diff --git a/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs b/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs
index 48cdb2c5..98b66f65 100644
--- a/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs
@@ -232,6 +232,64 @@ public sealed class MeshPipelineDeviceSeamTests
             System.Runtime.InteropServices.MemoryMarshal.Cast<byte, ushort>(indexBytes).ToArray());
     }
 
+    /// <summary>
+    /// Campaign OVERHAUL S1 (G1 FAIL 2026-09-02). Cell-shell batches upload in
+    /// ascending source surface index (retail's built-EnvCell subset order,
+    /// ConstructMesh @0x0059DFA0 / DrawMesh @0x0059D4A0), which differs from
+    /// the (Width,Height,Format) storage order the index segments used to be
+    /// filled in. The first fix ordered only the batch list, so batch i took
+    /// segment i's index range from a different batch: magenta walls,
+    /// stretched textures, missing faces in every dungeon. This pins that each
+    /// uploaded batch's FirstIndex points at ITS OWN indices in the arena, with
+    /// a fixture whose storage order is the reverse of its surface order.
+    /// </summary>
+    [Fact]
+    public void CellShellBatchesKeepTheirOwnIndexRangesWhenReorderedBySurfaceIndex()
+    {
+        using var device = new RecordingGpuDevice();
+        using ObjectMeshManager manager = Build(device, modernPath: true);
+        GlobalMeshBuffer arena = manager.GlobalBuffer!;
+
+        static AcDream.Content.TextureBatchData CellBatch(int slot, uint surfaceId, int size, ushort[] indices) => new()
+        {
+            Key = new AcDream.Content.TextureKey { SurfaceId = surfaceId },
+            TextureData = new byte[size * size * 4],
+            Indices = [.. indices],
+            IsCellShell = true,
+            SourceSurfaceIndex = slot,
+            CullMode = DatReaderWriter.Enums.CullMode.Clockwise,
+        };
+
+        // Two storage groups (two texture sizes) so dictionary insertion order
+        // puts slot 5 first; retail order is slot 2 first.
+        var mesh = new AcDream.Content.ObjectMeshData
+        {
+            ObjectId = 0x1_0000_0000UL | 0xF4180104UL,
+            Vertices = new VertexPositionNormalTexture[6],
+            TextureBatches =
+            {
+                [(8, 8, Chorizite.Core.Render.Enums.TextureFormat.RGBA8)] = [CellBatch(5, 0x08000005u, 8, [0, 1, 2])],
+                [(16, 16, Chorizite.Core.Render.Enums.TextureFormat.RGBA8)] = [CellBatch(2, 0x08000002u, 16, [3, 4, 5])],
+            },
+        };
+
+        ObjectRenderData data = Assert.IsType<ObjectRenderData>(manager.UploadMeshData(mesh));
+
+        Assert.Equal(2, data.Batches.Count);
+        Assert.Equal(0x08000002u, data.Batches[0].Key.SurfaceId); // ascending surface index wins
+        Assert.Equal(0x08000005u, data.Batches[1].Key.SurfaceId);
+
+        foreach (ObjectRenderBatch batch in data.Batches)
+        {
+            ushort[] expected = batch.Key.SurfaceId == 0x08000002u ? [3, 4, 5] : [0, 1, 2];
+            var bytes = new byte[batch.IndexCount * sizeof(ushort)];
+            arena.IndexStore!.Read((long)batch.FirstIndex * sizeof(ushort), bytes);
+            Assert.Equal(
+                expected,
+                System.Runtime.InteropServices.MemoryMarshal.Cast<byte, ushort>(bytes).ToArray());
+        }
+    }
+
     /// <summary>
     /// #429 allocation gate (I1 style). Completing a prepared mesh on the
     /// render thread must allocate near its retained pick-copy size