diff --git a/apps/backend/src/__tests__/cards.test.ts b/apps/backend/src/__tests__/cards.test.ts
index f110c0da..dd707054 100644
--- a/apps/backend/src/__tests__/cards.test.ts
+++ b/apps/backend/src/__tests__/cards.test.ts
@@ -1,4 +1,4 @@
-import Fastify, { type FastifyInstance } from 'fastify';
+import Fastify, { type FastifyInstance, type FastifyRequest } from 'fastify';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 
 import { cardRoutes } from '../routes/cards.js';
@@ -48,10 +48,14 @@ const mockPrisma = {
 // against the same mock client, preserving existing per-operation mocks.
 function wireTransaction(): void {
   mockPrisma.$transaction.mockImplementation(
-    async (callback: (tx: typeof mockPrisma) => Promise<unknown>) => callback(mockPrisma),
+    async (callback: (tx: typeof mockPrisma) => Promise<unknown>, _options?: unknown) => callback(mockPrisma),
   );
 }
 
+async function buildApp(): Promise<FastifyInstance> {
+  const app = Fastify({ logger: false });
+  app.decorate('prisma', mockPrisma);
+  app.decorate('authenticate', async (request: FastifyRequest & { user?: { id: string } }) => {
 async function buildApp():Promise<FastifyInstance> {
   const app = Fastify({ logger: false });
   app.decorate('prisma', mockPrisma as unknown as PrismaClient);
@@ -182,6 +186,55 @@ describe('POST /api/cards — link ownership validation', () => {
 
     expect(res.statusCode).toBe(500);
   });
+
+  it('wraps creation in a Serializable transaction to prevent race conditions', async () => {
+    mockPrisma.platformLink.findMany.mockResolvedValue([{ id: OWNED_LINK_ID }]);
+    mockPrisma.card.count.mockResolvedValue(0);
+    mockPrisma.card.create.mockResolvedValue({ ...mockCard, cardLinks: [] });
+
+    const app = await buildApp();
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/cards',
+      payload: { title: 'Test Card', linkIds: [OWNED_LINK_ID] },
+    });
+
+    expect(res.statusCode).toBe(201);
+    expect(mockPrisma.$transaction).toHaveBeenCalledWith(
+      expect.any(Function),
+      { isolationLevel: 'Serializable' }
+    );
+  });
+
+  it('retries the transaction on P2034 serialization failure', async () => {
+    mockPrisma.platformLink.findMany.mockResolvedValue([]);
+    
+    // First attempt fails with P2034 (serialization conflict)
+    // Second attempt succeeds
+    const error = new Error('Serialization failure') as Error & { code: string };
+    error.code = 'P2034';
+    
+    // We mock $transaction to fail once, then succeed
+    mockPrisma.$transaction
+      .mockRejectedValueOnce(error)
+      .mockImplementationOnce(
+        async (callback: (tx: typeof mockPrisma) => Promise<unknown>) => callback(mockPrisma)
+      );
+
+    mockPrisma.card.count.mockResolvedValue(1); // second attempt sees count > 0
+    mockPrisma.card.create.mockResolvedValue({ ...mockCard, isDefault: false, cardLinks: [] });
+
+    const app = await buildApp();
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/cards',
+      payload: { title: 'Test Card', linkIds: [] },
+    });
+
+    expect(res.statusCode).toBe(201);
+    expect(res.json().isDefault).toBe(false);
+    expect(mockPrisma.$transaction).toHaveBeenCalledTimes(2);
+  });
 });
 
 // ─────────────────────────────────────────────────────────────────────────────
diff --git a/apps/backend/src/services/cardService.ts b/apps/backend/src/services/cardService.ts
index a9721783..1bc4ce03 100644
--- a/apps/backend/src/services/cardService.ts
+++ b/apps/backend/src/services/cardService.ts
@@ -1,93 +1,174 @@
-import type { FastifyInstance } from 'fastify'
-import type { Prisma } from '@prisma/client'
+import type { Prisma } from '@prisma/client';
+import type { FastifyInstance } from 'fastify';
+
+type CardLinkResponse = { platformLink: unknown };
+type RawCard = { id: string; title: string; isDefault: boolean; cardLinks: CardLinkResponse[] };
+type CardResponse = { id: string; title: string; isDefault: boolean; links: unknown[] };
+
+function mapCard(card: RawCard): CardResponse {
+  return {
+    id: card.id,
+    title: card.title,
+    isDefault: card.isDefault,
+    links: card.cardLinks.map((cardLink) => cardLink.platformLink),
+  };
+}
 
-export async function listCards(app: FastifyInstance, userId: string) {
-  const cards = await app.prisma.card.findMany({
+export async function listCards(app: FastifyInstance, userId: string): Promise<CardResponse[]> {
+  const cards = (await app.prisma.card.findMany({
     where: { userId },
     take: 50,
     include: { cardLinks: { include: { platformLink: true }, orderBy: { displayOrder: 'asc' } } },
     orderBy: { createdAt: 'asc' },
-  })
+  })) as unknown as RawCard[];
 
