Pre-registration (DRAFT) — sim spawn-v2: randomize the disk and the boat

2026-08-16, ~10:3xZ. Owner steering 09:16Z: “Both the disc and boat should be placed randomly.” Status: DRAFT — the CPU slice of queue item sim-spawn-v2-randomization. Finalization (frozen constants from the measured envelope + objection window) comes before any sim change lands or any GPU stage launches; the owner’s priority call (asked in-channel 09:50Z: does this outrank the token-legs report?) sequences the GPU slices.

Plain words. Today’s simulator always puts the wooden disk in the same spot and drops the toy boat in a small patch right in front of it, so every episode looks alike: boat on the same side, ~9–10 cm from the goal. That was a deliberate prototype choice — the patch is where the arm is strong and where the boat can’t land on the parked gripper — but it means a policy could pass our evals while only ever having seen one corner of the problem. Spawn-v2 places both objects randomly: the disk anywhere the arm can comfortably work, the boat anywhere in a ring around the disk. “Comfortably work” is not a hand-drawn box: during the scripted-expert work we measured exactly where the arm’s shoulder servo runs out of static torque and where its inverse kinematics can put the jaws, and those measurements — not guesses — define the allowed region. Everything trained or evaluated so far keeps its numbers under the old protocol (frozen, labeled spawn-v1); new demos and the new headline eval move to spawn-v2.

§1 Spawn-v1, exactly (the thing being replaced)

sim/so101_sim.py: disk fixed at (0.22, 0.11) m, radius 0.04 m (success = boat base inside the disk radius, upright, still, not held). Boat: SPAWN_X (0.195, 0.27) × SPAWN_Y (-0.005, 0.04), uniform yaw — a 7.5 × 4.5 cm band in front of the disk, mean boat→disk distance ~9.5 cm. The band’s documented rationale: (a) the comfortable-reach envelope around the menagerie pickup keyframe (~0.22 m forward); (b) jaw clearance — spawns nearer than x ≈ 0.17 landed the boat ON the parked jaw tips (x ≈ 0.155) for ~4% of seeds; (c) layout match to the original rig recordings. All three carry into v2 as constraints, not as a fixed band.

§2 Spawn-v2 design

Two placements per episode, both from the episode’s spawn RNG stream:

1. Disk: uniform over the measured workspace region W — the set of table positions where the scripted expert’s IK solves a grasp-height jaw-pad pose with residual < 1 mm AND the sysid’d shoulder servo’s static gravity moment at that pose stays under a registered fraction of its force limit (the stage-A torque wall, SERVO_SYSID + the nullspace posture machinery, used here as an instrument). W is precomputed once by the reachability probe (§3) and frozen as an explicit polar-grid mask constant — the sampler draws from the mask, so the envelope is inspectable data, not a runtime solver call.
2. Boat: uniform over the full annulus around the drawn disk center — r ∈ [r_min, r_max], θ ∈ [0, 2π), uniform yaw — rejection sampled against: (a) boat pose itself inside W (the arm must be able to grasp and the disk to receive); (b) min separation: r_min ≥ disk radius 0.04 + hull half-length 0.03 + margin (boat must not start touching the disk); (c) parked-jaw clearance: hull keep-out around the settled home pose’s jaw tips (the measured ~4% failure of v1); (d) table bounds with hull margin. r_max is a drawn-distance cap so episode length stays in the phase-clock budget the scripted expert and evals assume — pinned at finalization from the measured expert traverse envelope.

Determinism + protocol versioning. Disk draws add draws to the spawn stream, so v2 seeds are NOT stream-compatible with v1 — by design, as a registered protocol break: spawn_version becomes an explicit sim parameter; "v1" reproduces today’s draw order bit-identically (oracle-guarded), "v2" is the new protocol. Every banked v1 read stays frozen and labeled; no v1 number is ever compared against a v2 number in a verdict.

