23. Action-space: chunk-wise delta-joint — queued

Tag: action-space · idea #23 · index

Opened 2026-08-09 from lit 0819 (Action-space design, 2602.23408 — ICML 2026, code+data released and verified). We predict absolute joint positions in chunks because the codebase we started from did; that choice was never measured. This paper measured it in our exact policy class.

Hypothesis. Retraining the action expert to predict chunk-wise delta joint targets (a = q_target − q_chunk_start; never step-to-step increments) improves policy quality at zero architecture/data cost. Their evidence: flow-matching + joint-space + chunk-wise delta is the best cell overall — 88.0 vs 79.6 for our absolute-joint configuration (+8.4pp real-robot, robust across 100–500 demos and 300–1200 epochs, direction confirmed in RoboTwin sim); step-wise delta is dominated ~10pp empirically and O(k) noise-amplification theoretically.

Expected effect. Unknown offline; +5–15pp rollout-flavored if their result transfers. Two caveats carried loudly: (1) nothing in the paper runs on hobby-servo hardware; (2) both headline mechanisms are deployment-time effects a per-frame offline MAE partly cannot see — their chunk-wise-delta and absolute cells are identical in decode error propagation yet differ 8–15pp in rollouts, the cleanest evidence yet that action-space rankings can invert between per-frame error and rollouts (standing caveat now attached to any panel-based action-space claim).

Bonus hypothesis worth logging at the read: chunk-wise delta subtracts per-rig calibration offsets — plausibly worth more on our multi-rig community corpus than on their single lab arm.

Cost. One tiny-config probe run, then one full training run (existing data, existing trunk; normalization recomputed on delta stats).

Cheapest falsification. Pre-registered rules: chunk-wise only (never spend a run on step-wise); decode predictions back to absolute joint space before panel scoring, paired per-frame CI95 vs the absolute-joint baseline; offline win = necessary-not-sufficient (MAE plausibly flatters delta via better-conditioned targets while missing drift), offline loss = strong evidence against switching. The definitive read is rollout-flavored — a Squint-substrate relative screen (#16) once a sim-adaptation arm exists.

Record.

• 2026-08-09 — opened from the 0819 deep read; no arm queued yet (needs its own pre-reg; venue = any post-adamc box window or a local tiny-config probe first).