-  return cards.map((card: any) => ({ id: card.id, title: card.title, isDefault: card.isDefault, links: card.cardLinks.map((cl: any) => cl.platformLink) }))
+  return cards.map(mapCard);
 }
 
-export async function createCard(app: FastifyInstance, userId: string, body: { title: string; linkIds: string[] }) {
+export async function createCard(app: FastifyInstance, userId: string, body: { title: string; linkIds: string[] }): Promise<CardResponse> {
   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 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 cardCount = await app.prisma.card.count({ where: { userId } })
+  const maxRetries = 3;
+  for (let attempt = 1; attempt <= maxRetries; attempt++) {
+    try {
+      const card = (await app.prisma.$transaction(
+        async (tx: Prisma.TransactionClient) => {
+          const cardCount = await tx.card.count({ where: { userId } });
+
+          return tx.card.create({
+            data: {
+              userId,
+              title: body.title,
+              isDefault: cardCount === 0,
+              cardLinks: {
+                create: body.linkIds.map((linkId, index) => ({ platformLinkId: linkId, displayOrder: index })),
+              },
+            },
+            include: { cardLinks: { include: { platformLink: true }, orderBy: { displayOrder: 'asc' } } },
+          });
+        },
+        {
+          isolationLevel: 'Serializable',
+        },
+      )) as unknown as RawCard;
+
+      return mapCard(card);
+    } catch (error: unknown) {
+      if (
+        typeof error === 'object' &&
+        error !== null &&
+        'code' in error &&
+        (error as { code: string }).code === 'P2034' &&
+        attempt < maxRetries
+      ) {
+        continue;
+      }
 
-  const card = await app.prisma.card.create({
-    data: {
-      userId,
-      title: body.title,
-      isDefault: cardCount === 0,
-      cardLinks: { create: body.linkIds.map((linkId, index) => ({ platformLinkId: linkId, displayOrder: index })) },
-    },
-    include: { cardLinks: { include: { platformLink: true }, orderBy: { displayOrder: 'asc' } } },
-  })
+      app.log.error(error);
+      throw error;
+    }
+  }
 
-  return { id: card.id, title: card.title, isDefault: card.isDefault, links: card.cardLinks.map((cl: any) => cl.platformLink) }
+  throw new Error('Failed to create card after retrying serialization conflicts');
 }
 
-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
+export async function updateCard(
+  app: FastifyInstance,
+  userId: string,
+  id: string,
+  body: { title?: string; linkIds?: string[] },
+): Promise<CardResponse | null> {
+  const existing = await app.prisma.card.findFirst({ where: { id, userId } });
+  if (!existing) {
+    return null;
+  }
 
   if (body.title) {
-    await app.prisma.card.update({ where: { id }, data: { title: body.title } })
+    await app.prisma.card.update({ where: { id }, data: { title: body.title } });
   }
 
   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 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
+    const linkIds = body.linkIds;
     await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
-      await tx.cardLink.deleteMany({ where: { cardId: id } })
+      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' } } },
+  })) as unknown as RawCard | null;
+
+  if (!updated) {
+    return null;
   }
 
-  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) }
+  return mapCard(updated);
 }
 
-export async function deleteCard(app: FastifyInstance, userId: string, id: string) {
+export async function deleteCard(app: FastifyInstance, userId: string, id: string): Promise<null> {
   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' })
+    const existing = await tx.card.findFirst({ where: { id, userId } });
+    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' })
+    const userCardCount = await tx.card.count({ where: { userId } });
+    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' } })
+      const oldestRemainingCard = await tx.card.findFirst({
+        where: { userId, id: { not: id } },
+        orderBy: { createdAt: 'asc' },
+      });
+
       if (oldestRemainingCard) {
-        await tx.card.update({ where: { id: oldestRemainingCard.id }, data: { isDefault: true } })
+        await tx.card.update({ where: { id: oldestRemainingCard.id }, data: { isDefault: true } });
       }
     }
 
-    await tx.card.delete({ where: { id } })
-    return null
-  })
+    await tx.card.delete({ where: { id } });
+    return null;
+  });
 }
 
-export async function setDefaultCard(app: FastifyInstance, userId: string, id: string) {
-  const existing = await app.prisma.card.findFirst({ where: { id, userId } })
-  if (!existing) return null
+export async function setDefaultCard(app: FastifyInstance, userId: string, id: string): Promise<{ message: string } | null> {
+  const existing = await app.prisma.card.findFirst({ where: { id, userId } });
+  if (!existing) {
+    return null;
+  }
 
   await app.prisma.$transaction(async (tx: Prisma.TransactionClient) => {
-    await tx.card.updateMany({ where: { userId }, data: { isDefault: false } })
-    await tx.card.update({ where: { id }, data: { isDefault: true } })
-  })
+    await tx.card.updateMany({ where: { userId }, data: { isDefault: false } });
+    await tx.card.update({ where: { id }, data: { isDefault: true } });
+  });
 
-  return { message: 'Default card updated' }
+  return { message: 'Default card updated' };
 }