Rejection sampling is bounded: the sampler refuses (loud error, not a silent retry-forever) if acceptance over the first N draws falls below a registered floor — a degenerate mask should fail the oracle suite, not stall an eval.

§3 Instrument: the reachability probe (CPU, this queue item’s slice)

sim/spawn_v2.py (sampler + mask constants) + fontaine/scripts/spawn_v2_reachability_probe.py (the instrument): sweep a polar grid over the table’s reachable quadrant, run the stage-A IK (jaw-pad-midpoint space, wrist locked to the P4 pitch, 4 free dofs, nullspace posture pull) at grasp height, record IK residual + static shoulder moment fraction; emit the W mask + a chart (the measured envelope is itself a finding — the same torque wall the learned policies face). Oracles (all CPU): v1 bit-compat under spawn_version="v1"; v2 determinism (seed → identical placements); constraint invariants on 10k draws (separation, keep-out, bounds, in-W); acceptance-floor refusal on a degenerate mask.

§3.1 Instrument v0 — first measured fields (added ~10:3xZ, same session)

The probe ran (CPU, ~2–4 min; reports/analysis__spawn_v2_reachability_v0.json): 1 cm Cartesian grid, 2196 cells, the stage-A grasp solve at GRASP_Z = 0.014 m with the pan pre-swung to each cell’s bearing and a radially-facing hull. The instrument took two iterations to get honest, and the interim readings are kept here because they shaped the design:

• v0 used ExpertPlanner.solve_grasp as-is and read 425 cells under the 1 mm bar — but arranged in ring-bands. On that speckled mask the sampler’s measured tail hit 194 of the 200-draw refusal bar (mean 7.2 attempts over 2000 episodes): edge disks saw a mostly-rejected annulus. A morphological clean then collapsed the bands to 66 cells and the sampler correctly refused outright — the loud-refusal design did its job and flagged the instrument.
• Root cause: solve_ik stops at its 2 mm SITE tolerance, so whether a cell’s pad residual lands under 1 mm was stopping luck, not reachability. The rings were a solver artifact, not arm physics.
• v1 re-solves with an instrument-grade tolerance (0.2 mm, doubled iteration budget, local to the probe — stage-A behavior untouched).

The v1 facts the finalization constants will be cut from:

1. 1105 cells sit inside the 1 mm residual bar, and the one-pass neighbor clean + largest-component step leaves a solid 977-cell region (~977 cm² vs the v1 band’s 34 cm² — ~29×), containing the v1 band and disk comfortably. On the cleaned mask the sampler measures mean 2.4 boat attempts, p99 10, max 35 of the 200-draw refusal bar over 5000 episodes — the tail is gone.
2. Static torque does not bind: over the whole reachable field the shoulder’s static gravity moment peaks at 0.25 of the 3.478 force limit — the nullspace posture pull keeps solves out of the straight-arm poses that saturated the servo in stage A. The torque bound in W is a backstop, not the working constraint; the residual bar does the work.
3. The sampler (sim/spawn_v2.py, 7 CPU oracles) is standalone — SO101Sim.reset is untouched until this pre-reg finalizes.

§4 Registered consequences (the expensive part, priced)

The current joint checkpoint’s 44/100 (and the whole route-C chain) stays the frozen band-protocol read. The scripted expert’s pan-arc traverse was designed around a fixed disk bearing; A′ is a genuine re-validation, not a formality — if it fails its gate, the expert gets ONE registered robustness amendment (the v1 ladder’s precedent) before any F-verdict.

§5 What finalization must pin — with proposed values (measured, not yet frozen)

From the v1 instrument, the freeze candidates now have numbers (sim/spawn_v2.py DRAFT constants, sources commented at each):

Still genuinely open: A′ seed band + gate arithmetic; whether B′–D′ inherit the stage-B/C frozen recipes verbatim or re-open any knob (default: verbatim). Finalization = a registered post freezing this table + the objection window, after the owner’s two calls: priority vs the token-legs report (asked 09:50Z), and the C′ route choice.

