diff --git a/openhands/app_server/sandbox/docker_sandbox_service.py b/openhands/app_server/sandbox/docker_sandbox_service.py
index 0cdb81c10d..e31a6ac222 100644
--- a/openhands/app_server/sandbox/docker_sandbox_service.py
+++ b/openhands/app_server/sandbox/docker_sandbox_service.py
@@ -74,6 +74,7 @@ class DockerSandboxService(SandboxService):
     exposed_ports: list[ExposedPort]
     health_check_path: str | None
     httpx_client: httpx.AsyncClient
+    max_num_sandboxes: int
     docker_client: docker.DockerClient = field(default_factory=get_docker_client)
 
     def _find_unused_port(self) -> int:
@@ -251,6 +252,9 @@ class DockerSandboxService(SandboxService):
 
     async def start_sandbox(self, sandbox_spec_id: str | None = None) -> SandboxInfo:
         """Start a new sandbox."""
+        # Enforce sandbox limits by cleaning up old sandboxes
+        await self.pause_old_sandboxes(self.max_num_sandboxes - 1)
+
         if sandbox_spec_id is None:
             sandbox_spec = await self.sandbox_spec_service.get_default_sandbox_spec()
         else:
@@ -321,6 +325,9 @@ class DockerSandboxService(SandboxService):
 
     async def resume_sandbox(self, sandbox_id: str) -> bool:
         """Resume a paused sandbox."""
+        # Enforce sandbox limits by cleaning up old sandboxes
+        await self.pause_old_sandboxes(self.max_num_sandboxes - 1)
+
         try:
             if not sandbox_id.startswith(self.container_name_prefix):
                 return False
@@ -383,6 +390,10 @@ class DockerSandboxServiceInjector(SandboxServiceInjector):
     container_url_pattern: str = 'http://localhost:{port}'
     host_port: int = 3000
     container_name_prefix: str = 'oh-agent-server-'
+    max_num_sandboxes: int = Field(
+        default=5,
+        description='Maximum number of sandboxes allowed to run simultaneously',
+    )
     mounts: list[VolumeMount] = Field(default_factory=list)
     exposed_ports: list[ExposedPort] = Field(
         default_factory=lambda: [
@@ -446,4 +457,5 @@ class DockerSandboxServiceInjector(SandboxServiceInjector):
                 exposed_ports=self.exposed_ports,
                 health_check_path=self.health_check_path,
                 httpx_client=httpx_client,
+                max_num_sandboxes=self.max_num_sandboxes,
             )
diff --git a/openhands/app_server/sandbox/remote_sandbox_service.py b/openhands/app_server/sandbox/remote_sandbox_service.py
index 9601e13b35..e3c69d6aa8 100644
--- a/openhands/app_server/sandbox/remote_sandbox_service.py
+++ b/openhands/app_server/sandbox/remote_sandbox_service.py
@@ -95,6 +95,7 @@ class RemoteSandboxService(SandboxService):
     resource_factor: int
     runtime_class: str | None
     start_sandbox_timeout: int
+    max_num_sandboxes: int
     user_context: UserContext
     httpx_client: httpx.AsyncClient
     db_session: AsyncSession
@@ -268,6 +269,9 @@ class RemoteSandboxService(SandboxService):
     async def start_sandbox(self, sandbox_spec_id: str | None = None) -> SandboxInfo:
         """Start a new sandbox by creating a remote runtime."""
         try:
+            # Enforce sandbox limits by cleaning up old sandboxes
+            await self.pause_old_sandboxes(self.max_num_sandboxes - 1)
+
             # Get sandbox spec
             if sandbox_spec_id is None:
                 sandbox_spec = (
@@ -338,6 +342,9 @@ class RemoteSandboxService(SandboxService):
 
     async def resume_sandbox(self, sandbox_id: str) -> bool:
         """Resume a paused sandbox."""
+        # Enforce sandbox limits by cleaning up old sandboxes
+        await self.pause_old_sandboxes(self.max_num_sandboxes - 1)
+
         try:
             if not await self._get_stored_sandbox(sandbox_id):
                 return False
@@ -588,6 +595,10 @@ class RemoteSandboxServiceInjector(SandboxServiceInjector):
             'be in an error state.'
         ),
     )
+    max_num_sandboxes: int = Field(
+        default=10,
+        description='Maximum number of sandboxes allowed to run simultaneously',
+    )
 
