diff --git a/src/utils/exam-canvas/model.test.ts b/src/utils/exam-canvas/model.test.ts
index 52b40b7..4ae7670 100644
--- a/src/utils/exam-canvas/model.test.ts
+++ b/src/utils/exam-canvas/model.test.ts
@@ -62,4 +62,30 @@ describe('exam setup canvas serialization', () => {
     expect(shapes.map((s) => s.kind).sort()).toEqual(['boundary', 'furniture', 'part', 'response'])
     expect(shapes.find((s) => s.kind === 'part')).toMatchObject({ id: 'p1', x: 1, y: 2, w: 3, h: 4 })
   })
+
+  it('round-trips boundary y as page-local with page_index while rendering at full page width', () => {
+    const pages = [
+      { pageNumber: 1, x: 0, y: 0, w: 780, h: 1000 },
+      { pageNumber: 2, x: 0, y: 1000, w: 820, h: 1200 },
+    ]
+    const shapes = shapesFromTemplate({
+      ...template,
+      page_count: 2,
+      boundaries: [
+        { id: 'b1', template_id: 'tpl-1', question_id: 'q1', label: 'Q1 start', page_index: 1, y: 160, bounds: { x: 48, y: 160, w: 700, h: 8 }, source: 'manual', confirmed: true },
+        { id: 'b2', template_id: 'tpl-1', question_id: 'q1', label: 'Q1 end', page_index: 1, y: 720, bounds: null, source: 'manual', confirmed: true },
+      ],
+    }, pages)
+
+    expect(shapes[0]).toMatchObject({ kind: 'boundary', x: 0, y: 1160, w: 820, h: 8 })
+    expect(shapes[1]).toMatchObject({ kind: 'boundary', x: 0, y: 1720, w: 820, h: 8 })
+
+    const payload = serializeCanvasShapes(template, shapes, pages)
+    expect(payload.boundaries.map((b) => ({ page_index: b.page_index, y: b.y }))).toEqual([
+      { page_index: 1, y: 160 },
+      { page_index: 1, y: 720 },
+    ])
+    expect(payload.boundaries.every((b) => b.bounds.w === 820)).toBe(true)
+  })
+
 })