The landscape of game development is constantly evolving, driven by innovation, community, and accessibility. In a significant move set to reshape how developers interact with a powerful platform, Facepunch Studios, the acclaimed creators of Garry’s Mod and Rust, has announced that their ambitious game engine and platform, S&box, is now open source. This decision, revealed in November 2025, marks a pivotal moment, transforming S&box into a community-driven powerhouse under the permissive MIT License.
This article serves as a comprehensive guide to understanding S&box’s transition to open source, exploring its technical underpinnings, the opportunities it unlocks for developers, and its potential impact on the broader game development ecosystem.
From Garry’s Mod Successor to Open Platform
S&box (pronounced “Sandbox”) was conceived as a spiritual successor to the legendary Garry’s Mod, aiming to transcend its predecessor’s capabilities and offer a modern, intuitive environment for creating and playing games. Developed by Facepunch Studios, S&box has been a long-term project, meticulously crafted to incorporate lessons learned from Source 1, Garry’s Mod, Unity, and Unreal Engine.
Initially, S&box underwent several foundational shifts, including an early prototype built on Unreal Engine 4 before Facepunch ultimately transitioned to Valve’s Source 2 engine in 2021. This move was strategic, allowing them to leverage Source 2’s powerful underlying systems while building a robust, high-level C# layer on top. The journey has been one of continuous evolution, with Facepunch focusing on building a platform they themselves would want to use for decades to come.
The decision to open source S&box’s high-level systems under the MIT License is a bold statement, reflecting Facepunch’s commitment to fostering a vibrant, collaborative development environment. This isn’t merely a code dump; it’s an invitation for the global developer community to actively participate in shaping the future of S&box.
Under the Hood: The S&box Open Source Architecture
At its core, S&box is a powerful hybrid. While it relies on Valve’s proprietary Source 2 engine for lower-level systems like rendering and physics, Facepunch has open-sourced all of its high-level systems. This includes the entire editor, networking stack, scene system, user interface (UI), and the extensive C# API that developers interact with. This distinction is crucial: you’re not getting the Source 2 engine itself, but rather the comprehensive framework and tools built on top of it that make S&box so versatile.
S&box leverages modern C# 14, powered by .NET 10, offering developers a familiar and highly performant programming environment. The engine features a component-based system, a paradigm that will be familiar to developers coming from Unity or Godot, enabling intuitive and flexible game creation.
Key technical features that benefit from this open-source approach include:
- C# API: A first-rate API designed for both beginners and experienced professionals, providing seamless integration with the underlying engine.
- Hotloading: A standout feature that allows developers to see code changes take effect immediately without the need for compilation or restarting the game, drastically accelerating iteration times.
- Box3D Physics: A modified version of Valve’s Rubikon physics engine, optimized and decoupled from Valve-specific code for quicker updates.
- Scene System: Similar to those found in other modern engines, enabling rapid prototyping and visual organization of game elements.
- Built-in Tools: S&box offers a rich suite of tools for content creation, including:
- Hammer: For sophisticated level design.
- Terrain System: For crafting expansive open worlds.
- ActionGraph: A visual scripting system for creating interactive experiences without writing code.
- ShaderGraph: For creating custom shaders and visual effects.
Here’s a simplified example of how a C# component might interact within S&box, showcasing its intuitive structure:
Here’s a simplified example of how a C# component might interact within S&box, showcasing its intuitive structure: