Pre-reg + results: ftrig MolmoAct2 rerun on flipped-mount physics — 18/20 bit-identical, the bracket is innocent

Registered 2026-08-12 15:3xZ (work session; real date -u at write: 15:35). Owner prio 15:27:11Z: “Prio: can we re-run the 20 episodes on the flipped camera physics? Use many parallel workers so it goes fast.” Design confirmed in-channel 15:28Z. Rough/exploratory pass — this note pins the settings and the asterisk before launch; no registered claim gates on it.

Plain words

This morning’s 20-episode look at the owner’s MolmoAct2 checkpoint ran with the simulator’s wrist-camera bracket mounted upside down — a bug the owner spotted in the videos, now fixed (the bracket points at the ceiling, like the real arm). The owner wants the same 20 episodes re-run on the fixed physics, fast, using parallel simulation workers. Because our parallel mode is known to produce slightly different numbers than the trusted sequential mode (a floating-point batching effect), we run BOTH the old physics and the fixed physics through the same parallel mode and compare within it, episode by episode — that way the one thing that changes between the two runs is the bracket fix, and the comparison is fair even though the absolute numbers carry an asterisk.

Design (frozen before launch)

• Two arms, both sim.rollout_sim_parallel at --workers 8, same checkpoint (~/marius-convert-gate/converted/molmoact2_rig_r1_step2000), same settings as the banked sequential run (euler-10, horizon 30, replans 15, bf16 expert, v3 frames, seeds 0–19, videos on):
  • post-flip: flipped-mount physics (d5cf9fd, the registered geometry) — the number the owner asked for.
  • pre-flip: --no-mount-flip (new flag, this session): the mirrored-Menagerie bracket, physics-verified to reproduce the pre-flip settled bracket height exactly (camera_box2 40.2 mm at home — the probe-measured pre-flip value; flipped: 156.6 mm).
• The sanctioned read is the paired per-seed delta WITHIN the parallel path (pre-reg parallel rollouts frozen rule, applied): the parallel oracle FAILED 14:37Z (batched bf16 decode diverges), so parallel rows are never registered-comparable to the banked sequential rows. Cross-arm comparisons here are parallel-vs-parallel only; both arms inherit the identical scheduler, seed partition, and stable-key noise.
• Reads (exploratory, no gate): paired progress_final_cm delta (flip effect), knock-away count change (pre-flip sequential had 4/20 ≥1 cm; hypothesis: bracket-table collisions contributed), approach count, videos side-by-side for the worst movers. Incidental datum: parallel-vs-sequential drift on the pre-flip arm (same physics as the banked run, different decode path).
• Instrument changes this session (committed before launch): flip_camera_mount constructor toggle on SO101Sim (default True = registered geometry; CPU probe verifies the mirror restores all 3 mount geoms and the 40.2 mm settled height), and the parallel driver gains the sequential driver’s merged-stats fallback (converted checkpoints carry no per-dataset table) + --no-mount-flip + mount_flip recorded in the rows JSON. Harness oracle (tests/test_sim_parallel_rollouts.py) 5/5 green after the change.
• Gate: ≤0.5 GPU-h total (est. ~2× 5–8 min at workers=8). Outputs under outputs/sim/ftrig_eval20_flip_parallel/{postflip,preflip}/.
• What would change our mind about the flip: nothing here — the flip is registered on physics evidence (replay control loss 0.0831→0.0751, below-table sweep 31.9%→1.4%). This rerun asks whether the policy’s observed behavior (knock-aways, grinding) moves with it; a null is a real answer (the bracket wasn’t the binding constraint on THIS policy’s failures).

Results (same session, 15:42–15:59Z) — SUPERSEDED, see the correction below

Kept verbatim for the record: this readout measured only the collision-box half of the flip. The owner spotted (16:07Z) that the videos showed the bracket unmoved — a MuJoCo sameframe compile optimization was making the kinematics ignore the runtime pose edit on the visual mesh. The corrected run is in the next section.

The pre-registered null is the answer, almost exactly: 18/20 seeds are BIT-IDENTICAL across the two physics — this policy’s rollouts almost never touch the bracket. The 2 seeds where the bracket did engage both IMPROVED post-flip. The knock-aways are jaw-side, not bracket-side.

