2026-08-16 — Smoother demos v1.1: rate-bounded expert commands, and the tail-budget artifact that was taxing v1

Owner steering 16:53Z: “the action traces are very jumpy — smoother overall?” Answer: yes — landed same-session (dbc0731), and the instrumented read found a v1 yield bug on the way: kept% goes UP (45.8 → 54.2 on the 120-seed bench) while the largest commanded step drops 293° → 10°/tick.

Why the traces were jumpy

The scripted expert’s recorded action is its commanded absolute joint target. The loaded carry phases already slewed at 1.5–2.5°/tick (a full-speed swing slips the pinch — measured back at P4), but every unloaded phase commanded one-shot absolute targets: the lift is a −35° shoulder step in one tick, the retreat home swing is the whole joint-space distance in one tick, and a roll-branch flip can jump wrist_roll by hundreds of degrees. The servos low-pass all of that in the flight path, but the dataset records the commands — so the visualizer shows step functions, and an SFT policy is asked to imitate them.

The fix that worked — and the one that didn’t

An output-stage feedforward slew limiter now rate-bounds every commanded channel (SLEW_ARM_DEG = 10, SLEW_JAW_DEG = 12, None restores the legacy one-shot expert). Feedforward from the last command, never from the measured pose — pose-referenced slewing compounds servo lag into a crawl (measured earlier today: parked-at- home collapsed to 3%).

Rate matters more than expected:

• 6°/tick (first try) collapsed placement 59.2 → 40.0%. The instrument showed every lost seed dying at the 600-tick main clock: recovery moves (jam-flips, pinch-miss retries) get too slow and the episode runs out of budget. Not a physics failure at all.
• 10°/tick recovers placement to 58.3% (baseline 59.2, n=120 — within noise) and still caps every step at 10°.

The surprise: v1 was throwing away ~10% of its yield

The attribution instrument (per-episode: which success() term failed, what was still moving, boat displacement over the tail) showed the baseline losing 13 of 120 episodes to a measurement artifact: success() has no gripper-open term, so it can fire mid-settle while the boat is still pinched. The 150-tick retreat tail then has to cover settle + open + retreat (~135–155 ticks) — 33 of 71 placed episodes exhausted it mid-home-swing, and the post-tail stillness re-verify (max|qvel| < 0.5 includes arm dofs) demoted 13 of them with the boat perfectly placed on the disk.

Doubling the tail budget to 300 fixes it for both experts (artifact 13 → 3; parked-of-placed 53.5 → 94.3%) without touching the success bar — fast episodes still exit the moment the arm is home and quiet, so their recordings are unchanged. This budget was costing shipped v1 real data: same protocol, same physics, kept 45.8 → 55.0 just from letting the tail finish.

v1.1 defaults (landed, oracle-tested)

120 seeds each, spawn v2.1 + mix70, seeds 1000–1119; measurement harness fontaine/scripts/smooth_expert_measure.py, raw rows in fontaine/notes/smooth_*.json. Success protocol untouched; the still-bar’s arm-dof inclusion is a possible protocol change but is not needed — the tail budget alone kills the artifact.

Applies to v1.1 regens; the shipped v1 5k stays as-is (owner: “in-flight 5k fine”). The two 17:07Z bug reports were verdict-posted the same session:

• Top-cam cylinder: it does composite in (segmentation: ~1,350 px) — but the measured real-vs-sim read vindicates the report. The real rig’s wooden disk is 78% brighter than its table (ratio 1.78, visible side wall + drop shadow, ~3,400 px); the sim disk is slightly darker than its surround (ratio 0.95) — isoluminant camouflage. Calibration attempt (same session) found the real constraint: the raw-rendered disk is already near saturation (228/255) — the v3 composite’s per-episode affine (gain ~0.55, fitted on table statistics) compresses any foreground white to ≤~1.1× the plate, so the real 1.78 is unreachable by material alone (measured: v1 0.88 mean, brightened material 0.87). Revised v1.1 proposal: exempt the disk’s segmentation mask from the episode affine (keep the global grade) — predicted ~1.5; a composite- semantics change under the disk_appearance="realcal" flag, owner sign-off pending (fontaine/scripts/disk_contrast_probe.py is the instrument).
• Episode-boundary frame: the merged dataset is exact — data indices contiguous across all 5,000 episodes, per-episode video spans equal length/fps, zero overlaps, ffprobe frame counts match summed lengths (file-000: 13,643 = 13,643). The stray end-frame is the visualizer rendering the exclusive to_timestamp inclusively.