     async def inject(
         self, state: InjectorState, request: Request | None = None
@@ -628,6 +639,7 @@ class RemoteSandboxServiceInjector(SandboxServiceInjector):
                 resource_factor=self.resource_factor,
                 runtime_class=self.runtime_class,
                 start_sandbox_timeout=self.start_sandbox_timeout,
+                max_num_sandboxes=self.max_num_sandboxes,
                 user_context=user_context,
                 httpx_client=httpx_client,
                 db_session=db_session,
diff --git a/openhands/app_server/sandbox/sandbox_service.py b/openhands/app_server/sandbox/sandbox_service.py
index 526823b348..ad3d285309 100644
--- a/openhands/app_server/sandbox/sandbox_service.py
+++ b/openhands/app_server/sandbox/sandbox_service.py
@@ -1,7 +1,11 @@
 import asyncio
 from abc import ABC, abstractmethod
 
-from openhands.app_server.sandbox.sandbox_models import SandboxInfo, SandboxPage
+from openhands.app_server.sandbox.sandbox_models import (
+    SandboxInfo,
+    SandboxPage,
+    SandboxStatus,
+)
 from openhands.app_server.services.injector import Injector
 from openhands.sdk.utils.models import DiscriminatedUnionMixin
 
@@ -60,6 +64,61 @@ class SandboxService(ABC):
         Return False if the sandbox did not exist.
         """
 
+    async def pause_old_sandboxes(self, max_num_sandboxes: int) -> list[str]:
+        """Stop the oldest sandboxes if there are more than max_num_sandboxes running.
+
+        Args:
+            max_num_sandboxes: Maximum number of sandboxes to keep running
+
+        Returns:
+            List of sandbox IDs that were paused
+        """
+        if max_num_sandboxes <= 0:
+            raise ValueError('max_num_sandboxes must be greater than 0')
+
+        # Get all sandboxes (we'll search through all pages)
+        all_sandboxes = []
+        page_id = None
+
+        while True:
+            page = await self.search_sandboxes(page_id=page_id, limit=100)
+            all_sandboxes.extend(page.items)
+
+            if page.next_page_id is None:
+                break
+            page_id = page.next_page_id
+
+        # Filter to only running sandboxes
+        running_sandboxes = [
+            sandbox
+            for sandbox in all_sandboxes
+            if sandbox.status == SandboxStatus.RUNNING
+        ]
+
+        # If we're within the limit, no cleanup needed
+        if len(running_sandboxes) <= max_num_sandboxes:
+            return []
+
+        # Sort by creation time (oldest first)
+        running_sandboxes.sort(key=lambda x: x.created_at)
+
+        # Determine how many to pause
+        num_to_pause = len(running_sandboxes) - max_num_sandboxes
+        sandboxes_to_pause = running_sandboxes[:num_to_pause]
+
+        # Stop the oldest sandboxes
+        paused_sandbox_ids = []
+        for sandbox in sandboxes_to_pause:
+            try:
+                success = await self.pause_sandbox(sandbox.id)
+                if success:
+                    paused_sandbox_ids.append(sandbox.id)
+            except Exception:
+                # Continue trying to pause other sandboxes even if one fails
+                pass
+
+        return paused_sandbox_ids
+
 
 class SandboxServiceInjector(DiscriminatedUnionMixin, Injector[SandboxService], ABC):
     pass
diff --git a/tests/unit/app_server/test_docker_sandbox_service.py b/tests/unit/app_server/test_docker_sandbox_service.py
index 211ff023b2..b3fc2fcd41 100644
--- a/tests/unit/app_server/test_docker_sandbox_service.py
+++ b/tests/unit/app_server/test_docker_sandbox_service.py
@@ -80,6 +80,7 @@ def service(mock_sandbox_spec_service, mock_httpx_client, mock_docker_client):
         ],
         health_check_path='/health',
         httpx_client=mock_httpx_client,
+        max_num_sandboxes=3,
         docker_client=mock_docker_client,
     )
 
