Box3D: the open source 3D physics engine by Erin Catto

The creator of Box2D just launched Box3D, an open source 3D physics engine for video games. The announcement dropped on June 30, 2026, and the dev community reacted immediately. Here’s what it actually changes.

Box3D, an open source 3D physics engine born from a real need

Erin Catto didn’t build Box3D to pad a portfolio. He needed it for his own game, The Legend of California, an open-world survival title in development since 2022 at Kintsugiyama. Unreal’s native physics engine, Chaos, was causing concrete problems: trees teleporting on screen during collisions, no support for gyroscopic torques, and poor handling of large-scale entities on the server side.

Faced with those limitations, Catto first considered forking Jolt. His friend Dirk Gregorius, who had designed Rubikon (Valve’s internal physics engine for Half-Life: Alyx), suggested another route: forking a slimmed-down version of Rubikon. That fork gradually absorbed the architecture and algorithms of Box2D, and eventually became Box3D.

What Box3D brings technically

Box3D takes the DNA of Box2D and extends it into the third dimension. The added features include:

Triangle mesh collision
Heightfield collision
Baked composites for loading thousands of shapes as a single optimized structure
Sub-stepping solver and continuous collision detection
Wide SIMD support for the contact solver
Multi-threading via configurable hooks
Large world support with doubles for position
Cross-platform determinism and a record/replay system

The API is in C17, which makes integration into existing engines straightforward. The GitHub repo includes CMake presets ready for Visual Studio, VS Code, and CLion. Access is direct, with no unnecessary friction.

Why baked composites deserve attention

A stronghold in The Legend of California can contain around 50,000 separate collision meshes. Loading each one individually is expensive in both memory and time. The baked composites system groups those shapes into a single structure loaded all at once. It’s a concrete answer to a real production problem, not a theoretical feature.

Who is using Box3D today

At the time of the announcement, Box3D is already integrated into several projects:

The Legend of California (Kintsugiyama), Erin Catto’s own game
s&box, the game platform by Facepunch Studios
Esoterica, an open source game engine led by Bobby Anguelov
A 1000-player multiplayer space game by Glenn Fiedler

This isn’t an announcement without real adoption. The engine is running in production on active projects from day one of its public launch.

The Valve and Ragnarok connection

The announcement post mentions that Dirk Gregorius is continuing to evolve Rubikon inside a new engine called Ragnarok, intended for future Valve games. This is secondary information, not directly confirmed by Valve, but it comes from Erin Catto in his official post. Worth keeping an eye on.

Maturity and roadmap

Catto is upfront about it: Box3D is still in alpha. A v0.1 tag is coming soon, with a v1.0 as the long-term target. Planned areas of work include improving character movement, mitigating ghost collisions, and optimizing the joint solver.

The project will be open to pull requests, likely with a CLA. Full documentation is in progress. A written manual and Doxygen comments in the headers are already in place. The recommended starting point is the HelloWorld test in the repo.

Kintsugiyama has agreed to let Erin Catto work on Box3D as part of his employment, which gives the project realistic long-term support. This isn’t a side project going nowhere.

Key takeaways

Box3D is an open source 3D physics engine developed by Erin Catto, creator of Box2D, available on GitHub since June 30, 2026.
It inherits Box2D’s architecture and adds concrete 3D features: triangle mesh, heightfield, baked composites, SIMD, multi-threading.
It’s already used in production on several projects, including s&box (Facepunch Studios).
The engine is in alpha and v1.0 is still ahead, but the codebase is solid and documented.
The Valve/Ragnarok signal is worth watching, even though no official Valve announcement confirms it at this stage.

If you’re working on a game engine or a project that needs reliable 3D physics, Box3D is worth a look right now. Feel free to share your feedback or follow the blog for future coverage.