flagship demo · designed in polykrom
A small bipedal T-rex robot, designed from an empty file by an AI agent working in Polykrom, then checked standing and walking in Polykrom and in MuJoCo.
- servo joints
- 17
- parts
- 163
- all in
- 1.68 kg
- walked in MuJoCo, no fall
- 78.7 mm
Status. A design polish pass and the rebuild on PolyCore are in progress; the design and the numbers here will change.
the model
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The agent's design, exported from Polykrom, playing its Walk study: 272 steps keyed only on the 17 joints. A motion study can't move a free body, so here the body is held still and the legs step under it; the walk with the body free is MuJoCo's, in the results below.
what it is
| Joints | 17 revolute: six per leg (hip yaw, roll and pitch, knee, ankle pitch and roll), neck pitch, head pitch, head yaw, jaw and tail yaw |
|---|---|
| Actuators | 16 Feetech STS3215 serial servos, and an SG90 micro servo for the jaw |
| Parts | 163, including 84 socket-head screws |
| Mass | 1.68 kg (1.676 kg measured in Polykrom) |
| Compute | Raspberry Pi Zero 2 W |
| IMU | BNO085 |
| Power | 2S 1000 mAh LiPo |
| Shells | printed PLA in lime, with a black armour plate, dorsal spikes and claws, off-white teeth and TPU foot pads |
what was verified
24.7 mm
smallest stability margin through the whole walk, against 5 mm required (Polykrom statics)
78.7 mm
walked forward in MuJoCo with the body free: two cycles at 0.08 s per step, no fall, no drift, at most 0.41° of tilt
60%
peak knee torque of its 1.91 N·m rating in that walk; nothing else in the legs above 36%
48.4 mm
standing stability margin, the hardest-working servo at 2.4% of stall
The MuJoCo walk is being re-run with the corrected mass (Polykrom #238). The model it walked on was about 413 g light, because some printed parts exported as meshes with gaps (#216), so its servo loads read low. The Polykrom statics above use the full 1.68 kg.
how it was built
An AI agent built TinyRex through Polykrom's MCP server, with Polykrom's tools as its only tools: no shell, no file access, no hand edits. The work was split into staged briefs. Each stage is a fresh headless Claude Code session that opens the design the stage before saved, does one job, verifies it and reports back. The briefs, the harness and every stage's report are in the repository.
| Stage | What the agent did | Tool calls | Cost, USD |
|---|---|---|---|
| 1. Torso | lofted chest shell, armour plate and spikes, Pi, IMU and battery on standoffs and a strap, two hip yaw servos | 91 | 5.30 |
| 2. Left leg | six servos on printed brackets, thigh, shin and a three-toed clawed foot; swept every joint for collisions | 68 | 6.75 |
| 3. Right leg | mirrored the leg with real, unmirrored servos turned to match; checked joint isolation and interference | 89 | 5.47 |
| 4. Head, tail, arms | neck and head servos, a skull with teeth and an SG90 jaw, a tapering tail, tiny arms | 219 | ≥ 3.89 |
| 5. Stand | standing pose, statics on two feet and on one, and four faults left by earlier stages fixed | 93 | 4.90 |
| 6. Walk | a 272-step gait worked out from the leg geometry, statics over the gait, interference, MuJoCo | 42 | 1.68 |
| 7. Export | MJCF, the render, the bill of materials and a MuJoCo standing check | 63 | 2.56 |
Stage 4 ran in two sessions (69 and 150 tool calls): the first hit a usage limit, the second the harness's three-hour timeout after saving its work, so only the first session's cost was recorded. Stage 5 checked what it left and repaired it.
get it
- Bill of materialsCSV: 71 rows covering all 163 parts
- github.com/wckdboy/tinyrexthe design, the briefs, the harness and the stage reports
- Polykromthe CAD it was built in, in the browser (source)
- PolyCorethe Rust geometry kernel TinyRex will be rebuilt on
Code is Apache-2.0; the design, renders and animation are CC-BY-4.0. Feetech, Raspberry Pi and MuJoCo are named for what they are; none of them is affiliated with TinyRex.