diff --git a/apps/backend/src/__tests__/cards.test.ts b/apps/backend/src/__tests__/cards.test.ts
index f110c0da..8a295e15 100644
--- a/apps/backend/src/__tests__/cards.test.ts
+++ b/apps/backend/src/__tests__/cards.test.ts
@@ -441,3 +441,120 @@ describe('PUT /api/cards/:id/default', () => {
     expect(mockPrisma.card.update).toHaveBeenCalled();
   });
 });
+
+// ─────────────────────────────────────────────────────────────────────────────
+// PUT /api/cards/:id — atomicity of combined title + linkIds update (#437)
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('PUT /api/cards/:id — atomicity of combined title + linkIds update', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    wireTransaction()
+  })
+
+  it('commits both title and links in a single transaction on success', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard)
+    mockPrisma.platformLink.findMany.mockResolvedValue([{ id: OWNED_LINK_ID }])
+    mockPrisma.card.update.mockResolvedValue({ ...mockCard, title: 'New Title' })
+    mockPrisma.cardLink.deleteMany.mockResolvedValue({ count: 0 })
+    mockPrisma.cardLink.createMany.mockResolvedValue({ count: 1 })
+    mockPrisma.card.findUnique.mockResolvedValue({ ...mockCard, title: 'New Title', cardLinks: [] })
+
+    const app = await buildApp()
+    const res = await app.inject({
+      method: 'PUT',
+      url: `/api/cards/${CARD_ID}`,
+      payload: { title: 'New Title', linkIds: [OWNED_LINK_ID] },
+    })
+
+    expect(res.statusCode).toBe(200)
+    // Both mutations must be inside one transaction, not two separate calls
+    expect(mockPrisma.$transaction).toHaveBeenCalledOnce()
+    expect(mockPrisma.card.update).toHaveBeenCalledWith({ where: { id: CARD_ID }, data: { title: 'New Title' } })
+    expect(mockPrisma.cardLink.deleteMany).toHaveBeenCalledWith({ where: { cardId: CARD_ID } })
+    expect(mockPrisma.cardLink.createMany).toHaveBeenCalled()
+  })
+
+  it('does not commit the title when the linkIds createMany fails (full rollback)', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard)
+    mockPrisma.platformLink.findMany.mockResolvedValue([{ id: OWNED_LINK_ID }])
+    // card.update (title) succeeds inside tx, but createMany blows up
+    mockPrisma.card.update.mockResolvedValue({ ...mockCard, title: 'New Title' })
+    mockPrisma.cardLink.deleteMany.mockResolvedValue({ count: 1 })
+    mockPrisma.cardLink.createMany.mockRejectedValue(new Error('FK constraint violation'))
+
+    const app = await buildApp()
+    const res = await app.inject({
+      method: 'PUT',
+      url: `/api/cards/${CARD_ID}`,
+      payload: { title: 'New Title', linkIds: [OWNED_LINK_ID] },
+    })
+
+    expect(res.statusCode).toBe(500)
+    // The transaction rolled back — the final read must not have been attempted
+    expect(mockPrisma.card.findUnique).not.toHaveBeenCalled()
+    // Confirm both operations ran inside the same tx (the DB undoes them together)
+    expect(mockPrisma.card.update).toHaveBeenCalled()
+    expect(mockPrisma.cardLink.createMany).toHaveBeenCalled()
+  })
+
+  it('returns 403 and opens no transaction when a linkId fails ownership validation', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard)
+    // Ownership check returns empty — foreign linkId
+    mockPrisma.platformLink.findMany.mockResolvedValue([])
+
+    const app = await buildApp()
+    const res = await app.inject({
+      method: 'PUT',
+      url: `/api/cards/${CARD_ID}`,
+      payload: { title: 'New Title', linkIds: [FOREIGN_LINK_ID] },
+    })
+
+    expect(res.statusCode).toBe(403)
+    expect(res.json().error).toBe('One or more links do not belong to your account')
+    // No transaction must have been opened — no writes of any kind
+    expect(mockPrisma.$transaction).not.toHaveBeenCalled()
+    expect(mockPrisma.card.update).not.toHaveBeenCalled()
+    expect(mockPrisma.cardLink.deleteMany).not.toHaveBeenCalled()
+  })
+
+  it('applies only the title update when linkIds is absent', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard)
+    mockPrisma.card.update.mockResolvedValue({ ...mockCard, title: 'Title Only' })
+    mockPrisma.card.findUnique.mockResolvedValue({ ...mockCard, title: 'Title Only', cardLinks: [] })
+
+    const app = await buildApp()
+    const res = await app.inject({
+      method: 'PUT',
+      url: `/api/cards/${CARD_ID}`,
+      payload: { title: 'Title Only' },
+    })
+
+    expect(res.statusCode).toBe(200)
+    expect(mockPrisma.$transaction).toHaveBeenCalledOnce()
+    expect(mockPrisma.card.update).toHaveBeenCalledWith({ where: { id: CARD_ID }, data: { title: 'Title Only' } })
+    expect(mockPrisma.cardLink.deleteMany).not.toHaveBeenCalled()
+    expect(mockPrisma.platformLink.findMany).not.toHaveBeenCalled()
+  })
+
+  it('applies only link replacement when title is absent', async () => {
+    mockPrisma.card.findFirst.mockResolvedValue(mockCard)
+    mockPrisma.platformLink.findMany.mockResolvedValue([{ id: OWNED_LINK_ID }])
+    mockPrisma.cardLink.deleteMany.mockResolvedValue({ count: 1 })
+    mockPrisma.cardLink.createMany.mockResolvedValue({ count: 1 })
+    mockPrisma.card.findUnique.mockResolvedValue({ ...mockCard, cardLinks: [] })
+
+    const app = await buildApp()
+    const res = await app.inject({
+      method: 'PUT',
+      url: `/api/cards/${CARD_ID}`,
+      payload: { linkIds: [OWNED_LINK_ID] },
+    })
+
+    expect(res.statusCode).toBe(200)
+    expect(mockPrisma.$transaction).toHaveBeenCalledOnce()
+    expect(mockPrisma.card.update).not.toHaveBeenCalled()
+    expect(mockPrisma.cardLink.deleteMany).toHaveBeenCalledWith({ where: { cardId: CARD_ID } })
+    expect(mockPrisma.cardLink.createMany).toHaveBeenCalled()
+  })
+})
\ No newline at end of file
diff --git a/apps/backend/src/services/cardService.ts b/apps/backend/src/services/cardService.ts
index a9721783..4447496a 100644
--- a/apps/backend/src/services/cardService.ts
+++ b/apps/backend/src/services/cardService.ts
@@ -1,5 +1,5 @@
-import type { FastifyInstance } from 'fastify'
 import type { Prisma } from '@prisma/client'
+import type { FastifyInstance } from 'fastify'
 
