diff --git a/apps/api/src/device-agent/device-registration.helpers.spec.ts b/apps/api/src/device-agent/device-registration.helpers.spec.ts
new file mode 100644
index 0000000000..2e2f6a8862
--- /dev/null
+++ b/apps/api/src/device-agent/device-registration.helpers.spec.ts
@@ -0,0 +1,157 @@
+jest.mock('@db', () => ({
+  db: {
+    device: {
+      findUnique: jest.fn(),
+      findFirst: jest.fn(),
+      update: jest.fn(),
+      create: jest.fn(),
+    },
+  },
+}));
+
+import { db } from '@db';
+import {
+  registerWithSerial,
+  registerWithoutSerial,
+} from './device-registration.helpers';
+import type { RegisterDeviceDto } from './dto/register-device.dto';
+
+const mockDb = db as jest.Mocked<typeof db>;
+
+const orgId = 'org_test';
+const member = { id: 'mem_test' };
+
+function makeDto(
+  overrides: Partial<RegisterDeviceDto> = {},
+): RegisterDeviceDto {
+  return {
+    organizationId: orgId,
+    hostname: 'my-laptop.local',
+    name: 'My Laptop',
+    platform: 'macos',
+    osVersion: '15.0',
+    serialNumber: 'ABC123',
+    hardwareModel: 'MacBookPro18,1',
+    agentVersion: '1.0.0',
+    ...overrides,
+  };
+}
+
+describe('registerWithSerial — orphan adoption', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('adopts an existing serial-less row for the same hostname+member instead of creating a duplicate', async () => {
+    // The bug scenario: agent first registered without a serial (e.g. cold-
+    // boot `system_profiler` returned empty), creating a row with
+    // serialNumber=null. A later registration succeeds in reading the
+    // serial. Without adoption, registerWithSerial would create a brand-new
+    // row and the old one would stay orphaned.
+    (mockDb.device.findUnique as jest.Mock).mockResolvedValue(null);
+    (mockDb.device.findFirst as jest.Mock).mockResolvedValue({
+      id: 'dev_orphan',
+    });
+    (mockDb.device.update as jest.Mock).mockResolvedValue({
+      id: 'dev_orphan',
+    });
+
+    const dto = makeDto();
+    await registerWithSerial({ member, dto });
+
+    expect(mockDb.device.findFirst).toHaveBeenCalledWith({
+      where: {
+        hostname: dto.hostname,
+        memberId: member.id,
+        organizationId: orgId,
+        serialNumber: null,
+      },
+      select: { id: true },
+    });
+    expect(mockDb.device.update).toHaveBeenCalledWith({
+      where: { id: 'dev_orphan' },
+      data: expect.objectContaining({
+        serialNumber: dto.serialNumber,
+        hostname: dto.hostname,
+      }),
+    });
+    expect(mockDb.device.create).not.toHaveBeenCalled();
+  });
+
+  it('creates a fresh row when no orphan exists', async () => {
+    (mockDb.device.findUnique as jest.Mock).mockResolvedValue(null);
+    (mockDb.device.findFirst as jest.Mock).mockResolvedValue(null);
+    (mockDb.device.create as jest.Mock).mockResolvedValue({ id: 'dev_new' });
+
+    const dto = makeDto();
+    await registerWithSerial({ member, dto });
+
+    expect(mockDb.device.update).not.toHaveBeenCalled();
+    expect(mockDb.device.create).toHaveBeenCalledWith({
+      data: expect.objectContaining({
+        serialNumber: dto.serialNumber,
+        memberId: member.id,
+        organizationId: orgId,
+      }),
+    });
+  });
+
+  it('updates the existing serial-match row without looking for an orphan', async () => {
+    // Plain re-registration of an already-known device — must not trigger
+    // the orphan lookup or do anything other than an in-place update.
+    (mockDb.device.findUnique as jest.Mock).mockResolvedValue({
+      id: 'dev_existing',
+      memberId: member.id,
+    });
+    (mockDb.device.update as jest.Mock).mockResolvedValue({
+      id: 'dev_existing',
+    });
+
+    const dto = makeDto();
+    await registerWithSerial({ member, dto });
+
+    expect(mockDb.device.findFirst).not.toHaveBeenCalled();
+    expect(mockDb.device.create).not.toHaveBeenCalled();
+    expect(mockDb.device.update).toHaveBeenCalledWith({
+      where: { id: 'dev_existing' },
+      data: expect.objectContaining({ hostname: dto.hostname }),
+    });
+  });
+
+  it('only adopts an orphan that belongs to the same member', async () => {
+    // Safety: the orphan lookup is scoped by memberId, so another member's
+    // serial-less row for the same hostname must not be hijacked.