@@ -354,7 +355,12 @@ class TestDockerSandboxService:
 
         service.docker_client.containers.run.return_value = mock_container
 
-        with patch.object(service, '_find_unused_port', side_effect=[12345, 12346]):
+        with (
+            patch.object(service, '_find_unused_port', side_effect=[12345, 12346]),
+            patch.object(
+                service, 'pause_old_sandboxes', return_value=[]
+            ) as mock_cleanup,
+        ):
             # Execute
             result = await service.start_sandbox()
 
@@ -362,6 +368,9 @@ class TestDockerSandboxService:
         assert result is not None
         assert result.id == 'oh-test-test_container_id'
 
+        # Verify cleanup was called with the correct limit
+        mock_cleanup.assert_called_once_with(2)
+
         # Verify container was created with correct parameters
         service.docker_client.containers.run.assert_called_once()
         call_args = service.docker_client.containers.run.call_args
@@ -395,7 +404,10 @@ class TestDockerSandboxService:
         }
         service.docker_client.containers.run.return_value = mock_container
 
-        with patch.object(service, '_find_unused_port', return_value=12345):
+        with (
+            patch.object(service, '_find_unused_port', return_value=12345),
+            patch.object(service, 'pause_old_sandboxes', return_value=[]),
+        ):
             # Execute
             await service.start_sandbox(sandbox_spec_id='custom-spec')
 
@@ -412,7 +424,10 @@ class TestDockerSandboxService:
         mock_sandbox_spec_service.get_sandbox_spec.return_value = None
 
         # Execute & Verify
-        with pytest.raises(ValueError, match='Sandbox Spec not found'):
+        with (
+            patch.object(service, 'pause_old_sandboxes', return_value=[]),
+            pytest.raises(ValueError, match='Sandbox Spec not found'),
+        ):
             await service.start_sandbox(sandbox_spec_id='nonexistent')
 
     async def test_start_sandbox_docker_error(self, service):
@@ -422,10 +437,12 @@ class TestDockerSandboxService:
             'Failed to create container'
         )
 
-        with patch.object(service, '_find_unused_port', return_value=12345):
-            # Execute & Verify
-            with pytest.raises(SandboxError, match='Failed to start container'):
-                await service.start_sandbox()
+        with (
+            patch.object(service, '_find_unused_port', return_value=12345),
+            patch.object(service, 'pause_old_sandboxes', return_value=[]),
+            pytest.raises(SandboxError, match='Failed to start container'),
+        ):
+            await service.start_sandbox()
 
     async def test_resume_sandbox_from_paused(self, service):
         """Test resuming a paused sandbox."""
@@ -434,13 +451,18 @@ class TestDockerSandboxService:
         mock_container.status = 'paused'
         service.docker_client.containers.get.return_value = mock_container
 
-        # Execute
-        result = await service.resume_sandbox('oh-test-abc123')
+        with patch.object(
+            service, 'pause_old_sandboxes', return_value=[]
+        ) as mock_cleanup:
+            # Execute
+            result = await service.resume_sandbox('oh-test-abc123')
 
         # Verify
         assert result is True
         mock_container.unpause.assert_called_once()
         mock_container.start.assert_not_called()
+        # Verify cleanup was called with the correct limit
+        mock_cleanup.assert_called_once_with(2)
 
     async def test_resume_sandbox_from_exited(self, service):
         """Test resuming an exited sandbox."""
@@ -449,22 +471,32 @@ class TestDockerSandboxService:
         mock_container.status = 'exited'
         service.docker_client.containers.get.return_value = mock_container
 
-        # Execute
-        result = await service.resume_sandbox('oh-test-abc123')
+        with patch.object(
+            service, 'pause_old_sandboxes', return_value=[]
+        ) as mock_cleanup:
+            # Execute
+            result = await service.resume_sandbox('oh-test-abc123')
 