• Paired flip effect (the sanctioned read): mean +0.072 cm, 18/20 exactly tied, 2/20 improved, 0 worsened. The movers: seed 15 +0.29 → +1.10 cm (pre-flip video) and seed 5 −5.51 → −4.90 cm — both low-reaching episodes where the old bracket ground the table.
• Interpretation: the flip matters where the probes said it does — real-pose replay fidelity (control loss −62%, the servo read) — but this policy fails before it reaches the poses the bracket used to block. Its 6 knock-aways persist unchanged under both geometries: the jaws shove the boat, the bracket is innocent. The MolmoAct2 diagnosis from the sequential run stands unchanged on the fixed physics.
• Physics-difference sanity check: bit-identity on 18 seeds is the expected signature — the mount geoms only enter the dynamics through contact (their mass never moved, a compile-time residual noted at the flip), so trajectories that never collide with the bracket are unchanged to the bit.
• Incidental determinism datum: a launcher-flag slip ran the post-flip config twice first — the two runs came out bit-identical on all 20 rows, so lockstep-parallel at workers=8 is exactly reproducible run-to-run (a useful property for the GRPO probe).
• Parallel-vs-sequential drift, quantified at outcome level (both pre-flip physics, same 20 seeds): mean −0.37 cm, 11/20 seeds moved

  0.1 cm, max 6.0 cm (seed 15 flipped sign, seed 17 recovered 4 cm). The 14:37Z oracle FAIL was not cosmetic — cross-path comparisons stay barred, the asterisk on this page’s absolute numbers is real.

• Cost: 3 × 5.4 min arms ≈ 0.27 / 0.5 GPU-h (incl. the accidental replicate). All rows + 40 videos under /ftrig_eval20_flip_parallel/.

Correction (16:07–16:4xZ): the render never flipped — and fixing it revealed the real effect

The owner, comparing the two videos (16:07Z): the bracket still pointed at the table in both. Root cause, probe-confirmed: MuJoCo’s compiler stamps geoms whose frame coincides with an already-computed frame with a sameframe fast path, and mj_kinematics then never reads geom_pos/geom_quat again for them. The bracket’s visual mesh carried flag 2 (frame ≡ the mount body’s inertial frame), so the load-time flip edit was written into the model and silently ignored every render. camera_box1 carried flag 3 (rotation-only skip — harmless: a 180° flip maps a box onto itself); camera_box2 carried flag 0 and moved correctly. Net: physics flipped, appearance didn’t. The one-line fix: clear geom_sameframe on the edited geoms. Verified: the mesh’s settled world position moves from (74, 10, 48) mm — jaw side, table-ward — to (137, −22, 149) mm, wrapped around the camera at (150, 0, 150), ceiling side, matching the hand-computed prediction. Stills: fixed · broken.

The post-flip arm was then re-run (same 20 seeds, same driver). The planned bit-identity oracle failed — correctly: 13/20 seeds changed. The policy is vision-driven and the bracket is visible in the top camera; un-sticking the mesh changed the policy’s input. (The real frames do show the bracket ceiling-side — the fixed render closes an appearance gap, it doesn’t add one.) So the section above was a physics-only read, and the TRUE flip effect is:

• The two catastrophes dissolve: seed 4 −12.3 → −0.05 cm, seed 5 −5.5 → +0.1. Two seeds worsen (s11 −2.4, s14 −2.2); 9 tie exactly. The CI crosses zero — n=20 rough read, as registered.
• Character shift: less shoving, more freezing — several pre-flip movers now end at exactly 0.00. Plausibly the visible bracket makes frames more like the training distribution and the policy defers to its (frozen-ish) prior; the encoder-OOD probe follow-up named in the sequential pre-reg would adjudicate.
• Physics-side claims are untouched by the correction (the mesh never collides): replay control loss −62%, sweep 31.9%→1.4%, and the box-only paired read above stand as what they measured.
• Lesson registered for every future runtime geom edit: clear geom_sameframe after editing geom_pos/geom_quat — the compiler’s fast path silently swallows the edit otherwise. Audited the existing runtime edits: cameras (_repose_wrist_cam) have no such flag; material/light edits are unaffected.
• Corrected totals: 4 arms ≈ 0.36 / 0.5 GPU-h. Corrected rows + videos under /ftrig_eval20_flip_parallel/postflip_v2/.

Owner extension (16:37Z): the step-500 checkpoint, same seeds

The owner asked for the rig fine-tune’s step-500 checkpoint through the same read. Converted fresh (bijou.convert_molmoact2, same recipe/norm tag as step-2000, kept at outputs/converted/molmoact2_rig_r1_step500), then the same 20 seeds on the fixed post-flip sim, same parallel driver:

Paired per-seed (500 − 2000): +0.48 cm, CI95 [−0.06, +1.13] — 9 better / 3 worse / 8 tied. step-500 dissolves step-2000’s two worst episodes (s11 −3.6 → −0.25, s14 −5.0 → +0.03) and posts the day’s best approach (seed 0 +1.59 cm); its one knock-away is seed 1 (−1.8). Not CI-clean at n=20, but directionally: the extra 1500 fine-tune steps are not buying sim-side competence — the earlier checkpoint engages more and shoves less, consistent with the fine-tune narrowing toward rig appearance (sim frames sit further from step-2000’s distribution). Both checkpoints freeze on the same ~8–10 seeds. Day total across all 5 arms: ~0.45 / 0.5 GPU-h.