diff --git a/src/AcDream.App/Rendering/Wb/GlobalMeshBuffer.cs b/src/AcDream.App/Rendering/Wb/GlobalMeshBuffer.cs
index 9211cb68..34811e7e 100644
--- a/src/AcDream.App/Rendering/Wb/GlobalMeshBuffer.cs
+++ b/src/AcDream.App/Rendering/Wb/GlobalMeshBuffer.cs
@@ -286,9 +286,15 @@ public sealed class GlobalMeshBuffer : IDisposable
         }
     }
 
+    /// <summary>
+    /// #429: index batches arrive as (offset, count) segments of one shared
+    /// index array — the caller's retained CPU pick copy — instead of one
+    /// managed array per batch. The bytes staged per batch are unchanged.
+    /// </summary>
     internal GlobalMeshAllocation UploadMesh(
         VertexPositionNormalTexture[] vertices,
-        IReadOnlyList<ushort[]> indexBatches)
+        ushort[] indices,
+        ReadOnlySpan<(int Offset, int Count)> indexBatches)
     {
         ObjectDisposedException.ThrowIf(_disposed, this);
         _retirementLedger.RetryPendingPublications();
@@ -296,16 +302,21 @@ public sealed class GlobalMeshBuffer : IDisposable
         if (_migration is not null)
             throw new InvalidOperationException("A mesh upload cannot mutate the arena while a backing-buffer migration is in progress.");
         ArgumentNullException.ThrowIfNull(vertices);
-        ArgumentNullException.ThrowIfNull(indexBatches);
+        ArgumentNullException.ThrowIfNull(indices);
         if (vertices.Length == 0)
             throw new ArgumentException("A global mesh allocation requires vertices.", nameof(vertices));
 
         int totalIndices = 0;
-        for (int i = 0; i < indexBatches.Count; i++)
+        for (int i = 0; i < indexBatches.Length; i++)
         {
-            ushort[] batch = indexBatches[i]
-                ?? throw new ArgumentException("Index batches cannot contain null.", nameof(indexBatches));
-            totalIndices = checked(totalIndices + batch.Length);
+            (int offset, int count) = indexBatches[i];
+            if (offset < 0 || count <= 0 || (long)offset + count > indices.Length)
+            {
+                throw new ArgumentException(
+                    $"Index batch {i} ({offset}, {count}) is outside the shared index array ({indices.Length}).",
+                    nameof(indexBatches));
+            }
+            totalIndices = checked(totalIndices + count);
         }
         if (totalIndices == 0)
             throw new ArgumentException("A global mesh allocation requires indices.", nameof(indexBatches));
@@ -322,7 +333,7 @@ public sealed class GlobalMeshBuffer : IDisposable
             throw;
         }
 
-        var firstIndices = new int[indexBatches.Count];
+        var firstIndices = new int[indexBatches.Length];
         try
         {
             // IGpuBuffer.Upload stages through a neutral binding point of the
@@ -337,18 +348,15 @@ public sealed class GlobalMeshBuffer : IDisposable
 
             IGpuBuffer indexStore = RequireStore(_indexBuffer);
             int indexOffset = indexRange.Offset;
-            for (int i = 0; i < indexBatches.Count; i++)
+            for (int i = 0; i < indexBatches.Length; i++)
             {
-                ushort[] batch = indexBatches[i];
+                (int offset, int count) = indexBatches[i];
                 firstIndices[i] = indexOffset;
-                if (batch.Length > 0)
-                {
-                    long indexOffsetBytes = checked((long)indexOffset * sizeof(ushort));
-                    indexStore.Upload(
-                        indexOffsetBytes,
-                        MemoryMarshal.AsBytes(new ReadOnlySpan<ushort>(batch)));
-                    indexOffset = checked(indexOffset + batch.Length);
-                }
+                long indexOffsetBytes = checked((long)indexOffset * sizeof(ushort));
+                indexStore.Upload(
+                    indexOffsetBytes,
+                    MemoryMarshal.AsBytes(new ReadOnlySpan<ushort>(indices, offset, count)));
+                indexOffset = checked(indexOffset + count);
             }
         }
         catch
diff --git a/src/AcDream.App/Rendering/Wb/ObjectMeshManager.cs b/src/AcDream.App/Rendering/Wb/ObjectMeshManager.cs
index 25a6bc42..f8030062 100644
--- a/src/AcDream.App/Rendering/Wb/ObjectMeshManager.cs
+++ b/src/AcDream.App/Rendering/Wb/ObjectMeshManager.cs
@@ -2045,13 +2045,47 @@ namespace AcDream.App.Rendering.Wb
         {
             if (meshData.Vertices.Length == 0) return null;
 