+    (mockDb.device.findUnique as jest.Mock).mockResolvedValue(null);
+    (mockDb.device.findFirst as jest.Mock).mockResolvedValue(null);
+    (mockDb.device.create as jest.Mock).mockResolvedValue({ id: 'dev_new' });
+
+    const dto = makeDto();
+    await registerWithSerial({ member, dto });
+
+    const call = (mockDb.device.findFirst as jest.Mock).mock.calls[0]?.[0];
+    expect(call?.where.memberId).toBe(member.id);
+    expect(call?.where.serialNumber).toBeNull();
+  });
+});
+
+describe('registerWithoutSerial — unchanged behavior', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('updates the matching null-serial row when one exists', async () => {
+    (mockDb.device.findFirst as jest.Mock).mockResolvedValue({
+      id: 'dev_null',
+    });
+    (mockDb.device.update as jest.Mock).mockResolvedValue({ id: 'dev_null' });
+
+    const dto = makeDto({ serialNumber: undefined });
+    await registerWithoutSerial({ member, dto });
+
+    expect(mockDb.device.update).toHaveBeenCalledWith({
+      where: { id: 'dev_null' },
+      data: expect.any(Object),
+    });
+    expect(mockDb.device.create).not.toHaveBeenCalled();
+  });
+});
diff --git a/apps/api/src/device-agent/device-registration.helpers.ts b/apps/api/src/device-agent/device-registration.helpers.ts
index e170c4e8b9..57c6e5ac57 100644
--- a/apps/api/src/device-agent/device-registration.helpers.ts
+++ b/apps/api/src/device-agent/device-registration.helpers.ts
@@ -46,6 +46,33 @@ export async function registerWithSerial({
     });
   }
 
+  // Adopt any prior serial-less registration for the same physical device
+  // before creating a new row. The agent's serial extraction can return
+  // undefined on a cold boot (e.g. macOS `system_profiler` cache not yet
+  // built) and a real value on a subsequent boot — without this, the second
+  // registration creates a duplicate while the first row stays orphaned and
+  // never receives another check-in (frozen at its old compliance state).
+  const orphan = await db.device.findFirst({
+    where: {
+      hostname: dto.hostname,
+      memberId: member.id,
+      organizationId: dto.organizationId,
+      serialNumber: null,
+    },
+    select: { id: true },
+  });
+
+  if (orphan) {
+    return db.device.update({
+      where: { id: orphan.id },
+      data: {
+        ...updateData,
+        hostname: dto.hostname,
+        serialNumber: dto.serialNumber!,
+      },
+    });
+  }
+
   return db.device.create({
     data: {
       ...updateData,
diff --git a/packages/device-agent/src/main/device-info.ts b/packages/device-agent/src/main/device-info.ts
index 8b5811977c..a4c0f74593 100644
--- a/packages/device-agent/src/main/device-info.ts
+++ b/packages/device-agent/src/main/device-info.ts
@@ -73,11 +73,19 @@ function getOSVersion(platform: DevicePlatform): string {
 function getSerialNumber(platform: DevicePlatform): string | undefined {
   try {
     if (platform === 'macos') {
+      // `$NF` (last field) handles both "Serial Number: ABC" (3 fields) and
+      // "Serial Number (system): ABC" (4 fields, newer macOS). The fixed
+      // `$4` we used before silently returned empty on the 3-field variant,
+      // which made the agent register without a serial and later create a
+      // duplicate row once `system_profiler`'s cache warmed up and produced
+      // the 4-field variant. `exit` stops after the first match so any
+      // sub-component "Serial Number" lines added by future hardware can't
+      // smuggle a second value into the output.
       return (
-        execSync("system_profiler SPHardwareDataType | awk '/Serial Number/{print $4}'", {
-          encoding: 'utf-8',
-          timeout: 5000,
-        }).trim() || undefined
+        execSync(
+          "system_profiler SPHardwareDataType | awk '/Serial Number/{print $NF; exit}'",
+          { encoding: 'utf-8', timeout: 5000 },
+        ).trim() || undefined
       );
     }
     if (platform === 'linux') {