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Alloy vs Newton: which should I use?

Use the Alloy physics engine when you want Newton's solvers behind one scene API that picks the solver for each object: you name a material, and Alloy routes it, couples it to the rest and checks every world. It adds a particle liquid, tactile images and cutting. Install it with pip install alloy-engine.

What Alloy adds

  • One scene API over the solvers. Cloth and soft bodies step in Newton's VBD, sand and pastes in Newton's implicit MPM, robots in MuJoCo Warp; Alloy chooses from the material and shape, and sim.routes() gives the reasons. You do not wire solvers together yourself.
  • A particle liquid of its own (DFSPH), which pours, fills containers and floats bodies. Newton's liquid is grid-based implicit MPM.
  • Tactile images from gel fingertip sensors in every world (the package's tactile examples), and cutting of soft objects with a knife. Newton's public sensors and solvers list neither.
  • Health in every world. sim.health() grades each world OK, DEGRADED or INVALID and names a fix for every flag: non-finite values, penetration, speed, energy jumps. Newton warns when a contact estimate is large.
  • It is made for coding agents: the docs ship inside the package (python -m alloy.docs), every error carries its fix, and alloy mcp serves the engine as MCP tools (agents). It ships ready to use: nine ready-made environments, nine robots (arms, a hand, a quadruped, two humanoids), 90 materials and a library of 5,423 objects (What ships).

Sand and a rigid block: Alloy picks the solvers

import alloy

scene = alloy.Scene(dt=0.01)
scene.add_ground()
scene.add_open_box("tray", inner=(0.1, 0.1, 0.05), wall=0.01, pos=(0, 0, 0))
scene.add_box("sand", size=(0.06, 0.06, 0.03), pos=(0, 0, 0.03), material="sand_silica_dry")
scene.add_box("block", size=(0.05, 0.05, 0.05), pos=(0.3, 0, 0.025), mass=0.5)
sim = alloy.build(scene, device="cpu")
for name in ("sand", "block"):
    print(name, sim.routes()[name]["route"], sim.routes()[name]["solver"])
sim.run(0.1)
print(sim.check()["ok"])

sand grain_mpm mpm
block rigid mujoco
True

Both objects are add_box calls; only the material differs. Alloy sent the sand to Newton's implicit MPM solver and the block to MuJoCo, and the world passed its health check. It ran on a CPU with no GPU.

Table: Alloy and Newton

One rule for every engine: "yes" means the feature is in the engine's public release and documented; "not documented" means our survey has no public source for it. A note in brackets says where a "yes" holds. The other engines' cells were read from their public code and docs on 2026-11-09, not run by us. The scorecard file, bench/scorecard/row6/capabilities.jsonl in Alloy's source repository, has 26 rows and also grades how complete each feature is; this table shows the 22 rows Alloy's public package documents.

Capability Alloy Newton
Particle fluids yes no
Cutting yes no
Heat yes not documented
Tactile sensing yes no
Liquids yes yes
Sand and grains yes yes
MPM (pastes, snow-like materials) yes yes
Rigid bodies with liquid yes yes
Soft bodies with liquid yes yes
Different step sizes per solver yes yes
Cloth yes yes
Soft bodies (volumetric FEM) yes yes
Rigid bodies with cloth yes yes
Rigid bodies with soft bodies yes yes
Thin shells (paper, film, sheet metal) yes yes
Plasticity (dents, permanent bends) yes yes
Cables and rods yes yes
Per-world health checks yes yes
Many worlds on one GPU yes yes
Batched camera rendering yes yes
Same result on a re-run yes (CPU) yes (opt-in, most solvers)
Double precision yes (CPU) yes (Kamino linear algebra only)

When Newton is the better fit

  • You want the solvers themselves, or want to change their internals: Alloy is built on Newton, so go to it directly.
  • You need differentiable simulation, inverse kinematics, Newton's own sensors and viewers, or its other solvers: they are in Newton's API.

Keep in mind

Alloy's limits are listed on one page, each with its size and a workaround: Keep in mind.

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