diff --git a/docs/superpowers/specs/2026-05-04-sp11-reward-as-controlled-subsystem.md b/docs/superpowers/specs/2026-05-04-sp11-reward-as-controlled-subsystem.md
index 9b9eb2e6b..5bb99082e 100644
--- a/docs/superpowers/specs/2026-05-04-sp11-reward-as-controlled-subsystem.md
+++ b/docs/superpowers/specs/2026-05-04-sp11-reward-as-controlled-subsystem.md
@@ -562,8 +562,33 @@ After SP10's last slot at 339:
 | `[352..358)` | `REWARD_COMPONENT_GRAD_RATIO_BASE` (6 canaries) | 6 |
 | `358` | `SABOTEUR_ENGAGEMENT_RATE_INDEX` (canary) | 1 |
 | `359` | `PNL_REWARD_MAGNITUDE_EMA_INDEX` (canary) | 1 |
+| `360` | `POPART_COMPONENT_MAG_EMA_INDEX` (canary, NEW B1b fix-up) | 1 |
 
-Total: **20 new slots**. Bump `SP5_SLOT_END = 360`, `ISV_TOTAL_DIM = 360`.
+Total: **21 new slots**. Bump `SP5_SLOT_END = 361`, `ISV_TOTAL_DIM = 361`.
+
+**Why slot 360 (amendment 2026-05-04 post-B1b smoke):** B1b's structural
+refactor revealed that pre-SP11's `reward_components_per_sample[+0]`
+(which feeds slot 63 = `REWARD_POPART_EMA_INDEX`) was being overloaded
+to mean both "PopArt's normalization input" (= total reward magnitude
+EMA) AND "popart-component-specific magnitude". Pre-SP11 these
+collapsed to the same value because reward composition was inline
+accumulation with popart as the dominant component.
+
+Post-B1b with explicit per-component decomposition, the SP11 mag-ratio
+canary reading slot 63 saw the inflated TOTAL-reward magnitude as
+"popart's signal", and the controller redistributed weight toward
+popart (w_pop ≈ 2.0) based on a contaminated ratio. Symptom: B1b smoke
+showed sharpe ≈ 3 vs B1a baseline ≈ 30, traced to the controller
+amplifying a wrongly-attributed signal while attenuating the actually-
+firing components.
+
+Resolution: allocate slot 360 for popart-component-specific magnitude
+EMA. Slot 63 stays as PopArt's input (= total reward magnitude,
+pre-SP11 invariant preserved). The SP11 mag-ratio canary reads slot
+360 (popart-specific) + slots 64..68 (existing cf/trail/micro/opp_cost/
+bonus) to compute the 6-component ratios. No change to PopArt's
+existing readers; one new producer kernel + one mag-ratio canary
+signature change.
 
 ## 5. Producer kernels (4 total)