diff --git a/src/app/App.tsx b/src/app/App.tsx
index e7be2afa..d18ffd3d 100644
--- a/src/app/App.tsx
+++ b/src/app/App.tsx
@@ -942,7 +942,12 @@ export function App({ store, initialStatusMessage }: AppProps) {
           </div>
 
           <div className="toolbar__group">
-            <button className="toolbar__button toolbar__button--accent" type="button" data-testid="create-box-brush" onClick={handleCreateBoxBrush}>
+            <button
+              className="toolbar__button toolbar__button--accent"
+              type="button"
+              data-testid="create-box-brush"
+              onClick={() => handleCreateBoxBrush()}
+            >
               Create Box
             </button>
             <button className="toolbar__button" type="button" data-testid="place-player-start-toolbar" onClick={handleSelectOrPlacePlayerStart}>
@@ -958,21 +963,6 @@ export function App({ store, initialStatusMessage }: AppProps) {
             </button>
           </div>
 
-          <div className="toolbar__group">
-            <button className="toolbar__button" type="button" disabled={!editorState.storageAvailable} onClick={handleSaveDraft}>
-              Save Draft
-            </button>
-            <button className="toolbar__button" type="button" disabled={!editorState.storageAvailable} onClick={handleLoadDraft}>
-              Load Draft
-            </button>
-            <button className="toolbar__button" type="button" onClick={handleExportJson}>
-              Export JSON
-            </button>
-            <button className="toolbar__button" type="button" onClick={handleImportButtonClick}>
-              Import JSON
-            </button>
-          </div>
-
           <div className="toolbar__group">
             <button className="toolbar__button" type="button" disabled={!editorState.canUndo} onClick={() => store.undo()}>
               Undo
diff --git a/tests/domain/build-runtime-scene.test.ts b/tests/domain/build-runtime-scene.test.ts
index fe31ed2a..d514852c 100644
--- a/tests/domain/build-runtime-scene.test.ts
+++ b/tests/domain/build-runtime-scene.test.ts
@@ -139,4 +139,12 @@ describe("buildRuntimeSceneFromDocument", () => {
       yawDegrees: 180
     });
   });
+
+  it("blocks first-person runtime builds when PlayerStart is missing", () => {
+    expect(() =>
+      buildRuntimeSceneFromDocument(createEmptySceneDocument({ name: "Missing Player Start" }), {
+        navigationMode: "firstPerson"
+      })
+    ).toThrow("First-person run requires an authored Player Start");
+  });
 });
diff --git a/tests/domain/scene-document-validation.test.ts b/tests/domain/scene-document-validation.test.ts
new file mode 100644
index 00000000..e2348647
--- /dev/null
+++ b/tests/domain/scene-document-validation.test.ts
@@ -0,0 +1,119 @@
+import { describe, expect, it } from "vitest";
+
+import { createBoxBrush } from "../../src/document/brushes";
+import { createEmptySceneDocument } from "../../src/document/scene-document";
+import { validateSceneDocument } from "../../src/document/scene-document-validation";
+import { createPlayerStartEntity } from "../../src/entities/entity-instances";
+
+describe("validateSceneDocument", () => {
+  it("accepts a valid first-room document", () => {
+    const brush = createBoxBrush({
+      id: "brush-room-shell"
+    });
+    const playerStart = createPlayerStartEntity({
+      id: "entity-player-start-main"
+    });
+    const document = {
+      ...createEmptySceneDocument({ name: "First Room" }),
+      brushes: {
+        [brush.id]: brush
+      },
+      entities: {
+        [playerStart.id]: playerStart
+      }
+    };
+
+    const validation = validateSceneDocument(document);
+
+    expect(validation.errors).toEqual([]);
+    expect(validation.warnings).toEqual([]);
+  });
+
+  it("detects duplicate authored ids across collections", () => {
+    const brush = createBoxBrush({
+      id: "shared-room-id"
+    });
+    const playerStart = createPlayerStartEntity({
+      id: "shared-room-id"
+    });
+    const document = {
