diff --git a/src/AcDream.App/Rendering/Wb/EnvCellLandblockBuild.cs b/src/AcDream.App/Rendering/Wb/EnvCellLandblockBuild.cs
index 36d3878a..7ba5dde7 100644
--- a/src/AcDream.App/Rendering/Wb/EnvCellLandblockBuild.cs
+++ b/src/AcDream.App/Rendering/Wb/EnvCellLandblockBuild.cs
@@ -1,5 +1,6 @@
 using System.Collections.Immutable;
 using System.Numerics;
+using AcDream.Core.Rendering.Wb;
 using DatReaderWriter.DBObjs;
 using DatReaderWriter.Enums;
 using DatReaderWriter.Types;
@@ -125,14 +126,7 @@ public sealed class EnvCellLandblockBuildBuilder
         uint environmentId,
         ushort cellStructure,
         IReadOnlyList<ushort> surfaces)
-    {
-        var hash = 17L;
-        hash = hash * 31 + (int)environmentId;
-        hash = hash * 31 + cellStructure;
-        foreach (var surface in surfaces)
-            hash = hash * 31 + surface;
-        return (ulong)hash | 0x2_0000_0000UL;
-    }
+        => EnvCellGeometryIdentity.Compute(environmentId, cellStructure, surfaces);
 
     private static WbBoundingBox ComputeLocalBounds(CellStruct cellStruct)
     {
diff --git a/src/AcDream.Core/Rendering/Wb/EnvCellGeometryIdentity.cs b/src/AcDream.Core/Rendering/Wb/EnvCellGeometryIdentity.cs
new file mode 100644
index 00000000..f03ba64d
--- /dev/null
+++ b/src/AcDream.Core/Rendering/Wb/EnvCellGeometryIdentity.cs
@@ -0,0 +1,29 @@
+using System.Collections.Generic;
+
+namespace AcDream.Core.Rendering.Wb;
+
+/// <summary>
+/// Computes WorldBuilder's content identity for an EnvCell shell.
+/// Equal environment, cell-structure, and ordered surface tuples share one
+/// prepared geometry payload even when they occur under different cell IDs.
+/// </summary>
+public static class EnvCellGeometryIdentity
+{
+    /// <summary>
+    /// Source: WorldBuilder <c>EnvCellRenderManager.GetEnvCellGeomId</c>,
+    /// lines 94-103. Bit 33 separates content identities from per-cell IDs,
+    /// whose synthetic geometry requests use bit 32.
+    /// </summary>
+    public static ulong Compute(
+        uint environmentId,
+        ushort cellStructure,
+        IReadOnlyList<ushort> surfaces)
+    {
+        long hash = 17;
+        hash = unchecked(hash * 31 + environmentId);
+        hash = unchecked(hash * 31 + cellStructure);
+        for (int i = 0; i < surfaces.Count; i++)
+            hash = unchecked(hash * 31 + surfaces[i]);
+        return (ulong)hash | 0x2_0000_0000UL;
+    }
+}
diff --git a/tests/AcDream.Core.Tests/Rendering/Wb/EnvCellGeometryIdentityTests.cs b/tests/AcDream.Core.Tests/Rendering/Wb/EnvCellGeometryIdentityTests.cs
new file mode 100644
index 00000000..ebb4c10a
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Rendering/Wb/EnvCellGeometryIdentityTests.cs
@@ -0,0 +1,42 @@
+using AcDream.Core.Rendering.Wb;
+
+namespace AcDream.Core.Tests.Rendering.Wb;
+
+public sealed class EnvCellGeometryIdentityTests
+{
+    [Fact]
+    public void Compute_EmptySurfaces_MatchesExtractedWorldBuilderValue()
+    {
+        Assert.Equal(
+            0x2_0000_47D6UL,
+            EnvCellGeometryIdentity.Compute(0x42, 7, Array.Empty<ushort>()));
+    }
+
+    [Fact]
+    public void Compute_OrderedSurfaces_MatchesExtractedWorldBuilderValue()
+    {
+        Assert.Equal(
+            0x2_20A7_A46CUL,
+            EnvCellGeometryIdentity.Compute(0x42, 7, new ushort[] { 1, 2, 3 }));
+    }
+
+    [Fact]
+    public void Compute_SurfaceOrderIsPartOfIdentity()
+    {
+        var forward = EnvCellGeometryIdentity.Compute(1, 1, new ushort[] { 10, 20 });
+        var reverse = EnvCellGeometryIdentity.Compute(1, 1, new ushort[] { 20, 10 });
+
+        Assert.NotEqual(forward, reverse);
+    }
+
+    [Fact]
+    public void Compute_AlwaysUsesDeduplicatedGeometryNamespace()
+    {
+        var id = EnvCellGeometryIdentity.Compute(
+            uint.MaxValue,
+            ushort.MaxValue,
+            new ushort[] { ushort.MaxValue, ushort.MaxValue });
+
+        Assert.NotEqual(0UL, id & 0x2_0000_0000UL);
+    }
+}