diff --git a/crates/ml/src/cuda_pipeline/state_layout.cuh b/crates/ml/src/cuda_pipeline/state_layout.cuh
index 8fc5a509c..a7a2ce12b 100644
--- a/crates/ml/src/cuda_pipeline/state_layout.cuh
+++ b/crates/ml/src/cuda_pipeline/state_layout.cuh
@@ -115,6 +115,15 @@
 #define DIR_FLAT                     3   // close all positions to zero
 #define NUM_DIRECTIONS               4
 
+// ────────────────────────────────────────────────────────────────────────────
+// Magnitude action sub-indices (mag_idx in [0..NUM_MAGNITUDES)).
+// Invariant 8: every magnitude value has a named constant.
+// ────────────────────────────────────────────────────────────────────────────
+#define MAG_QUARTER                  0   // 0.25× max_position
+#define MAG_HALF                     1   // 0.50× max_position
+#define MAG_FULL                     2   // 1.00× max_position
+#define NUM_MAGNITUDES               3
+
 // ── Compile-time checks ──
 static_assert(SL_PADDING_START + SL_PADDING_DIM == SL_STATE_DIM,
     "State layout dimensions must sum to SL_STATE_DIM");
diff --git a/crates/ml/src/cuda_pipeline/trade_physics.cuh b/crates/ml/src/cuda_pipeline/trade_physics.cuh
index d7f7f134f..4742ccd20 100644
--- a/crates/ml/src/cuda_pipeline/trade_physics.cuh
+++ b/crates/ml/src/cuda_pipeline/trade_physics.cuh
@@ -70,7 +70,7 @@ __device__ __forceinline__ float compute_target_position_4branch(
      * Returns NaN as safety net — any arithmetic with NaN propagates visibly. */
     if (dir_idx == DIR_HOLD) return __int_as_float(0x7FC00000); /* NaN sentinel */
     float direction = (dir_idx == DIR_SHORT) ? -1.0f : (dir_idx == DIR_LONG) ? 1.0f : 0.0f;
-    float magnitude = (mag_idx == 0) ? 0.25f : (mag_idx == 1) ? 0.5f : 1.0f;
+    float magnitude = (mag_idx == MAG_QUARTER) ? 0.25f : (mag_idx == MAG_HALF) ? 0.5f : 1.0f;
     return direction * magnitude * max_position;
 }
 
diff --git a/docs/dqn-named-dims.md b/docs/dqn-named-dims.md
index 5ec25d2e9..f0514b9e7 100644
--- a/docs/dqn-named-dims.md
+++ b/docs/dqn-named-dims.md
@@ -122,4 +122,5 @@ From the trade_plan MLP output.
 - Task 4B (plan_isv[0..6) constants): commit 741cb48d5
 - Task 4C (plan_params[0..6) constants): commit 0ac83479e
 - Task 4D (BRANCH_* constants): commit 11755632b
-- Task 4E (DIR_* direction sub-indices): commit TBD
+- Task 4E (DIR_* direction sub-indices): commit c64fde0be
+- Task 4F (MAG_* magnitude sub-indices): commit TBD