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Josh Creek
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Stage 5 blocked after nine attempts and ~540M steps, every one on productive_air_touch_fraction. Instrumenting the environment rather than retuning the reward again found three separate causes, none of which was the policy's competence. The gate could not register the behaviour. productive_air_touch_fraction divides by TOTAL touches in the episode, so a strong ground game dilutes it for identical aerial play. Stage 4's entire purpose is improving that ground game (it took forward_motion_fraction 0.24 -> 0.48), so Stage 4's success drove Stage 5's gate toward zero and the two stages were working against each other. It also explains why every non-zero reading in the whole lineage came from degenerate episodes whose single touch happened to be aerial: per-episode 1.0, which is exactly 0.0100 once meaned over SB3's 100-episode buffer, and 0.0100 was every run's observed maximum. Replaced with productive_air_touch_episode_fraction, which asks whether the episode contained a productive aerial at all and cannot be diluted by ground play. The bar was never derived from anything. AIR_TOUCH_HEIGHT was 5.0 and four rounds of aerial mechanisms were built on top of it without anyone measuring where the ball goes. New ball-altitude telemetry over normal match play: the ball averages ~1.6m, the average episode's peak is ~2.4m, and it clears 5m for ~5% of ticks. Lowered to 3.0, this project's existing airborne threshold, with _place_air_intercept's band retuned 8-14m -> 6-10m. Simulated against real physics the pair strictly dominates the old one: 67.8% reach (was 53.2%), 57.3% above-bar touches (was 41.2%), 5.2m of climb instead of 8.2m. The band could not be lowered alone -- at a 5m bar, 8-14m was optimal and 5-8m collapses above-bar touches to 4.3%. This reverses Round 9's explicit "AIR_TOUCH_HEIGHT stays 5.0"; that objection was about comparability, and a metric that read 0.0 for nine attempts has no history to protect. Pre-2026-08-24 air-touch figures are not comparable with later ones. Note AIR_TOUCH_HEIGHT also gates air_touch_bonus_weight's payout, so unlike Round 9 this DOES change the reward function and the usual "don't resume a policy shaped by a different reward balance" rule is engaged rather than exempt. Resuming retry2 anyway is justified on narrower grounds: the changed term has never once fired (productive_air_touch_fraction exactly 0.0 across nine attempts, air_touch_fraction at ~0.0003 noise), so no learned value estimate is attached to it, while the ground handling and scoring retry2 does know are untouched. The flip side is that at a 3m bar a fully-aligned aerial touch now pays 0.7 + 0.5 = 1.2 against a ground touch's 0.7, which is the intended incentive but is a live reward change -- if attempts show touch farming near 3m rather than genuine intercepts, cut air_touch_bonus_weight rather than raising the threshold back. The policy could not climb, and the entropy controller could not see it. Its target is a sum over heads, which read 21% of h_max -- on target -- while thrust_y alone sat at 14% of its own ceiling. The measured consequence was a policy commanding ~0.03 mean vertical thrust when hovering needs 0.408 (120/5 = 24 m/s^2 against 9.8 gravity), leaving it in free fall ~84% of every episode. Added --min-head-entropy-frac so one starved head raises ent_coef regardless of the aggregate, and --ent-coef-max because a probe pinned the old 0.05 ceiling for its entire duration with the head still starved. A 200k-step probe from retry2 with all three in place moved air_touch_fraction from 0/74 rollouts non-zero to 5/98, ent_coef 0.0102 -> 0.0416 and vertical_thrust_mean 0.031 -> 0.089, with goal_rate, upright_fraction and forward_motion_fraction all holding. The gate metric was still 0.0 at that scale, so its 0.02 floor is marked provisional in generation5.py and should be re-derived from attempt 1's tail rather than trusted. Stage 5 expands to 90M timesteps and MAX_RETRIES 4, its goal_rate floor drops 0.75 -> 0.72 (every attempt landed 0.7217-0.7369 and was failed by ~2-4% while winning its paired evaluations 54-25, 63-23 and 47-32), and state resumes from 20260823-1734-gen5-s5-intercepts-retry2 via resume_override. Verified: generation5.py --dry-run resolves the resume to retry2 with the new flags, 123 unit tests pass, probe artifacts removed.