diff --git a/src/runtime-three/runtime-scene-build.ts b/src/runtime-three/runtime-scene-build.ts
index 068183b9..2d7744d6 100644
--- a/src/runtime-three/runtime-scene-build.ts
+++ b/src/runtime-three/runtime-scene-build.ts
@@ -891,12 +891,12 @@ function buildRuntimeTerrain(
     sampleCountX: terrain.sampleCountX,
     sampleCountZ: terrain.sampleCountZ,
     cellSize: terrain.cellSize,
-    heights: [...terrain.heights],
+    heights: terrain.heights,
     layers: terrain.layers.map((layer) => ({
       materialId: layer.materialId,
       material: resolveRuntimeMaterial(document, layer.materialId)
     })),
-    paintWeights: [...terrain.paintWeights]
+    paintWeights: terrain.paintWeights
   };
 }
 
diff --git a/src/viewport-three/viewport-host.ts b/src/viewport-three/viewport-host.ts
index c72c8258..c9e652e6 100644
--- a/src/viewport-three/viewport-host.ts
+++ b/src/viewport-three/viewport-host.ts
@@ -9701,7 +9701,7 @@ export class ViewportHost {
 
     const committed =
       this.terrainBrushCommitHandler?.({
-        terrain: cloneTerrain(finalPreviewTerrain),
+        terrain: finalPreviewTerrain,
         commandLabel: getTerrainBrushCommandLabel(toolState.tool),
         tool: toolState.tool
       }) === true;
diff --git a/tests/geometry/terrain-brush.test.ts b/tests/geometry/terrain-brush.test.ts
index 08833f48..def4823c 100644
--- a/tests/geometry/terrain-brush.test.ts
+++ b/tests/geometry/terrain-brush.test.ts
@@ -6,6 +6,7 @@ import {
   getTerrainSampleLayerWeights
 } from "../../src/document/terrains";
 import {
+  applyTerrainBrushStampInPlace,
   applyTerrainBrushStamp,
   getTerrainBrushWeight,
   sampleTerrainHeightAtWorldPosition
@@ -112,6 +113,41 @@ describe("terrain brush geometry", () => {
     expect(getTerrainBrushWeight(2, 1, 0.5)).toBe(0);
   });
 
+  it("can stamp terrain in place and report dirty sample bounds", () => {
+    const terrain = createTerrain({
+      id: "terrain-stamp-in-place",
+      position: { x: 0, y: 0, z: 0 },
+      sampleCountX: 5,
+      sampleCountZ: 5,
+      cellSize: 1,
+      heights: new Array(25).fill(0)
+    });
+    const originalHeights = terrain.heights;
+    const originalPaintWeights = terrain.paintWeights;
+
+    const result = applyTerrainBrushStampInPlace({
+      terrain,
+      center: { x: 2, z: 2 },
+      settings: {
+        radius: 0.6,
+        strength: 0.5,
+        falloff: 0
+      },
+      tool: "raise"
+    });
+
+    expect(result.changed).toBe(true);
+    expect(result.dirtyBounds).toEqual({
+      minSampleX: 2,
+      maxSampleX: 2,
+      minSampleZ: 2,
+      maxSampleZ: 2
+    });
+    expect(terrain.heights).toBe(originalHeights);
+    expect(terrain.paintWeights).toBe(originalPaintWeights);
+    expect(terrain.heights[2 + 2 * 5]).toBeCloseTo(0.5);
+  });
+
   it("paints terrain layer weights toward the active layer while preserving a normalized blend", () => {
     const terrain = createTerrain({
       id: "terrain-paint",
diff --git a/tests/geometry/terrain-mesh.test.ts b/tests/geometry/terrain-mesh.test.ts
index 12d86d85..d623e1fb 100644
--- a/tests/geometry/terrain-mesh.test.ts
+++ b/tests/geometry/terrain-mesh.test.ts
@@ -3,6 +3,7 @@ import { describe, expect, it } from "vitest";
 import { createTerrain } from "../../src/document/terrains";
 import {
   buildTerrainDerivedMeshData,
+  buildTerrainLodChunkMeshData,
   buildTerrainLodMeshData,
   resolveTerrainLodLevelIndex,
   resolveTerrainLodLevelIndexWithHysteresis
@@ -131,6 +132,25 @@ describe("terrain mesh generation", () => {
     });
   });
 
+  it("can rebuild one terrain LoD chunk from its starting sample", () => {
+    const terrain = createTerrain({
+      sampleCountX: 130,
+      sampleCountZ: 70
+    });
+
+    const chunk = buildTerrainLodChunkMeshData(terrain, 64, 0);
+
+    expect(chunk).toMatchObject({
+      chunkX: 1,
+      chunkZ: 0,
+      startSampleX: 64,
+      startSampleZ: 0,
+      endSampleX: 128,
+      endSampleZ: 64
+    });
+    expect(chunk?.levels.length).toBeGreaterThan(1);
+  });
+
   it("generates smaller terrain LoD levels as stride increases", () => {
     const terrain = createTerrain({
       sampleCountX: 65,
diff --git a/tests/serialization/local-draft-storage.test.ts b/tests/serialization/local-draft-storage.test.ts
index 4a490278..2f6345fc 100644
--- a/tests/serialization/local-draft-storage.test.ts
+++ b/tests/serialization/local-draft-storage.test.ts
@@ -10,6 +10,7 @@ import {
   createEmptySceneDocument,
   createProjectDocumentFromSceneDocument
 } from "../../src/document/scene-document";
+import { createTerrain } from "../../src/document/terrains";
 import { serializeSceneDocument } from "../../src/serialization/scene-document-json";
 import {
   DEFAULT_SCENE_DRAFT_STORAGE_KEY,
@@ -246,6 +247,38 @@ describe("local draft storage", () => {
     });
   });
 
+  it("skips oversized terrain-heavy autosaves and clears stale drafts", () => {
+    const storage = new MemoryStorage();
+    storage.setItem(DEFAULT_SCENE_DRAFT_STORAGE_KEY, "stale draft");
+    const terrain = createTerrain({
+      id: "terrain-large-draft",
+      sampleCountX: 17,
+      sampleCountZ: 17
+    });
+    const document = {
+      ...createEmptyProjectDocument(),
+      scenes: {
+        "scene-main": {
+          ...createEmptyProjectScene({
+            id: "scene-main",
+            name: "Terrain Draft"
+          }),
+          terrains: {
+            [terrain.id]: terrain
+          }
+        }
+      }
+    };
+
+    const result = saveSceneDocumentDraft(storage, document, null, undefined, {
+      maxSerializedBytes: 512
+    });
+
+    expect(result.status).toBe("skipped");
+    expect(result.message).toContain("Autosave skipped");
+    expect(storage.getItem(DEFAULT_SCENE_DRAFT_STORAGE_KEY)).toBeNull();
+  });
+
   it("loads older raw scene-document drafts without requiring viewport layout state", () => {
     const storage = new MemoryStorage();
     storage.setItem(