By ByteBot
Erin Catto announced Box3D on June 30, 2026 — an open-source, MIT-licensed 3D physics engine for games written in C17. If the name Box2D means anything to you, this is significant. Box2D is the 2D physics engine Catto released for free in 2006 that quietly ended up inside Angry Birds, IncrediBots, Limbo, and thousands of other games. Rovio built a $1 billion franchise on it. Catto got a hoodie. Box3D carries the same MIT spirit: free to use, no strings attached. The Hacker News post hit 417 points and 92 comments within 48 hours, with Glenn Fiedler of gafferongames.com endorsing the library directly in the thread.
Why Erin Catto Built Box3D: Real Frustration, Not Research
Box3D was not designed in a lab. Catto has been working at Kintsugiyama since 2022, building The Legend of California on Unreal Engine. Unreal’s native Chaos physics system had critical problems for the game — no support for gyroscopic torque simulation, and broken continuous collision detection when falling trees hit terrain meshes. Managing hundreds of thousands of server-side entities required a custom broad-phase solution Chaos couldn’t provide. So Catto built one.
That origin matters. Physics engines built to ship real games tend to solve real problems. Box3D is already proving it: the GitHub repository launched with confirmed adoption by Facepunch Studios’ s&box (the successor to Garry’s Mod), the Esoterica engine, and a 1000-player space multiplayer game. This is not a demo. It is a tool that came out of a studio fighting inadequate infrastructure — exactly how Box2D was born too.
What’s Inside Box3D
The technical feature set covers what game developers actually need. Box3D ships with continuous collision detection for fast-moving objects, collision shapes including convex hulls, capsules, spheres, triangle meshes, and height fields, plus the full set of joint types — revolute, prismatic, distance, wheel, and weld. Large-world support uses double precision for position to handle open-world scales without floating-point drift.
The standout feature, noted repeatedly across the Hacker News discussion, is cross-platform determinism. Box3D is designed to produce identical results across thread counts and platforms. It explicitly rejects -ffast-math compiler optimizations — shortcuts that break deterministic behavior. For networked games and replay systems, this is not a nice-to-have; it is table stakes. Moreover, the contact solver is multithreaded using SIMD and graph coloring to parallelize physics islands safely. The release binary is approximately 916KB on macOS — compact enough for WebAssembly compilation, something the community flagged immediately as significant for browser-based 3D games.
The C17 API mirrors Box2D closely:
b3WorldDef worldDef = b3DefaultWorldDef();
b3WorldId worldId = b3CreateWorld(&worldDef);
b3BodyDef bodyDef = b3DefaultBodyDef();
bodyDef.type = b3_dynamicBody;
bodyDef.position = (b3Vec3){0.0f, 4.0f, 0.0f};
b3BodyId bodyId = b3CreateBody(worldId, &bodyDef);
b3World_Step(worldId, timeStep, subStepCount);Related: Godot Bans AI Code: What Open Source Learned the Hard Way
Box3D vs Jolt vs Rapier: Where It Fits in 2026
The open-source 3D physics field in 2026 is not crowded. Jolt Physics is the current leader — C++, mature, deterministic, used in Horizon Forbidden West, with solid adoption since developers moved away from NVIDIA PhysX. Rapier is excellent if you are working in Rust or need a WASM-first option. Box3D occupies a different slot: C17 (not C++), a small binary footprint, WASM-feasible, and a clean API that avoids C++ template complexity. For C-native projects, for WASM game experiments, or for reinforcement learning environments that need fast 3D deterministic physics, Box3D is a genuine option that did not exist last week.
However, this is not a “switch from Jolt” announcement. If Jolt is already working in your C++ game, there is no reason to migrate. Box3D fills a niche — and it fills it well — but it is alpha software. The API will change. Catto does not accept pull requests; issues and GitHub Discussions only. The correct posture for now: evaluate it, build experiments, lock a commit hash for anything serious, and watch for v0.1.
Key Takeaways
- Box3D is Erin Catto’s MIT-licensed 3D physics engine, announced June 30, 2026 — built out of real game development frustrations with Unreal’s Chaos physics, not a theoretical exercise
- Cross-platform determinism across thread counts and platforms is its most notable differentiator — critical for networked games and replay systems
- At ~916KB release binary and written in C17, Box3D is the most practical option for WebAssembly game targets that need 3D physics — neither Jolt nor Rapier fills that gap as cleanly
- Already adopted by Facepunch Studios (s&box) and multiple other projects before the public announcement
- Alpha status is real: v0.1 is not yet tagged, the API is subject to change, and contributions are issues-only — evaluate it, but don’t couple a production build tightly to the current API without locking a commit
