diff --git a/src/__tests__/api/sync/preview.test.ts b/src/__tests__/api/sync/preview.test.ts
new file mode 100644
index 0000000..45e18ef
--- /dev/null
+++ b/src/__tests__/api/sync/preview.test.ts
@@ -0,0 +1,63 @@
+import { GET } from '@/app/api/sync/preview/route'
+import { NextRequest } from 'next/server'
+
+jest.mock('@/lib/googleSheets', () => ({
+  readSheetProducts: jest.fn(),
+}))
+jest.mock('@/lib/shopifySync', () => ({
+  fetchAllShopifyProducts: jest.fn(),
+}))
+jest.mock('@/lib/syncDiff', () => ({
+  computeDiff: jest.fn(),
+}))
+
+import { readSheetProducts } from '@/lib/googleSheets'
+import { fetchAllShopifyProducts } from '@/lib/shopifySync'
+import { computeDiff } from '@/lib/syncDiff'
+
+const mockSheets = readSheetProducts as jest.Mock
+const mockShopify = fetchAllShopifyProducts as jest.Mock
+const mockDiff = computeDiff as jest.Mock
+
+describe('GET /api/sync/preview', () => {
+  beforeEach(() => jest.clearAllMocks())
+
+  it('retourne le DiffResult complet', async () => {
+    const fakeSheetProducts = [{ ref: 'REF-1', handle: 'ref-1', title: 'T', description: '', price: '10.00', stock: 1, status: 'active' }]
+    const fakeShopifyProducts = []
+    const fakeDiff = {
+      changes: [{ type: 'create', ref: 'REF-1', sheetProduct: fakeSheetProducts[0] }],
+      summary: { create: 1, update: 0, deactivate: 0, unchanged: 0 },
+    }
+
+    mockSheets.mockResolvedValue(fakeSheetProducts)
+    mockShopify.mockResolvedValue(fakeShopifyProducts)
+    mockDiff.mockReturnValue(fakeDiff)
+
+    const req = new NextRequest('http://localhost/api/sync/preview')
+    const res = await GET(req)
+
+    expect(res.status).toBe(200)
+    const data = await res.json()
+    expect(data.summary.create).toBe(1)
+    expect(data.changes).toHaveLength(1)
+    expect(mockDiff).toHaveBeenCalledWith(fakeSheetProducts, fakeShopifyProducts)
+  })
+
+  it('retourne 500 si Google Sheets échoue', async () => {
+    mockSheets.mockRejectedValue(new Error('Google Auth error'))
+    const req = new NextRequest('http://localhost/api/sync/preview')
+    const res = await GET(req)
+    expect(res.status).toBe(500)
+    const data = await res.json()
+    expect(data.error).toBeTruthy()
+  })
+
+  it('retourne 500 si Shopify échoue', async () => {
+    mockSheets.mockResolvedValue([])
+    mockShopify.mockRejectedValue(new Error('Shopify error'))
+    const req = new NextRequest('http://localhost/api/sync/preview')
+    const res = await GET(req)
+    expect(res.status).toBe(500)
+  })
+})
diff --git a/src/app/api/sync/preview/route.ts b/src/app/api/sync/preview/route.ts
new file mode 100644
index 0000000..dadf785
--- /dev/null
+++ b/src/app/api/sync/preview/route.ts
@@ -0,0 +1,19 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { readSheetProducts } from '@/lib/googleSheets'
+import { fetchAllShopifyProducts } from '@/lib/shopifySync'
+import { computeDiff } from '@/lib/syncDiff'
+
+export async function GET(_req: NextRequest) {
+  try {
+    const [sheetProducts, shopifyProducts] = await Promise.all([
+      readSheetProducts(),
+      fetchAllShopifyProducts(),
+    ])
+
+    const diff = computeDiff(sheetProducts, shopifyProducts)
+    return NextResponse.json(diff)
+  } catch (err) {
+    const message = err instanceof Error ? err.message : 'Erreur inconnue'
+    return NextResponse.json({ error: message }, { status: 500 })
+  }
+}