispatches CREATE event to all modules

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Security & Sandboxing

PolymodHandler implements strict security rules to prevent malicious mods:

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Blacklisted Classes

These classes cannot be imported in mod scripts:
  • Sys - System command execution
  • sys.* - File system access
  • cpp.Lib - Native library loading
  • haxe.Unserializer - Arbitrary class instantiation
  • polymod.* - Framework manipulation
  • API classes that could enable cheating

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Sandboxed Alternatives

Safe alternatives are provided: 
// Restricted file access
Polymod.addImportAlias('funkin.util.FileUtil', funkin.util.FileUtilSandboxed);

// Read-only leaderboard access
Polymod.addImportAlias('funkin.api.newgrounds.Leaderboards', 
  funkin.api.newgrounds.Leaderboards.LeaderboardsSandboxed);

// Safe reflection utilities
Polymod.addImportAlias('Reflect', funkin.util.ReflectUtil);
Polymod.addImportAlias('Type', funkin.util.ReflectUtil);

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Default Imports

Commonly used classes are automatically available: 
import funkin.Assets;
import funkin.Paths;
import funkin.Preferences;
import funkin.util.Constants;
import flixel.FlxG;

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Mod Folder Structure

mods/
└── my-mod/
    ├── _polymod_meta.json    # Mod metadata (required)
    ├── _polymod_icon.png     # Mod icon (optional)
    ├── data/                  # Data files (JSON, text)
    ├── images/                # Image assets
    ├── sounds/                # Sound effects
    ├── music/                 # Music tracks
    ├── songs/                 # Chart data
    └── scripts/               # Script files (.hxc)

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Mod Metadata

Every mod requires a _polymod_meta.json file: 
{
  "title": "My Cool Mod",
  "description": "Adds new content to the game",
  "contributors": [
    {"name": "YourName"}
  ],
  "api_version": "0.8.0",
  "mod_version": "1.0.0",
  "license": "Apache-2.0"
}

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Parse Rules

PolymodHandler configures how different file types are merged: 
// Text files support line-based merging
output.addType('txt', TextFileFormat.LINES);

// Script files support plaintext merging
output.addType('hscript', TextFileFormat.PLAINTEXT);
output.addType('hxc', TextFileFormat.PLAINTEXT);
This allows mods to append to or modify existing game data.

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Error Handling

Errors during mod loading are handled by PolymodErrorHandler:
  • Script Parse Errors: Syntax errors in scripts show a popup
  • Script Runtime Exceptions: Runtime errors show a popup
  • Import Errors: Missing or blacklisted imports are reported
  • Version Mismatches: Incompatible mods are skipped

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Example: Checking Loaded Mods

import funkin.modding.PolymodHandler;

class MyScript {
  public static function main() {
    // Check what mods are loaded
    trace('Loaded mods:');
    for (modId in PolymodHandler.loadedModIds) {
      trace('  - $modId');
    }
    
    // Get detailed info about all available mods
    var allMods = PolymodHandler.getAllMods();
    trace('\nTotal mods found: ${allMods.length}');
    
    for (mod in allMods) {
      var status = PolymodHandler.loadedModIds.contains(mod.id) ? "LOADED" : "DISABLED";
      trace('[$status] ${mod.title} (${mod.id}) - v${mod.modVersion}');
    }
  }
}

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