S&box Gamemode Development Skill

This skill teaches you how to develop gamemodes in S&box (Facepunch's game engine on Source 2).

When to Use This Skill

• Learning S&box game development from scratch
• Building a new gamemode or game feature
• Debugging multiplayer/networking issues
• Understanding component lifecycle and patterns
• Avoiding common pitfalls in S&box development

Core Concepts

Component Architecture

S&box uses a Unity/Godot-like component system. All game behavior lives in Components:

public sealed class MyComponent : Component
{
    [Property] public float Speed { get; set; } = 100f;  // Exposed in editor
    [RequireComponent] Rigidbody Body { get; set; }      // Auto-linked
    
    protected override void OnStart() { }        // First enable
    protected override void OnUpdate() { }       // Every frame
    protected override void OnFixedUpdate() { }  // Physics tick (50/sec)
}

Lifecycle order: OnLoad → OnAwake → OnStart → OnUpdate/OnFixedUpdate → OnDestroy

Input Handling

Define actions in ProjectSettings/Input.config, then query them:

if (Input.Pressed("Jump"))     // Just pressed this frame
if (Input.Down("Attack1"))     // Held down
if (Input.Released("Reload"))  // Just released

Vector3 move = Input.AnalogMove;  // WASD or left stick
Angles look = Input.AnalogLook;   // Mouse or right stick

Networking Fundamentals

Player Spawning (on host):

public sealed class GameManager : Component, Component.INetworkListener
{
    [Property] public GameObject PlayerPrefab { get; set; }
    
    public void OnActive(Connection connection)
    {
        var player = PlayerPrefab.Clone(SpawnPosition);
        player.NetworkSpawn(connection);  // Assign ownership
    }
}

Proxy Check (critical for input):

protected override void OnUpdate()
{
    if (IsProxy) return;  // Don't process input for other players!
    
    // Your input handling here
}

RPC Broadcasting:

[Rpc.Broadcast]
public void OnDamaged(float amount)
{
    // Runs on ALL clients
    PlayHitSound();
}

Physics & Raycasting

var tr = Scene.Trace
    .Ray(startPos, endPos)
    .WithCollisionRules("bullet")
    .UseHitboxes(true)
    .Run();

if (tr.Hit)
{
    Log.Info($"Hit {tr.GameObject} at {tr.HitPosition}");
}

UI with Razor

Create .razor files for UI panels:

@inherits PanelComponent

<root>
    <div class="hud">
        <label>Health: @Health</label>
    </div>
</root>

@code {
    [Property] public int Health { get; set; }
}

Common Pitfalls & Blindspots

1. Forgetting IsProxy Check

Problem: Input code runs for all players, causing chaos. Solution: Always check if (IsProxy) return; at the start of OnUpdate.

2. Not Calling NetworkSpawn()

Problem: Cloned objects don't appear on other clients. Solution: Always call go.NetworkSpawn() after cloning networked prefabs.

3. Using OnUpdate for Physics

Problem: Frame-rate dependent physics behavior. Solution: Use OnFixedUpdate for movement, forces, and physics logic.

4. Missing [Property] Attribute

Problem: Fields don't appear in the S&box editor. Solution: Add [Property] to expose fields in the inspector.

5. Null Reference on Components

Problem: GetComponent<T>() returns null. Solution: Use [RequireComponent] for guaranteed references, or check for null.

6. Scene References Breaking

Problem: References to scene objects become null after scene reload. Solution: Use [Property] references or find objects dynamically.

7. RPC Not Calling

Problem: [Rpc.Broadcast] method doesn't execute on clients. Solution: Ensure the method is on a networked GameObject and ownership is correct.

Debugging Tips

Log.Info("Debug message");           // Console output
Log.Warning("Something weird");      // Yellow warning
Log.Error("Something broke!");       // Red error

// Console commands
[ConCmd("debug_health")]
static void DebugHealth(int amount)
{
    Log.Info($"Setting health to {amount}");
}

Log files: <sbox-install>/logs/Log.log

Project Structure Best Practices

Code/
├── Components/          # Reusable components
│   ├── Player/         # Player-related
│   └── Weapons/        # Weapon systems
├── GameModes/          # Gamemode-specific logic
├── UI/                 # Razor panels
└── Managers/           # Singleton managers

Assets/
├── prefabs/            # Prefab GameObjects
├── scenes/             # Scene files
└── ui/                 # UI assets

Quick Reference

API References

• Docs: https://docs.facepunch.com/s/sbox-dev
• API Reference: https://sbox.game/api
• GitHub Issues: https://github.com/Facepunch/sbox-issues