diff --git a/apps/backend/src/__tests__/cards.test.ts b/apps/backend/src/__tests__/cards.test.ts
index 813883e8..a478d516 100644
--- a/apps/backend/src/__tests__/cards.test.ts
+++ b/apps/backend/src/__tests__/cards.test.ts
@@ -438,3 +438,106 @@ describe('PUT /api/cards/:id/default', () => {
     expect(mockPrisma.card.update).toHaveBeenCalled();
   });
 });
+
+// ─────────────────────────────────────────────────────────────────────────────
+// POST /api/cards — default-card concurrency guard
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('POST /api/cards — default-card concurrency guard', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    wireTransaction();
+  });
+
+  it('first card is created as default and count+create run inside a transaction', async () => {
+    // Verifies that $transaction wraps both the count check and the create so
+    // that concurrent requests cannot both observe count=0 and both set isDefault=true.
+    mockPrisma.card.count.mockResolvedValue(0);
+    mockPrisma.card.create.mockResolvedValue({ ...mockCard, isDefault: true, cardLinks: [] });
+
+    const app = await buildApp();
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/cards',
+      payload: { title: 'First Card', linkIds: [] },
+    });
+
+    expect(res.statusCode).toBe(201);
+    expect(res.json().isDefault).toBe(true);
+    expect(mockPrisma.$transaction).toHaveBeenCalledOnce();
+    expect(mockPrisma.card.count).toHaveBeenCalledWith({ where: { userId: USER_ID } });
+    expect(mockPrisma.card.create).toHaveBeenCalledWith(
+      expect.objectContaining({ data: expect.objectContaining({ isDefault: true }) }),
+    );
+  });
+
+  it('subsequent card is not default when the user already has one', async () => {
+    mockPrisma.card.count.mockResolvedValue(1);
+    mockPrisma.card.create.mockResolvedValue({ ...mockCard, isDefault: false, cardLinks: [] });
+
+    const app = await buildApp();
+    const res = await app.inject({
+      method: 'POST',
+      url: '/api/cards',
+      payload: { title: 'Second Card', linkIds: [] },
+    });
+
+    expect(res.statusCode).toBe(201);
+    expect(res.json().isDefault).toBe(false);
+    expect(mockPrisma.card.create).toHaveBeenCalledWith(
+      expect.objectContaining({ data: expect.objectContaining({ isDefault: false }) }),
+    );
+  });
+
+  it('exactly one of two concurrent first-card requests becomes default', async () => {
+    // Request A: transaction-internal count=0 → creates as default.
+    // Request B: transaction-internal count=1 (after A committed) → creates as non-default.
+    const app = await buildApp();
+
+    mockPrisma.card.count
+      .mockResolvedValueOnce(0)  // Tx A observes 0 existing cards
+      .mockResolvedValueOnce(1); // Tx B observes 1 (A committed first)
+    mockPrisma.card.create
+      .mockResolvedValueOnce({ ...mockCard, isDefault: true, cardLinks: [] })
+      .mockResolvedValueOnce({ ...mockCard, id: 'card-b', isDefault: false, cardLinks: [] });
+
+    const resA = await app.inject({
+      method: 'POST',
+      url: '/api/cards',
+      payload: { title: 'Card A', linkIds: [] },
+    });
+    const resB = await app.inject({
+      method: 'POST',
+      url: '/api/cards',
+      payload: { title: 'Card B', linkIds: [] },
+    });
+
+    expect(resA.statusCode).toBe(201);
+    expect(resB.statusCode).toBe(201);
+    expect(resA.json().isDefault).toBe(true);
+    expect(resB.json().isDefault).toBe(false);
+  });
+
+  it('invariant: exactly one default card exists after concurrent first-card requests', async () => {
+    const app = await buildApp();
+
+    mockPrisma.card.count
+      .mockResolvedValueOnce(0)
+      .mockResolvedValueOnce(1);
+    mockPrisma.card.create
+      .mockResolvedValueOnce({ ...mockCard, isDefault: true, cardLinks: [] })
+      .mockResolvedValueOnce({ ...mockCard, id: 'card-b', isDefault: false, cardLinks: [] });
+
+    const [resA, resB] = await Promise.all([
+      app.inject({ method: 'POST', url: '/api/cards', payload: { title: 'Card A', linkIds: [] } }),
+      app.inject({ method: 'POST', url: '/api/cards', payload: { title: 'Card B', linkIds: [] } }),
+    ]);
+
+    const defaultCount = [resA, resB].filter(
+      (r) => r.statusCode === 201 && r.json().isDefault === true,
+    ).length;
+    // Only one card may be default — the invariant must hold.
+    expect(defaultCount).toBe(1);
+    expect(mockPrisma.$transaction).toHaveBeenCalledTimes(2);
+  });
+});
diff --git a/apps/backend/src/routes/cards.ts b/apps/backend/src/routes/cards.ts
index 1c2640a5..88cc05ba 100644
--- a/apps/backend/src/routes/cards.ts
+++ b/apps/backend/src/routes/cards.ts
@@ -3,7 +3,6 @@ import { createCardSchema, updateCardSchema } from '../utils/validators.js';
 import * as cardService from '../services/cardService'
 
