diff --git a/docs/research/deepdives/r13-dynamic-lighting.md b/docs/research/deepdives/r13-dynamic-lighting.md
index 91067d1..b1321a5 100644
--- a/docs/research/deepdives/r13-dynamic-lighting.md
+++ b/docs/research/deepdives/r13-dynamic-lighting.md
@@ -46,7 +46,7 @@ public partial class LightInfo : IDatObjType {
 
 - **Practical consequence.** For indoor cells, retail sets directional sun to zero (the cell is windowless) and relies on the baked vertex colours for the ambient "floor". Any `LightInfo` inside the cell is additive.
 - **No cell has a separate ambient RGB field.** The only global ambient is `SkyTimeOfDay.AmbColor` / `AmbBright`, which is only applied outdoors.
-- **acdream action.** We need a `CellAmbientState` that holds the current `AmbColor * AmbBright` (outdoors, driven by sky dat) or a fixed dark RGB like `(0.10, 0.09, 0.08)` (indoors, approximating the dungeon "deep" tone) — then add active lights on top. See §12 for the C# class.
+- **acdream action.** We need a `CellAmbientState` that holds the current `AmbColor * AmbBright` (outdoors, driven by sky dat) or **a flat `(0.20, 0.20, 0.20)` neutral** (indoors) — then add active lights on top. The indoor constant is taken **directly from retail**: `CellManager::ChangePosition` (0x004559B0) calls `SmartBox::SetWorldAmbientLight(0.2f, 0xFFFFFFFF)` whenever `CObjCell::seen_outside == 0`. The early-2026 guess at `(0.10, 0.09, 0.08)` was eyeballed; the retail value is both brighter and neutral. See §12 for the C# class.
 
 ## 4. Torch lights and `WeenieType.LightSource`
 
diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index ff777f8..6f87a61 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -8319,7 +8319,13 @@ public sealed class GameWindow : IDisposable
     /// Derive the current sun (directional light, slot 0 of the UBO)
     /// from the interpolated <see cref="AcDream.Core.World.SkyKeyframe"/>,
     /// plus the cell ambient. Indoor cells force the sun intensity to
-    /// zero (r13 §13.7) and substitute a fixed dungeon-tone ambient.
+    /// zero and substitute a flat 0.2 white ambient — exact retail
+    /// behavior per <c>CellManager::ChangePosition</c> @ 0x004559B0,
+    /// which calls <c>SmartBox::SetWorldAmbientLight(0.2f, 0xFFFFFFFF)</c>
+    /// when the player's <c>CObjCell::seen_outside</c> flag is 0.
+    /// Indoor brightness then comes from per-cell point lights
+    /// (Setup.Lights on the cell's static objects, registered through
+    /// <see cref="AcDream.Core.Lighting.LightingHookSink"/>).
     /// </summary>
     private void UpdateSunFromSky(AcDream.Core.World.SkyKeyframe kf, bool cameraInsideCell)
     {
@@ -8330,7 +8336,8 @@ public sealed class GameWindow : IDisposable
 
         if (cameraInsideCell)
         {
-            // Dungeon default per r13 §3 — warm-dark ambient, no sun.
+            // Indoor default — retail's flat 0.2 neutral ambient, sun
+            // zeroed. See xref to retail decomp in the doc comment above.
             Lighting.Sun = new AcDream.Core.Lighting.LightSource
             {
                 Kind = AcDream.Core.Lighting.LightKind.Directional,
@@ -8340,7 +8347,7 @@ public sealed class GameWindow : IDisposable
                 Range = 1f,
             };
             Lighting.CurrentAmbient = new AcDream.Core.Lighting.CellAmbientState(
-                AmbientColor: new System.Numerics.Vector3(0.10f, 0.09f, 0.08f),
+                AmbientColor: new System.Numerics.Vector3(0.20f, 0.20f, 0.20f),
                 SunColor:     System.Numerics.Vector3.Zero,
                 SunDirection: sunToWorld);
         }