fs25-scripting-mod

FS25 Scripting Mod Development

Overview

FS25 mods extend the game via Lua scripts. Always reference the base game scripts to understand existing patterns before writing new code.

When to Use

• Writing or debugging Lua scripts for an FS25 mod
• Creating vehicle specializations or placeable extensions
• Overriding or extending base game behavior
• Understanding how a base game system works

Base Game Scripts

IMPORTANT: Before writing any mod script, search the base game scripts for how the game implements similar functionality. The decompiled scripts are the primary reference.

Base game scripts: dataS/scripts/ (relative to game data root)

The game ships compiled .l64 bytecode. Decompile with fs-luau-decompile from fs-utils:

cd fs-utils && cargo run --bin fs-luau-decompile -- path/to/script.l64
# Recursive:
cd fs-utils && cargo run --bin fs-luau-decompile -- -r path/to/scripts/

Key Directories

Directory	Contains
vehicles/specializations/	Vehicle specializations (Motorized, Drivable, Fillable, etc.)
placeables/specializations/	Placeable specializations
specialization/	SpecializationManager, TypeManager, SpecializationUtil
gui/	GUI elements, dialogs, screens
events/	Network event classes
economy/	Economy, prices, selling points
environment/	Weather, seasons, time
animals/	Animal husbandry system
field/	Field state, ownership, missions
fruits/	Fruit types and growth
internalMods/	DLC/internal mods (e.g. FS25_precisionFarming)

Important Base Files

File	Purpose
main.lua	Entry point, class system (source(), Class())
BaseMission.lua	Mission lifecycle, loading
FSBaseMission.lua	FS-specific mission logic
events.lua	Event registration
MessageCenter.lua	Pub/sub messaging system
MessageType.lua	Message type constants

Mod Structure

FS25_ModName/
  modDesc.xml          # Mod metadata, script entry points
  scripts/
    MySpecialization.lua
    events/
      MyEvent.lua

modDesc.xml Script Registration

<modDesc descVersion="106">
    <!-- Specialization type -->
    <specializations>
        <specialization name="mySpec" className="MySpecialization" filename="scripts/MySpecialization.lua" />
    </specializations>

    <!-- Attach to vehicle types -->
    <vehicleTypes>
        <type name="myVehicle" parent="baseDrivable">
            <specialization name="mySpec" />
        </type>
    </vehicleTypes>

    <!-- Or: standalone script -->
    <extraSourceFiles>
        <sourceFile filename="scripts/main.lua" />
    </extraSourceFiles>
</modDesc>

Specialization Pattern

The standard pattern for vehicle/placeable specializations:

MySpecialization = {}

function MySpecialization.prerequisitesPresent(specializations)
    return SpecializationUtil.hasSpecialization(Drivable, specializations)
end

function MySpecialization.registerEventListeners(vehicleType)
    SpecializationUtil.registerEventListener(vehicleType, "onLoad", MySpecialization)
    SpecializationUtil.registerEventListener(vehicleType, "onUpdate", MySpecialization)
    SpecializationUtil.registerEventListener(vehicleType, "onDelete", MySpecialization)
end

function MySpecialization.registerFunctions(vehicleType)
    SpecializationUtil.registerFunction(vehicleType, "myCustomFunction", MySpecialization.myCustomFunction)
end

function MySpecialization.registerOverwrittenFunctions(vehicleType)
    SpecializationUtil.registerOverwrittenFunction(vehicleType, "someBaseFunction", MySpecialization.someBaseFunction)
end

function MySpecialization:onLoad(savegame)
    self.spec_mySpecialization = {}
    -- init
end

function MySpecialization:onUpdate(dt, isActiveForInput, isActiveForInputIgnoreSelection, isSelected)
    -- per-frame logic
end

Overwriting Base Functions

Use registerOverwrittenFunction — calls your function with the original as first arg:

function MySpecialization.registerOverwrittenFunctions(vehicleType)
    SpecializationUtil.registerOverwrittenFunction(vehicleType, "getCanBeSelected", MySpecialization.getCanBeSelected)
end

function MySpecialization:getCanBeSelected(superFunc)
    if someCondition then
        return false
    end
    return superFunc(self)
end

Common Event Listeners

Event	When
onLoad	Vehicle/placeable loaded from XML
onPostLoad	After all specializations loaded
onDelete	Object being deleted
onUpdate	Every frame (active vehicles)
onUpdateTick	Every network tick
onDraw	Render pass
onReadStream / onWriteStream	Network sync (initial)
onReadUpdateStream / onWriteUpdateStream	Network sync (updates)
saveToXMLFile	Savegame write

Network Events

MyEvent = {}
local MyEvent_mt = Class(MyEvent, Event)

InitEventClass(MyEvent, "MyEvent")

function MyEvent.emptyNew()
    return Event.new(MyEvent_mt)
end

function MyEvent.new(vehicle, value)
    local self = MyEvent.emptyNew()
    self.vehicle = vehicle
    self.value = value
    return self
end

function MyEvent:readStream(streamId, connection)
    self.vehicle = NetworkUtil.readNodeObject(streamId)
    self.value = streamReadFloat32(streamId)
    self:run(connection)
end

function MyEvent:writeStream(streamId, connection)
    NetworkUtil.writeNodeObject(streamId, self.vehicle)
    streamWriteFloat32(streamId, self.value)
end

function MyEvent:run(connection)
    if self.vehicle ~= nil then
        self.vehicle:myFunction(self.value, true)
    end
end

Debugging

• Game log: Check log.txt in the game data folder for errors
• print(): Outputs to log and in-game console
• In-game console: ~ key, use gsToggleStats for performance overlay
• Common error: attempt to index nil usually means a specialization dependency is missing or spec_ table not initialized

Workflow

1. Identify what base game system to extend — search dataS/scripts/ for similar functionality
2. Read the relevant base specialization to understand the API
3. Write your specialization following the same patterns
4. Register in modDesc.xml
5. Test in-game, check log.txt for errors