-            var modernIndexBatches = meshData.TextureBatches.Values
-                .SelectMany(batches => batches)
-                .Where(batch => batch.Indices.Count != 0)
-                .Select(batch => batch.Indices.ToArray())
-                .ToArray();
+            // #429: allocation-exact conversion. The retained pick copies
+            // (CPUPositions/CPUIndices) are the only geometry-proportional
+            // allocations this method makes; the per-batch segments handed to
+            // the shared arena are (offset, count) views into the same
+            // retained CPUIndices array. The former LINQ chain materialized
+            // every batch twice (a per-batch ToArray plus an unsized
+            // SelectMany growth) — megabytes of transient garbage per mesh on
+            // the render thread.
+            int totalIndexCount = 0;
+            int nonEmptyBatchCount = 0;
+            foreach (List<TextureBatchData> formatBatches in meshData.TextureBatches.Values)
+            {
+                foreach (TextureBatchData formatBatch in formatBatches)
+                {
+                    if (formatBatch.Indices.Count == 0) continue;
+                    totalIndexCount = checked(totalIndexCount + formatBatch.Indices.Count);
+                    nonEmptyBatchCount++;
+                }
+            }
+
+            var cpuIndices = new ushort[totalIndexCount];
+            var indexSegments = new (int Offset, int Count)[nonEmptyBatchCount];
+            {
+                int fillOffset = 0;
+                int segmentIndex = 0;
+                foreach (List<TextureBatchData> formatBatches in meshData.TextureBatches.Values)
+                {
+                    foreach (TextureBatchData formatBatch in formatBatches)
+                    {
+                        int count = formatBatch.Indices.Count;
+                        if (count == 0) continue;
+                        CollectionsMarshal.AsSpan(formatBatch.Indices)
+                            .CopyTo(cpuIndices.AsSpan(fillOffset, count));
+                        indexSegments[segmentIndex++] = (fillOffset, count);
+                        fillOffset = checked(fillOffset + count);
+                    }
+                }
+            }
+
             GlobalMeshAllocation? globalAllocation = null;
-            var renderBatches = new List<ObjectRenderBatch>();
+            var renderBatches = new List<ObjectRenderBatch>(nonEmptyBatchCount);
             var acquiredTextures = new List<(TextureAtlasManager Atlas, TextureKey Key)>();
 
             try
@@ -2068,8 +2102,13 @@ namespace AcDream.App.Rendering.Wb
                 // or vertex/index buffer to build here — Vulkan bakes vertex input
                 // into the pipeline, and the shared arena's stores are bound once
                 // per pass (see WbDrawDispatcher.Rhi.cs's BindPipelineWithMesh).
-                if (GlobalBuffer is not null && modernIndexBatches.Length != 0)
-                    globalAllocation = GlobalBuffer.UploadMesh(meshData.Vertices, modernIndexBatches);
+                if (GlobalBuffer is not null && nonEmptyBatchCount != 0)
+                {
+                    globalAllocation = GlobalBuffer.UploadMesh(
+                        meshData.Vertices,
+                        cpuIndices,
+                        indexSegments);
+                }
 
                 foreach (var (format, batches) in meshData.TextureBatches)
                 {
@@ -2093,8 +2132,25 @@ namespace AcDream.App.Rendering.Wb
                         // family. Choosing an earlier reclaimed slot first
                         // duplicates a layer already resident in a later array
                         // every time portal churn revisits that texture.
-                        atlasManager = atlasList.FirstOrDefault(a => a.HasTexture(batch.Key))
-                            ?? atlasList.FirstOrDefault(a => a.AvailableSlots > 0);
+                        for (int i = 0; i < atlasList.Count; i++)
+                        {
+                            if (atlasList[i].HasTexture(batch.Key))
+                            {
+                                atlasManager = atlasList[i];
+                                break;
+                            }
+                        }
+                        if (atlasManager is null)
+                        {
+                            for (int i = 0; i < atlasList.Count; i++)
+                            {
+                                if (atlasList[i].AvailableSlots > 0)
+                                {
+                                    atlasManager = atlasList[i];
+                                    break;
+                                }
+                            }
+                        }
                         if (atlasManager == null)
                         {
                             atlasManager = new TextureAtlasManager(
@@ -2180,21 +2236,34 @@ namespace AcDream.App.Rendering.Wb
                     }
                 }
 
+                // Every renderBatches entry is one non-empty texture batch, so
+                // their index counts sum to exactly totalIndexCount.
                 long geometryBytes = checked(
                     (long)meshData.Vertices.Length * VertexPositionNormalTexture.Size
-                    + renderBatches.Sum(b => (long)b.IndexCount * sizeof(ushort)));
+                    + (long)totalIndexCount * sizeof(ushort));
+                bool hasCutoutSubset = false;
+                for (int i = 0; i < renderBatches.Count; i++)
+                {
+                    if (renderBatches[i].Translucency
+                        == AcDream.Core.Meshing.TranslucencyKind.ClipMap)
+                    {
+                        hasCutoutSubset = true;
+                        break;
+                    }
+                }
+                var cpuPositions = new Vector3[meshData.Vertices.Length];
+                for (int i = 0; i < cpuPositions.Length; i++)
+                    cpuPositions[i] = meshData.Vertices[i].Position;
                 var renderData = new ObjectRenderData
                 {
                     VertexCount = meshData.Vertices.Length,
                     Batches = renderBatches,
-                    HasCutoutSubset = renderBatches.Any(
-                        static batch => batch.Translucency
-                            == AcDream.Core.Meshing.TranslucencyKind.ClipMap),
+                    HasCutoutSubset = hasCutoutSubset,
                     GlobalAllocation = globalAllocation,
                     ParticleEmitters = meshData.ParticleEmitters,
                     DIDDegrade = meshData.DIDDegrade,
-                    CPUPositions = meshData.Vertices.Select(v => v.Position).ToArray(),
-                    CPUIndices = meshData.TextureBatches.Values.SelectMany(l => l).SelectMany(b => b.Indices).ToArray(),
+                    CPUPositions = cpuPositions,
+                    CPUIndices = cpuIndices,
                     CPUEdgeLines = meshData.EdgeLines,
                     MemorySize = geometryBytes,
                     NonArenaGpuBytes = CalculateNonArenaGeometryBytes(
diff --git a/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs b/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs
index ff9f00a6..48cdb2c5 100644
--- a/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/Wb/MeshPipelineDeviceSeamTests.cs
@@ -207,7 +207,10 @@ public sealed class MeshPipelineDeviceSeamTests
         vertices[2].Position = new System.Numerics.Vector3(7f, 8f, 9f);
         ushort[] indices = [0, 1, 2];
 
