diff --git a/src/instructor/instructor.service.ts b/src/instructor/instructor.service.ts
index cebb4d5..2481597 100644
--- a/src/instructor/instructor.service.ts
+++ b/src/instructor/instructor.service.ts
@@ -2178,9 +2178,7 @@ export class InstructorService {
           organizationId,
           organizationName,
         ),
-        displayName: {
-          startsWith: baseName,
-        },
+        platformId: masterPlatform.id,
       },
       select: {
         displayName: true,
@@ -2227,7 +2225,11 @@ export class InstructorService {
       });
     } catch (error) {
       if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === 'P2002') {
-        throw new ConflictException('Platform name already exists');
+        const target = error.meta?.target;
+        if (Array.isArray(target) && target.includes('instance_no')) {
+          throw new ConflictException('플랫폼 등록번호가 중복되었습니다. 다시 등록해 주세요.');
+        }
+        throw new ConflictException('이미 사용 중인 플랫폼 이름입니다. 다른 이름을 입력해 주세요.');
       }
       throw error;
     }
diff --git a/src/instructor/platform-registration.spec.ts b/src/instructor/platform-registration.spec.ts
new file mode 100644
index 0000000..3ed41da
--- /dev/null
+++ b/src/instructor/platform-registration.spec.ts
@@ -0,0 +1,57 @@
+import { Prisma } from '@prisma/client';
+import { InstructorService } from './instructor.service';
+
+describe('platform registration', () => {
+  const input = { platformId: 'yahboom', agentNodeId: 'node-2', agentHostname: 'car-2' };
+  function setup() {
+    const registered = [
+      { platformId: 'yahboom', displayName: 'Custom car', instanceNo: 1 },
+      { platformId: 'mkmini', displayName: 'Yahboom MicroROS-Car-Pi5 #02', instanceNo: 2 },
+    ];
+    const prisma = {
+      platform: { findFirst: jest.fn().mockResolvedValue({ id: 'yahboom', displayName: 'Yahboom MicroROS-Car-Pi5' }) },
+      instructorPlatform: {
+        findMany: jest.fn(async ({ where }) => registered.filter(row =>
+          (!where.platformId || row.platformId === where.platformId)
+          && (!where.displayName?.startsWith || row.displayName.startsWith(where.displayName.startsWith)))),
+        create: jest.fn().mockResolvedValue({}),
+      },
+    };
+    const service = new InstructorService({} as never, {} as never, prisma as never, {} as never, {} as never, {} as never);
+    jest.spyOn(service as any, 'getInstructor').mockResolvedValue({
+      authUser: { primaryAffiliation: { organizationId: 'org-id', organizationName: 'org' } },
+    });
+    jest.spyOn(service, 'listPlatforms').mockResolvedValue({ platforms: [] } as never);
+    return { service, prisma };
+  }
+
+  it('counts renamed vehicles by platform ID, ignoring names of other types', async () => {
+    const { service, prisma } = setup();
+    await service.createPlatform('token', input);
+    expect(prisma.instructorPlatform.findMany).toHaveBeenCalledWith(expect.objectContaining({
+      where: expect.objectContaining({ platformId: 'yahboom', OR: expect.any(Array) }),
+    }));
+    expect(prisma.instructorPlatform.create).toHaveBeenCalledWith({
+      data: expect.objectContaining({ instanceNo: 2, displayName: 'Yahboom MicroROS-Car-Pi5 #02' }),
+    });
+  });
+
+  it('keeps a user-provided name while assigning the next internal number', async () => {
+    const { service, prisma } = setup();
+    await service.createPlatform('token', { ...input, displayName: 'Second car' });
+    expect(prisma.instructorPlatform.create).toHaveBeenCalledWith({
+      data: expect.objectContaining({ instanceNo: 2, displayName: 'Second car' }),
+    });
+  });
+
+  it.each([
+    [['organization_name', 'display_name'], '이미 사용 중인 플랫폼 이름'],
+    [['organization_name', 'platform_id', 'instance_no'], '플랫폼 등록번호가 중복'],
+  ])('distinguishes unique constraint failures for %j', async (target, message) => {
+    const { service, prisma } = setup();
+    prisma.instructorPlatform.create.mockRejectedValue(new Prisma.PrismaClientKnownRequestError('duplicate', {
+      code: 'P2002', clientVersion: '6', meta: { target },
+    }));
+    await expect(service.createPlatform('token', input)).rejects.toThrow(message);
+  });
+});