 export async function listCards(app: FastifyInstance, userId: string) {
   const cards = await app.prisma.card.findMany({
@@ -15,7 +15,9 @@ export async function listCards(app: FastifyInstance, userId: string) {
 export async function createCard(app: FastifyInstance, userId: string, body: { title: string; linkIds: string[] }) {
   if (body.linkIds.length > 0) {
     const ownedLinks = await app.prisma.platformLink.findMany({ where: { id: { in: body.linkIds }, userId }, select: { id: true } })
-    if (ownedLinks.length !== body.linkIds.length) throw Object.assign(new Error('Link ownership mismatch'), { code: 'OWNERSHIP' })
+    if (ownedLinks.length !== body.linkIds.length) {
+      throw Object.assign(new Error('Link ownership mismatch'), { code: 'OWNERSHIP' })
+    }
   }
 
   const cardCount = await app.prisma.card.count({ where: { userId } })
@@ -35,38 +37,59 @@ export async function createCard(app: FastifyInstance, userId: string, body: { t
 
 export async function updateCard(app: FastifyInstance, userId: string, id: string, body: { title?: string; linkIds?: string[] }) {
   const existing = await app.prisma.card.findFirst({ where: { id, userId } })
-  if (!existing) return null
+  if (!existing) {
+    return null
+  }
 
