TinyRex

A small bipedal T-rex robot, designed from an empty file by an AI agent working in Polykrom, then checked for standing and walking in Polykrom and in MuJoCo.

Status. The design here is the last completed stage (MJCF, render and BOM). A design polish pass (stage 8) is still running, and TinyRex will be rebuilt on PolyCore, the new Rust kernel, once it is ready. Expect the files and numbers below to change.

What it is

Joints	17 revolute joints: 6 per leg (hip yaw, hip roll, hip pitch, knee, ankle pitch, ankle 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 (BOM)
Mass	1.68 kg (1.676 kg measured in Polykrom)
Electronics	Raspberry Pi Zero 2 W, BNO085 IMU, 2S 1000 mAh LiPo, as envelopes at their real size and mass
Look	printed PLA shells in lime (#CCFF00); black (#111111) armour plate, dorsal spikes and claws; off-white teeth; a TPU pad under each foot

It is in the class of Pollen Robotics' Microduck and the Open Duck Mini, with a T-rex's big feet, tail and jaw.

How it was made

An AI agent built it through Polykrom's MCP server (polykrom-cli mcp --headless), 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 are in spec/stages.md, and every stage's own report is in results/reports/.

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, skull with teeth and an SG90 jaw, tapering tail, tiny arms	69 + 150	3.89 + not recorded
5. Stand	standing pose, statics on two feet and on one foot, fixed four faults left by earlier stages	93	4.90
6. Walk	a 272-step gait from the leg geometry, statics over the gait, interference, MuJoCo	42	1.68
7. Export	MJCF, the render, the BOM and a MuJoCo standing check	63	2.56

Stage 4 ran in two sessions: the first hit a usage limit, and the continuation hit the harness's three-hour timeout after saving its work, so it left no report. Stage 5 picked it up, checked it and repaired what it found.

Verified results

From the stage reports (stage 5, stage 6, stage 7):

Standing (Polykrom statics, both soles on z = 0). The centre of mass is inside the support polygon, 48.4 mm from its nearest edge; the hardest-working servo is at 2.4% of stall. With the weight shifted over one foot, the centre of mass stays 28.8 mm inside that foot.

Walking (Polykrom statics over the Walk study). Two 40.0 mm steps with 19.7 mm of foot clearance. The smallest stability margin through the gait is 24.7 mm (5 mm required); the highest servo load is 17% (right hip roll). No interference between moving parts, checked every 4 steps, and the stance foot stays within 0.14 mm of the ground.

Walking in MuJoCo (MJCF exported by Polykrom, base free, the Walk study replayed as servo targets, 2 cycles). At 0.08 s per step it walked 78.7 mm forward with no fall, no sideways drift and at most 0.41° of tilt. The knees peaked at 60% and 57% of their 1.91 N·m rating; nothing else in the legs went above 36%. At 0.05 s per step it also held, but the knees reached 97%. Standing in MuJoCo it holds with 0.08° of tilt.

Known issue: Polykrom #216. Nine lofted printed bodies (torso shell, thighs, feet, skull, jaw, claws) export to MJCF as meshes with small gaps, so MuJoCo gives them no mass or collision. The MuJoCo model weighs 1.263 kg against 1.676 kg in CAD: about 413 g is missing, and the real servo loads, the knees especially, will be higher than MuJoCo shows. The jaw servo also saturates briefly in every simulated run.

Repository

Path	What	Licence
designs/tinyrex.polykrom	the design after stage 7, with the Stand and Walk studies	CC-BY-4.0
results/	render, walk GIF, BOM and the stage reports (agent transcripts are not published)	CC-BY-4.0
spec/	the stage briefs: stages.py, which the harness reads, and stages.md to read	Apache-2.0
harness/	the scripts that run the stages	Apache-2.0

Open the design in Polykrom, or hand it to an agent with polykrom-cli mcp --in designs/tinyrex.polykrom.

Running it yourself

You need:

• Claude Code (claude on your PATH, signed in). Each stage runs claude -p with Polykrom's MCP server as its only allowed tool. A full run costs tens of dollars and takes hours.
• polykrom-cli, built from wckdboy/polykrom: cargo build --release -p solidcraft-cli --bin polykrom-cli (the binary lands in target/release/).
• Python 3, and for the MuJoCo checks a Python with mujoco and numpy (python3 -m venv mjenv && mjenv/bin/pip install mujoco numpy). Point Polykrom at it with POLYKROM_MUJOCO_PYTHON; otherwise it uses python3.

Then:

export POLYKROM_CLI=/path/to/polykrom/target/release/polykrom-cli
export POLYKROM_MUJOCO_PYTHON=$PWD/mjenv/bin/python
harness/run_all.sh                 # every stage in order, into ./work
START=5 harness/run_all.sh         # from stage index 5, s6_walk (it opens work/s5_stand.polykrom)
harness/resume.sh 3                # finish an interrupted stage 3 from its own save, then run the rest
python3 harness/run_stage.py "$POLYKROM_CLI" 0   # one stage

Stage indices start at 0 (s1_torso). Each stage writes <stage>.polykrom, a <stage>.json report and a <stage>.transcript.jsonl to the work directory (TINYREX_WORK, default ./work, ignored by git). TINYREX_TIMEOUT (default 10800 s) and TINYREX_MAX_TURNS (default 300) bound each stage. The agent's results will differ from run to run.

Related

• Polykrom: the CAD app TinyRex was built in, open in the browser at cad.polykrom.app. Its robot dog is an earlier showcase built the same way.
• PolyCore: the Rust geometry kernel Polykrom is moving to.

Licence

• Code (harness/, spec/): Apache-2.0.
• Design and media (designs/, results/): CC-BY-4.0. Credit "TinyRex by wckdboy, github.com/wckdboy/tinyrex".