§6 FINALIZED (2026-08-16, same day)

The §5 table is frozen at its proposed values, unchanged. Chain of record: owner approved the v1-dataset protocol built on this table 12:21:03Z (“agree with v1 with just the boat upright in the annulus”); the objection window was set in-channel 12:22:14Z as “flag it before the box lands”; the A100 box landed 12:25:56Z with no objection — window closed. What landed with the freeze:

• W is committed data: sim/spawn_v2_mask.json — the 977-cell cleaned mask from the §3.1 v1 instrument read, loaded by WorkspaceMask.frozen() with the cell count pinned (a drifted asset refuses at sim construction).
• Integration: SO101Sim(spawn_version="v2") — disk uniform over W (a static geom moved on the model each reset; success() and the scripted expert read the live disk_center), boat via the annulus sampler, all on the spawn stream in the sampler’s pinned draw order. spawn_version="v1" (the default) is bit-identical to the pre-change code — verified qpos-digest-equal against the pre-change tree at reset, and the existing appearance/spawn-stream oracles all pass unchanged (check.py 950).
• Measured, not assumed: a disk drawn at the worst case — directly on the parked-jaw keep-out center (0.155, 0) — makes zero contacts with the homed arm (the parked gripper sits ~10 cm up; the disk is 12 mm tall), so the boat keep-out needs no disk twin.
• Sequencing note: A′ (expert re-validation under spawn-v2) merges into the v1 dataset generation itself — the sharded collection measures the expert’s spawn-v2 success rate on thousands of seeds as it generates; the first-shards read is the go/no-go telemetry.

§7 A′ FAILED on v2 as frozen → registered amendment v2.1 (same day)

A′ ran immediately (the A100 box landed) and failed hard: 19.8% expert success on 600 unrendered spawn-v2 seeds (first rendered smoke agreed: 3/22). This is the §4 registered risk realized — and the instrument, not the expert, is the culprit:

• Failure geometry: success is a cliff in boat distance from base — 48.3% below r_base 0.26 m (the v1 band’s radius), 6.3% at 0.26–0.34, 0.8% beyond. Bearing doesn’t matter (0–4% across ±70° at far radius). Failed episodes loop jam-flip → recover → approach: the measured pads never come within the 3.5 cm jam threshold of the solve target, which the expert misreads as a mechanical jam. (reports/analysis__spawn_v2_expert_probe.json, phase traces included; droop-clip A/B at ±8 cm: no effect, 19.2%.)
• The instrument was wrong about torque: §3.1’s static-moment field read ≤0.25 of forcerange across W. Direct measurement (spawn_v2_hold_probe.py: solve the expert’s own grasp IK, teleport onto the solution, hold under physics) shows the sysid’d shoulder-lift servo saturated (force fraction 1.00) holding extended poses, with steady-state pad sag growing from ~3 mm at r_base 0.20 to ~20 mm at 0.36. The 1 mm IK-residual bar measured kinematic reachability; the actuator cannot statically serve the outer half of W. This is real arm physics (the same servo the rig runs), not a sim artifact.
• Amendment v2.1 (spawn_version="v2.1", the ONE registered robustness amendment): keep the annulus geometry, full ±180° yaw, uniform-in-W draws — constrain both placements to the measured competence bands: boat r_base ∈ [0.16, 0.27], disk r_base ∈ [0.18, 0.32]. Cut from the 600-seed field (68.2% on the post-hoc joint band, n=110); the v2.1 sampler measures 56.0% end-to-end on 400 fresh seeds (edges of the bands are weaker than the interior — the numbers above are the honest sampler-weighted rate). v2 as frozen stays oracle-pinned and unused; v1 bit-compat untouched.
• Coverage vs v1 remains a step change: disk anywhere in a 56 cm² annular band across all bearings (v1: one fixed point), boat in the full annulus around it at 8–19 cm separation with free yaw (v1: a 34 cm² patch in front of one disk).
• The far-radius region is not lost, just deferred: reaching it needs either a stronger shoulder (hardware) or a non-prehensile/regrasp strategy (the side-spawn righting probe’s territory — same skill family, queued).