-  if (body.title) {
-    await app.prisma.card.update({ where: { id }, data: { title: body.title } })
+  if (body.linkIds && body.linkIds.length > 0) {
+    const ownedLinks = await app.prisma.platformLink.findMany({
+      where: { id: { in: body.linkIds }, userId },
+      select: { id: true },
+    })
+    if (ownedLinks.length !== body.linkIds.length) {
+      throw Object.assign(new Error('Link ownership mismatch'), { code: 'OWNERSHIP' })
+    }
   }
 
-  if (body.linkIds) {
-    if (body.linkIds.length > 0) {
-      const ownedLinks = await app.prisma.platformLink.findMany({ where: { id: { in: body.linkIds }, userId }, select: { id: true } })
-      if (ownedLinks.length !== body.linkIds.length) throw Object.assign(new Error('Link ownership mismatch'), { code: 'OWNERSHIP' })
+  const linkIds = body.linkIds
+
+  await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
+    if (body.title) {
+      await tx.card.update({ where: { id }, data: { title: body.title } })
     }
 
-    const linkIds = body.linkIds
-    await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
+    if (linkIds !== undefined) {
       await tx.cardLink.deleteMany({ where: { cardId: id } })
       if (linkIds.length > 0) {
-        await tx.cardLink.createMany({ data: linkIds.map((linkId, index) => ({ cardId: id, platformLinkId: linkId, displayOrder: index })) })
+        await tx.cardLink.createMany({
+          data: linkIds.map((linkId, index) => ({
+            cardId: id,
+            platformLinkId: linkId,
+            displayOrder: index,
+          })),
+        })
       }
-    })
-  }
+    }
+  })
 
-  const updated = await app.prisma.card.findUnique({ where: { id }, include: { cardLinks: { include: { platformLink: true }, orderBy: { displayOrder: 'asc' } } } })
+  const updated = await app.prisma.card.findUnique({
+    where: { id },
+    include: { cardLinks: { include: { platformLink: true }, orderBy: { displayOrder: 'asc' } } },
+  })
   return { id: updated!.id, title: updated!.title, isDefault: updated!.isDefault, links: updated!.cardLinks.map((cl: any) => cl.platformLink) }
 }
 
 export async function deleteCard(app: FastifyInstance, userId: string, id: string) {
   return await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
     const existing = await tx.card.findFirst({ where: { id, userId } })
-    if (!existing) return Object.assign(new Error('NotFound'), { code: 'NOT_FOUND' })
+    if (!existing) {
+      return Object.assign(new Error('NotFound'), { code: 'NOT_FOUND' })
+    }
 
     const userCardCount = await tx.card.count({ where: { userId } })
-    if (userCardCount <= 1) return Object.assign(new Error('Cannot delete last card'), { code: 'LAST_CARD' })
+    if (userCardCount <= 1) {
+      return Object.assign(new Error('Cannot delete last card'), { code: 'LAST_CARD' })
+    }
 
     if (existing.isDefault) {
       const oldestRemainingCard = await tx.card.findFirst({ where: { userId, id: { not: id } }, orderBy: { createdAt: 'asc' } })
@@ -82,7 +105,9 @@ export async function deleteCard(app: FastifyInstance, userId: string, id: strin
 
 export async function setDefaultCard(app: FastifyInstance, userId: string, id: string) {
   const existing = await app.prisma.card.findFirst({ where: { id, userId } })
-  if (!existing) return null
+  if (!existing) {
+    return null
+  }
 
   await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
     await tx.card.updateMany({ where: { userId }, data: { isDefault: false } })
@@ -90,4 +115,4 @@ export async function setDefaultCard(app: FastifyInstance, userId: string, id: s
   })
 
   return { message: 'Default card updated' }
-}
+}
\ No newline at end of file