hopper-forward-v
Hopper (MuJoCo Hopper-v5) · solved 8 of 15 agent attempts pass
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2026-10-23 sweep: 2/12 sessions pass (both Opus 5.5); seven others satisfy both rails and then hop in place on the free plant, which is a different equilibrium from forward locomotion on this plant, so the rails are a misleading decomposition here. The oracle is the passing design (6.9 m) and is the most robust controller of the study across the physics scenarios.
Gate. per-seed success is the `forward` rung of this benchmark's ladder ['alive', 'forward', 'brisk', 'fast'] (`--success-mode forward`), not the evaluator's default flag; the task is solved when the success rate over the grader's held-out seeds is `>= 0.9`; the component contract(s) `rail`, `balance` are evaluated first on the same seeds and reported as `rig:<name>:<rung>`; each contract must also clear `>= 0.9` (`--require-rigs`)
Axes: plant:hopper, scenario:standard_v0, gate:forward_3m, rl-reference:td3-3532, v-model:rails
Each video re-runs the attempt's final controller through the task's own evaluator on a public seed (never the grader's held-out ones). The table under each video is the official grade on the held-out seeds.
standard_v0| criterion | graded | needed |
|---|---|---|
| base:forward | 1.000 | ≥ 0.900 |
The agent's final message
The controller at `/app/work/controller.py` passes the full gate. On the official evaluator it solves 50 of 50 visible seeds (0-29 and 30-49) on the `forward` rung, and both component contracts (rail and balance) also pass 100 %. **What the controller does.** A contact-driven state machine: a short settle phase, then stance and flight. - **Stance:** the hip holds torso pitch with a PD loop. The ankle balances the whole body as an inverted pendulum by regulating the thigh's world angle so the foot sits slightly behind the hip, which keeps the thigh off its 0 rad limit where the hip loses authority. The knee holds stiff while descending, then thrusts at a constant command until it is nearly straight. - **Flight:** the hip runs a torso-pitch PD, not a thigh-angle PD, because with the leg three times heavier than the torso, hip torque in flight rotates the torso. The knee and ankle use soft PDs to the landing posture, with the knee straightened as speed grows so the foot lands further forward. - **Speed limiting:** above the desired speed both pitch set-points move nose-up, which shifts the centre of mass behind the foot. This fixed a speed runaway that made the weak-motor variant collapse. **Results on the nominal plant, 30 local seeds per configuration** | Configuration | Pass | Worst margin | |---|---|---| | Free system | 30/30 | x ≈ 9.6 m, |pitch| ≤ 0.11, height ≥ 1.08 m, 12 flights | | Rail rig | 30/30 | 17-18 flights | | Balance rig | 30/30 | |pitch| ≤ 0.07, 12 flights …
A foot-contact-driven state machine uses PD loops on torso pitch (hip), thigh lean (ankle), and knee landing angle, applies constant knee thrust on the rising stance, and shifts pitch/lean/knee targets with speed error to regulate forward velocity.
Shown: public seed 0 (standard_v0); this episode passes the forward rung. This episode: episode_steps 1000, x_final 9.62, min_height 1.11, max_abs_pitch 0.108, n_flights 12, total_reward 2202.
controller.py · agent transcript · trial hopper-forward-v__KnYNLRg
standard_v0| criterion | graded | needed |
|---|---|---|
| base:forward | 0.875 | ≥ 0.900 |
The agent's final message
Final controller is in place at `/app/work/controller.py`, verified by the official harness after its last edit. **Outcome** | Check | Result | |---|---| | Rig `rail` contract (seeds 0-19, official harness) | 1.00 | | Rig `balance` contract (seeds 0-19, official harness) | 1.00 | | Free system `forward`, seeds 0-19 (official harness) | 0.85 (seeds 1, 5, 18 fall late, after 3.9 to 4.1 m) | | Free system `forward`, seeds 0-399 (scratch harness, same env) | 368/400 = 0.92 | The scratch harness and the official verifier agree seed for seed. The held-out estimate is about 0.92 with noticeable block-to-block variance (0.85 to 0.96 per 20 to 100 seeds), so the 0.9 gate is expected to clear but not with a wide margin. The `reward.txt` in `/app/work/eval` reads 0 only because the CLI default gate is 1.0, not the task's 0.9. **The control law** is a three-phase state machine with pure PD plus feedforward, no learning: - Knee gives a shallow flat-foot hop (crouch to −0.35 rad, thrust toward straight). A deep crouch is impossible inside the pitch band because the hip cannot swing forward of the torso. - Hip runs a torso pitch PD in stance and flight. In flight it only re-pitches the light torso, which is enough to hold the band. - Ankle in thrust uses `push = u_ffa + k_push * (thigh - th_push)`: heel push when the leg is under the body, toe-off when the leg trails. Pivoting on the 0.26 m toe moves the ankle about 8 cm forward per hop and is the only mechanism that keeps the leg from …
A three-phase state machine (flight, compress, thrust) applies PD servos on hip, knee and ankle, with knee thrust feedforward and an ankle push that depends on thigh angle, while speed feedback adjusts landing foot angle and thrust to track a target forward velocity.
Shown: public seed 1 (standard_v0); this episode fails the forward rung. This episode: episode_steps 797, x_final 3.9, min_height 0.699, max_abs_pitch 0.177, n_flights 15, total_reward 1284. It was the first failing one of 2 public seeds tried.
controller.py · agent transcript · trial hopper-forward-v__NHB4LBu
standard_v0| criterion | graded | needed |
|---|---|---|
| base:forward | 0.000 | ≥ 0.900 |
The agent's final message
The controller does not pass the gate. The two component rigs pass and the full system stays alive, but it reaches only about 2.2 m against the 3 m `forward` threshold, so `forward` is 0/10 on seeds 0–9. It was run on the standard scenario only; I did not test the five perturbation scenarios. On seeds 0–9, standard scenario: | Check | Result | |---|---| | `rail` rig, `hops` | 100% (15–16 flights) | | `balance` rig, `hops` | 100% | | Full system, `alive` | 100% | | Full system, `forward` | 0%, final x about 2.1–2.3 m | The 17:48 `date` is about 56 minutes after my first command, so I stopped there. - **Rail rig:** when `rooty` is in `obs["locked_joints"]`, the controller switches to a separate vertical-hop law (crouch, full knee thrust, fold in flight). That is why the rail rig passes, but it keys on a rig flag, so the free-pitch controller never hops on the rail. - **Free-pitch hop controller:** a stance-down / stance-up / flight state machine. The hip runs an attitude loop with per-phase biases that keep the thigh off its stop, and p0 shifts with forward-speed error so the lean sets the speed. Its parameters came from a black-box evolution-strategy search on the simulator, not gradient-based learning. - **Faster alternative rejected:** a later parameter set reached 3 m or more on about half the seeds but fell on the rest, so I kept the one that never falls. Mixing the two sets also failed, either staying near 2.2 m or falling. - **Not finished:** the search was still imp…
A contact/flight state machine drives the knee through crouch then thrust and folds/extends in flight, while a hip PD attitude loop whose pitch target is shifted by forward-velocity error sets the hopping speed, with PD ankle control and clipped outputs.
Shown: public seed 0 (standard_v0); this episode fails the forward rung. This episode: episode_steps 1000, x_final 2.12, min_height 1.18, max_abs_pitch 0.18, n_flights 17, total_reward 1263. It was the first failing one of 1 public seeds tried.
controller.py · agent transcript · trial hopper-forward-v__9TYApXp