§7.1 Disk-collision fix + retreat tail (same day, pre-launch)

Two more changes landed before generation started, both measured:

1. The moved disk was a phantom. The disk is a world-body geom and MuJoCo builds the world’s midphase BVH at compile time — a disk moved via geom_pos keeps colliding at its compiled location while rendering at the new one (measured: boat rest z 0.0002 through the moved disk vs 0.0122 on it). Every v2/v2.1 read above therefore scored success against transient release states, not stable rests; the grasp-side findings (boat-radius cliff, servo saturation) are disk-independent and stand. Fix: the midphase is disabled for the moving-disk protocols only (v1 keeps default physics, bit-compat oracle still green). Re-measured v2.1 field: 53.8% (n=400) with genuine on-disk rests — statistically the same rate, now meaning what it claims.
2. Post-success retreat tail (owner steering 13:46Z): demos now record the expert retreating to the HOME rest pose after success — up-and-back 25 ticks, then a slew to home (the servo parks ~6° shy under gravity; ≤10° = parked). Success is re-verified after the tail, so a retreat that knocks the boat demotes the episode to a miss. Measured on 120 seeds: 48.3% kept (vs 53.8% without the tail requirement), 86% of kept episodes end parked (the rest end quiet mid-return at the 150-tick tail budget, still successes); median kept episode 272 ticks (~9 s).

§8 Side-spawn feasibility probe: measured NO-GO on push-righting (2026-08-16, same day)

The owner’s side-spawn ask (12:18:57Z: “place the boat on the side”) ran its CPU feasibility probe (sim/probe_side_spawn.py, commit a8973dd). Three phases, all n=120 demo-band seeds, all unrendered:

1. Side spawns are mechanically solid. reset(boat_start="side") (roll ±90° about the hull axis, drop from 3 cm, tripled settle; upright-mode spawn stream bit-identical, oracle-pinned) rests the boat on its hull side 120/120 — no self-righting, no capsize, settled |upright| ≈ 0.002, base z 15.6 mm.
2. The stock expert scores 0/120 against the upright > 0.9 success oracle, as predicted. Instructive detail: it pinches and carries the side-lying hull fine — 35% of episodes end within disk radius, boat still on its side (89% end with |upright| < 0.5). Grasping isn’t the missing capability; reorienting is.
3. Righting prototype: 0/120 — a measured NO-GO for quasistatic pushing. Six execution variants of the push-roll (sweep the raised hull edge toward the keel side so the boat tips keel-down): closed-jaw and open-jaw pad-space sweeps, keel-side press at two alignments, tip-space sweeps at z 0.022/0.024/0.029 and 1–1.5 mm/tick. Every variant ends the same way: the boat slides (6–7 cm of plow), it does not roll — peak upright 0.12. The transferable tipping moment (limited by table friction under a rounded hull) never beats the restoring moment.

Tool-geometry facts measured en route, now on record: pad-midpoint space has a physical floor at z ≈ 0.077 (the shoulder saturates with the jaw boxes doing the touching below — true during the stock expert’s own descend, whose GRASP_Z = 0.014 target is kinematic fiction the XY-alignment close rule papers over); the gripperframe site sits at the jaw-tip cluster, so site-space IK is tip-space; with the jaw axis rolled hull-parallel, the open moving jaw hangs below the tip and strikes the table first.

Consequence (the probe’s decision): side spawns stay out of v1.1 — no dataset slice. A viable righting design needs a different mechanism than quasistatic pushing: candidates are a dynamic flick (momentum beats the friction bound; harder to make demo-grade), a wedge-under-and-lift with the tip, or hardware/scene changes (higher friction mat). Any of these is a new probe with its own measured gate, not a tuning pass on this one. The reset extension and the probe harness stay landed for that next attempt.