diff --git a/src/app/features/scene/README.md b/src/app/features/scene/README.md
index deed806a..d265c332 100644
--- a/src/app/features/scene/README.md
+++ b/src/app/features/scene/README.md
@@ -38,10 +38,9 @@ computed from `SNOISE_LIPSCHITZ` (CultMath's constant for `snoise`) in `world.ts
 envelope (fbm plus the erosion amplitude) the step uses the envelope's bound alone and Phacelle is
 not evaluated.
 
-- The Phacelle gradient bound `phacelleGradBound` (6.3) is **an empirical probe, not a proof**:
-  the maximum `|grad|` of the cos component over 4,194,304 random points (5.04) times 1.25
-  (`harness/scene/drive.cjs phacelle`). Replace it with Libby's bound; until then the march is
-  only as safe as that probe.
+- The Phacelle gradient bound `phacelleGradBound` (7.0) is Libby's measured sup (6.63 at
+  |side| = 1.5, n = 0, CultLib PR #1 head 990521b3) plus margin: a measurement, not a proof.
+  The shader fixes roughness n = 0; a nonzero n needs the bound divided by (1 - n).
 - The bound makes grazing rays crawl. A ray that exhausts its steps while still below the highest
   terrain is ended as a distant surface (fog hides it). That is a visible approximation, not a
   hit proven by the march.
diff --git a/src/app/features/scene/world.ts b/src/app/features/scene/world.ts
index 82afdf94..a1c6cb57 100644
--- a/src/app/features/scene/world.ts
+++ b/src/app/features/scene/world.ts
@@ -23,11 +23,12 @@ export const WORLD = {
   // The stripe wave vector and normalization the terrain feeds cultmath_phacelle.
   erodeSide: [1.7, 0, 1] as const,
   erodeNorm: 0.5,
-  // Lipschitz bound of the phasor's cos component, NOT a proof: max |grad| over 4,194,304 random
-  // points of the z = 0 slice with the side and norm above was 5.04 (`drive.cjs phacelle`,
-  // 2026-10-31), x 1.25. |value| <= 1 holds by construction (the output is a normalized phasor).
-  // Replace with the bound Libby supplies; until then the march is as safe as that probe.
-  phacelleGradBound: 6.3,
+  // Bound on |grad| of the phasor's cos component for the side and norm above. Libby measured a
+  // sup of 6.63 at |side| = 1.5, n = 0 (CultLib PR #1 head 990521b3); 7.0 adds margin. The
+  // shader passes roughness n = 0 as a literal, so the doc envelope max(10, 1.25|side|)/(1-n)
+  // needs no 1/(1-n) factor here; if n ever becomes nonzero, divide this bound by (1 - n).
+  // |value| <= 1 holds by construction (the output is a normalized phasor).
+  phacelleGradBound: 7.0,
   // World tree at the origin.
   tree: {
     trunkHeight: 38,