-        GlobalMeshAllocation allocation = arena.UploadMesh(vertices, [indices]);
+        GlobalMeshAllocation allocation = arena.UploadMesh(
+            vertices,
+            indices,
+            [(0, indices.Length)]);
 
         Assert.Equal(3, allocation.Vertices.Length);
         Assert.Equal(3, allocation.Indices.Length);
@@ -229,6 +232,75 @@ public sealed class MeshPipelineDeviceSeamTests
             System.Runtime.InteropServices.MemoryMarshal.Cast<byte, ushort>(indexBytes).ToArray());
     }
 
+    /// <summary>
+    /// #429 allocation gate (I1 style). Completing a prepared mesh on the
+    /// render thread must allocate near its retained pick-copy size
+    /// (CPUPositions + CPUIndices), not multiples of it. The regression this
+    /// pins: the upload conversion ran LINQ chains — a per-batch
+    /// <c>Indices.ToArray()</c> plus an unsized <c>SelectMany().ToArray()</c>
+    /// — that materialized every index three-plus times in transient garbage
+    /// per completed mesh, on the render thread, up to the per-frame upload
+    /// budget.
+    /// </summary>
+    [Fact]
+    public void AWarmedMeshCompletionAllocatesNearItsRetainedCopySize()
+    {
+        using var device = new RecordingGpuDevice();
+        using ObjectMeshManager manager = Build(device, modernPath: true);
+
+        // Warm: an identically shaped mesh grows the arena, the atlas family,
+        // and every pool the completion path touches.
+        Assert.NotNull(manager.UploadMeshData(
+            CreateLargeMeshData(0x0100AA01u, surfaceSeed: 0x08000000u)));
+
+        ObjectMeshData meshData =
+            CreateLargeMeshData(0x0100AA02u, surfaceSeed: 0x08001000u);
+        long before = GC.GetAllocatedBytesForCurrentThread();
+        ObjectRenderData? uploaded = manager.UploadMeshData(meshData);
+        long allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+
+        Assert.NotNull(uploaded);
+        long retained =
+            (long)uploaded!.CPUIndices.Length * sizeof(ushort)
+            + (long)uploaded.CPUPositions.Length * 3 * sizeof(float);
+        // Sanity: the fixture is actually index-heavy enough to discriminate.
+        Assert.True(retained >= 480_000, $"fixture retained only {retained} bytes");
+        // The LINQ regression allocates over 3x the index bytes and fails
+        // this bound by more than a megabyte.
+        long bound = retained + retained / 2 + 128 * 1024;
+        Assert.True(
+            allocated < bound,
+            $"A warmed mesh completion allocated {allocated} bytes "
+            + $"(retained copies {retained}, bound {bound}).");
+    }
+
+    private static ObjectMeshData CreateLargeMeshData(ulong id, uint surfaceSeed)
+    {
+        const int vertexCount = 1024;
+        const int batchCount = 4;
+        const int indicesPerBatch = 60_000;
+        var data = new ObjectMeshData
+        {
+            ObjectId = id,
+            Vertices = new VertexPositionNormalTexture[vertexCount],
+        };
+        var batches = new System.Collections.Generic.List<TextureBatchData>(batchCount);
+        for (int b = 0; b < batchCount; b++)
+        {
+            var indices = new System.Collections.Generic.List<ushort>(indicesPerBatch);
+            for (int i = 0; i < indicesPerBatch; i++)
+                indices.Add((ushort)((i + b) % vertexCount));
+            batches.Add(new TextureBatchData
+            {
+                Key = new TextureKey { SurfaceId = surfaceSeed + (uint)b },
+                TextureData = new byte[8 * 8 * 4],
+                Indices = indices,
+            });
+        }
+        data.TextureBatches[(8, 8, TextureFormat.RGBA8)] = batches;
+        return data;
+    }
+
     /// <summary>
     /// The production Vulkan implementation of the seam, checked against the
     /// same surface. Its two capability flags answer true because what they