diff --git a/src/__tests__/lib/syncDiff.test.ts b/src/__tests__/lib/syncDiff.test.ts
new file mode 100644
index 0000000..e26c2fb
--- /dev/null
+++ b/src/__tests__/lib/syncDiff.test.ts
@@ -0,0 +1,97 @@
+import { computeDiff } from '@/lib/syncDiff'
+import type { SyncProduct, ShopifyProductFull } from '@/types/sync'
+
+const sheet = (ref: string, overrides: Partial<SyncProduct> = {}): SyncProduct => ({
+  ref,
+  handle: ref.toLowerCase().replace(/[^a-z0-9]+/g, '-').replace(/^-+|-+$/g, ''),
+  title: `Produit ${ref}`,
+  description: 'Desc',
+  price: '10.00',
+  stock: 5,
+  status: 'active',
+  ...overrides,
+})
+
+const shopify = (ref: string, shopifyId = '111', overrides: Partial<ShopifyProductFull> = {}): ShopifyProductFull => ({
+  shopifyId,
+  ...sheet(ref, overrides),
+  ...overrides,
+})
+
+describe('computeDiff', () => {
+  it('retourne vide si aucun produit des deux côtés', () => {
+    const { changes, summary } = computeDiff([], [])
+    expect(changes).toHaveLength(0)
+    expect(summary).toEqual({ create: 0, update: 0, deactivate: 0, unchanged: 0 })
+  })
+
+  it('marque comme create si produit présent dans Sheets mais absent de Shopify', () => {
+    const { changes, summary } = computeDiff([sheet('REF-1')], [])
+    expect(changes).toHaveLength(1)
+    expect(changes[0].type).toBe('create')
+    expect(changes[0].ref).toBe('REF-1')
+    expect(summary.create).toBe(1)
+  })
+
+  it('marque comme unchanged si produits identiques', () => {
+    const s = sheet('REF-2')
+    const sp = shopify('REF-2')
+    const { changes, summary } = computeDiff([s], [sp])
+    expect(changes[0].type).toBe('unchanged')
+    expect(summary.unchanged).toBe(1)
+  })
+
+  it('marque comme update si le titre diffère', () => {
+    const s = sheet('REF-3', { title: 'Nouveau titre' })
+    const sp = shopify('REF-3', '222', { title: 'Ancien titre' })
+    const { changes } = computeDiff([s], [sp])
+    expect(changes[0].type).toBe('update')
+    expect(changes[0].changedFields).toContainEqual({
+      field: 'title',
+      from: 'Ancien titre',
+      to: 'Nouveau titre',
+    })
+  })
+
+  it('marque comme update si le prix diffère', () => {
+    const s = sheet('REF-4', { price: '25.00' })
+    const sp = shopify('REF-4', '333', { price: '20.00' })
+    const { changes } = computeDiff([s], [sp])
+    expect(changes[0].type).toBe('update')
+    expect(changes[0].changedFields).toContainEqual({ field: 'price', from: '20.00', to: '25.00' })
+  })
+
+  it('marque comme update si le statut diffère', () => {
+    const s = sheet('REF-5', { status: 'draft' })
+    const sp = shopify('REF-5', '444', { status: 'active' })
+    const { changes } = computeDiff([s], [sp])
+    expect(changes[0].type).toBe('update')
+  })
+
+  it('marque comme deactivate si produit ACTIF dans Shopify absent des Sheets', () => {
+    const sp = shopify('REF-6', '555', { status: 'active' })
+    const { changes, summary } = computeDiff([], [sp])
+    expect(changes[0].type).toBe('deactivate')
+    expect(summary.deactivate).toBe(1)
+  })
+
+  it("ne marque PAS comme deactivate si le produit Shopify est déjà draft", () => {
+    const sp = shopify('REF-7', '666', { status: 'draft' })
+    const { changes } = computeDiff([], [sp])
+    expect(changes).toHaveLength(0)
+  })
+
+  it('calcule le résumé correctement sur un mix de changements', () => {
+    const sheetProducts = [sheet('REF-A'), sheet('REF-B', { title: 'Modifié' }), sheet('REF-C')]
+    const shopifyProducts = [
+      shopify('REF-B', '10', { title: 'Original' }),
+      shopify('REF-C', '11'),
+      shopify('REF-D', '12', { status: 'active' }),
+    ]
+    const { summary } = computeDiff(sheetProducts, shopifyProducts)
+    expect(summary.create).toBe(1)    // REF-A
+    expect(summary.update).toBe(1)    // REF-B
+    expect(summary.unchanged).toBe(1) // REF-C
+    expect(summary.deactivate).toBe(1) // REF-D
+  })
+})
diff --git a/src/lib/syncDiff.ts b/src/lib/syncDiff.ts
new file mode 100644
index 0000000..f13f423
--- /dev/null
+++ b/src/lib/syncDiff.ts
@@ -0,0 +1,65 @@
+import type { SyncProduct, ShopifyProductFull, SyncChange, DiffResult } from '@/types/sync'
+
+const COMPARABLE_FIELDS = ['title', 'description', 'price', 'status'] as const
+
+function getChangedFields(
+  sheet: SyncProduct,
+  shopify: ShopifyProductFull,
+): Array<{ field: string; from: string; to: string }> {
+  return COMPARABLE_FIELDS
+    .filter((field) => sheet[field] !== shopify[field])
+    .map((field) => ({
+      field,
+      from: String(shopify[field]),
+      to: String(sheet[field]),
+    }))
+}
+
+/**
+ * Calcule le diff entre les produits Sheets et Shopify.
+ * Matching par handle (ref normalisée).
+ */
+export function computeDiff(
+  sheetProducts: SyncProduct[],
+  shopifyProducts: ShopifyProductFull[],
+): DiffResult {
+  const shopifyMap = new Map(shopifyProducts.map((p) => [p.handle, p]))
+  const sheetHandles = new Set(sheetProducts.map((p) => p.handle))
+
+  const changes: SyncChange[] = []
+
+  for (const sheet of sheetProducts) {
+    const shopify = shopifyMap.get(sheet.handle)
+    if (!shopify) {
+      changes.push({ type: 'create', ref: sheet.ref, sheetProduct: sheet })
+    } else {
+      const changedFields = getChangedFields(sheet, shopify)
+      if (changedFields.length > 0) {
+        changes.push({
+          type: 'update',
+          ref: sheet.ref,
+          sheetProduct: sheet,
+          shopifyProduct: shopify,
+          changedFields,
+        })
+      } else {
+        changes.push({ type: 'unchanged', ref: sheet.ref, sheetProduct: sheet, shopifyProduct: shopify })
+      }
+    }
+  }
+
+  for (const shopify of shopifyProducts) {
+    if (shopify.status === 'active' && !sheetHandles.has(shopify.handle)) {
+      changes.push({ type: 'deactivate', ref: shopify.ref, shopifyProduct: shopify })
+    }
+  }
+
+  const summary = {
+    create: changes.filter((c) => c.type === 'create').length,
+    update: changes.filter((c) => c.type === 'update').length,
+    deactivate: changes.filter((c) => c.type === 'deactivate').length,
+    unchanged: changes.filter((c) => c.type === 'unchanged').length,
+  }
+
+  return { changes, summary }
+}