+      ...createEmptySceneDocument(),
+      brushes: {
+        [brush.id]: brush
+      },
+      entities: {
+        "entity-player-start-main": playerStart
+      }
+    };
+
+    const validation = validateSceneDocument(document);
+
+    expect(validation.errors).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          code: "entity-id-mismatch"
+        }),
+        expect.objectContaining({
+          code: "duplicate-authored-id"
+        })
+      ])
+    );
+  });
+
+  it("detects invalid box sizes and missing material references", () => {
+    const brush = createBoxBrush({
+      id: "brush-invalid"
+    });
+    brush.size.x = 0;
+    brush.faces.posZ.materialId = "material-that-does-not-exist";
+
+    const validation = validateSceneDocument({
+      ...createEmptySceneDocument(),
+      brushes: {
+        [brush.id]: brush
+      }
+    });
+
+    expect(validation.errors).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          code: "invalid-box-size",
+          path: "brushes.brush-invalid.size"
+        }),
+        expect.objectContaining({
+          code: "missing-material-ref",
+          path: "brushes.brush-invalid.faces.posZ.materialId"
+        })
+      ])
+    );
+  });
+
+  it("detects invalid Player Start values", () => {
+    const validation = validateSceneDocument({
+      ...createEmptySceneDocument(),
+      entities: {
+        "entity-player-start-main": {
+          id: "entity-player-start-main",
+          kind: "playerStart",
+          position: {
+            x: 0,
+            y: Number.NaN,
+            z: 0
+          },
+          yawDegrees: Number.NaN
+        }
+      }
+    });
+
+    expect(validation.errors).toEqual(
+      expect.arrayContaining([
+        expect.objectContaining({
+          code: "invalid-player-start-position"
+        }),
+        expect.objectContaining({
+          code: "invalid-player-start-yaw"
+        })
+      ])
+    );
+  });
+});
diff --git a/tests/serialization/scene-document-json.test.ts b/tests/serialization/scene-document-json.test.ts
index 25bc13d0..8e4359e8 100644
--- a/tests/serialization/scene-document-json.test.ts
+++ b/tests/serialization/scene-document-json.test.ts
@@ -206,4 +206,57 @@ describe("scene document JSON", () => {
       })
     ).toThrow("Unsupported scene document version");
   });
+
+  it("rejects duplicate authored ids after migration and validation", () => {
+    expect(() =>
+      parseSceneDocumentJson(
+        JSON.stringify({
+          version: 4,
+          name: "Duplicate Id Scene",
+          world: createEmptySceneDocument().world,
+          materials: createEmptySceneDocument().materials,
+          textures: {},
+          assets: {},
+          brushes: {
+            "brush-room-shell": {
+              id: "shared-id",
+              kind: "box",
+              center: {
+                x: 0,
+                y: 1,
+                z: 0
+              },
+              size: {
+                x: 2,
+                y: 2,
+                z: 2
+              },
+              faces: {
+                posX: { materialId: null, uv: createBoxBrush().faces.posX.uv },
+                negX: { materialId: null, uv: createBoxBrush().faces.negX.uv },
+                posY: { materialId: null, uv: createBoxBrush().faces.posY.uv },
+                negY: { materialId: null, uv: createBoxBrush().faces.negY.uv },
+                posZ: { materialId: null, uv: createBoxBrush().faces.posZ.uv },
+                negZ: { materialId: null, uv: createBoxBrush().faces.negZ.uv }
+              }
+            }
+          },
+          modelInstances: {},
+          entities: {
+            "entity-player-start-main": {
+              id: "shared-id",
+              kind: "playerStart",
+              position: {
+                x: 0,
+                y: 0,
+                z: 0
+              },
+              yawDegrees: 0
+            }
+          },
+          interactionLinks: {}
+        })
+      )
+    ).toThrow("Duplicate authored id shared-id");
+  });
 });