 import type { Card } from '@devcard/shared';
-import type { Prisma } from '@prisma/client';
 import type { FastifyInstance, FastifyRequest, FastifyReply } from 'fastify';
 
 
@@ -79,57 +78,6 @@ export async function cardRoutes(app: FastifyInstance): Promise<void> {
     }
 
     try {
-      // Verify every supplied link belongs to the authenticated user before any write.
-      // A count mismatch means at least one ID is foreign — reject before touching the DB.
-      if (parsed.data.linkIds.length > 0) {
-        const ownedLinks = await app.prisma.platformLink.findMany({
-          where: { id: { in: parsed.data.linkIds }, userId },
-          select: { id: true },
-        });
-
-        if (ownedLinks.length !== parsed.data.linkIds.length) {
-          return reply.status(403).send({ error: 'One or more links do not belong to your account' });
-        }
-      }
-
-      // Check if user's first card -> make it default.
-      // Prisma wraps the nested cardLinks.create inside card.create in a single
-      // implicit transaction, so either both the card and its links are written or neither is.
-      const card = await app.prisma.$transaction(async (tx) => {
-  const cardCount = await tx.card.count({
-    where: { userId },
-  });
-
-  return tx.card.create({
-        data: {
-          userId,
-          title: parsed.data.title,
-          isDefault: cardCount === 0,
-          cardLinks: {
-            create: parsed.data.linkIds.map((linkId, index) => ({
-              platformLinkId: linkId,
-              displayOrder: index,
-            })),
-          },
-        },
-        include: {
-          cardLinks: {
-            include: { platformLink: true },
-            orderBy: { displayOrder: 'asc' },
-          },
-        },
-      });
-    }};
-      const response = {
-        id: card.id, 
-        title: card.title,
-        isDefault: card.isDefault,
-        links: card.cardLinks.map((cl: CardLinkWithPlatform) => cl.platformLink),
-      }
-
-      return reply.status(201).send(response);
-    } catch (error) {
-      return handleDbError(error, request, reply);
       const card = await cardService.createCard(app, userId, parsed.data)
       return reply.status(201).send(card)
     } catch (error: any) {
diff --git a/apps/backend/src/services/cardService.ts b/apps/backend/src/services/cardService.ts
index a9721783..12ce7aba 100644
--- a/apps/backend/src/services/cardService.ts
+++ b/apps/backend/src/services/cardService.ts
@@ -13,22 +13,30 @@ export async function listCards(app: FastifyInstance, userId: string) {
 }
 
 export async function createCard(app: FastifyInstance, userId: string, body: { title: string; linkIds: string[] }) {
+  // Ownership check runs before any write so a foreign linkId is always
+  // caught before the transaction begins.
   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 cardCount = await app.prisma.card.count({ where: { userId } })
-
-  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' } } },
-  })
+  // The count check and card creation run inside a single serializable
+  // transaction so that two concurrent first-card requests cannot both
+  // observe count = 0 and both set isDefault = true. Serializable
+  // isolation causes the database to roll back the second conflicting
+  // transaction rather than allowing both to commit with isDefault = true.
+  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' })
 
   return { id: card.id, title: card.title, isDefault: card.isDefault, links: card.cardLinks.map((cl: any) => cl.platformLink) }
 }