         # Verify
         assert result is True
         mock_container.start.assert_called_once()
         mock_container.unpause.assert_not_called()
+        # Verify cleanup was called with the correct limit
+        mock_cleanup.assert_called_once_with(2)
 
     async def test_resume_sandbox_wrong_prefix(self, service):
         """Test resuming sandbox with wrong prefix."""
-        # Execute
-        result = await service.resume_sandbox('wrong-prefix-abc123')
+        with patch.object(
+            service, 'pause_old_sandboxes', return_value=[]
+        ) as mock_cleanup:
+            # Execute
+            result = await service.resume_sandbox('wrong-prefix-abc123')
 
         # Verify
         assert result is False
         service.docker_client.containers.get.assert_not_called()
+        # Verify cleanup was still called
+        mock_cleanup.assert_called_once_with(2)
 
     async def test_resume_sandbox_not_found(self, service):
         """Test resuming non-existent sandbox."""
@@ -473,11 +505,16 @@ class TestDockerSandboxService:
             'Container not found'
         )
 
-        # Execute
-        result = await service.resume_sandbox('oh-test-abc123')
+        with patch.object(
+            service, 'pause_old_sandboxes', return_value=[]
+        ) as mock_cleanup:
+            # Execute
+            result = await service.resume_sandbox('oh-test-abc123')
 
         # Verify
         assert result is False
+        # Verify cleanup was still called
+        mock_cleanup.assert_called_once_with(2)
 
     async def test_pause_sandbox_success(self, service):
         """Test pausing a running sandbox."""
diff --git a/tests/unit/app_server/test_sandbox_service.py b/tests/unit/app_server/test_sandbox_service.py
new file mode 100644
index 0000000000..9a65131821
--- /dev/null
+++ b/tests/unit/app_server/test_sandbox_service.py
@@ -0,0 +1,353 @@
+"""Tests for SandboxService base class.
+
+This module tests the SandboxService base class implementation, focusing on:
+- pause_old_sandboxes method functionality
+- Proper handling of pagination when searching sandboxes
+- Correct filtering of running vs non-running sandboxes
+- Proper sorting by creation time (oldest first)
+- Error handling and edge cases
+"""
+
+from datetime import datetime, timedelta, timezone
+from unittest.mock import AsyncMock
+
+import pytest
+
+from openhands.app_server.sandbox.sandbox_models import (
+    SandboxInfo,
+    SandboxPage,
+    SandboxStatus,
+)
+from openhands.app_server.sandbox.sandbox_service import SandboxService
+
+
+class MockSandboxService(SandboxService):
+    """Mock implementation of SandboxService for testing."""
+
+    def __init__(self):
+        self.search_sandboxes_mock = AsyncMock()
+        self.get_sandbox_mock = AsyncMock()
+        self.start_sandbox_mock = AsyncMock()
+        self.resume_sandbox_mock = AsyncMock()
+        self.pause_sandbox_mock = AsyncMock()
+        self.delete_sandbox_mock = AsyncMock()
+
+    async def search_sandboxes(
+        self, page_id: str | None = None, limit: int = 100
+    ) -> SandboxPage:
+        return await self.search_sandboxes_mock(page_id=page_id, limit=limit)
+
+    async def get_sandbox(self, sandbox_id: str) -> SandboxInfo | None:
+        return await self.get_sandbox_mock(sandbox_id)
+
+    async def start_sandbox(self, sandbox_spec_id: str | None = None) -> SandboxInfo:
+        return await self.start_sandbox_mock(sandbox_spec_id)
+
+    async def resume_sandbox(self, sandbox_id: str) -> bool:
+        return await self.resume_sandbox_mock(sandbox_id)
+
+    async def pause_sandbox(self, sandbox_id: str) -> bool:
+        return await self.pause_sandbox_mock(sandbox_id)
+
+    async def delete_sandbox(self, sandbox_id: str) -> bool:
+        return await self.delete_sandbox_mock(sandbox_id)
+
+
+def create_sandbox_info(
+    sandbox_id: str,
+    status: SandboxStatus,
+    created_at: datetime,
+    created_by_user_id: str | None = None,
+    sandbox_spec_id: str = 'test-spec',
+) -> SandboxInfo:
+    """Helper function to create SandboxInfo objects for testing."""
+    return SandboxInfo(
+        id=sandbox_id,
+        created_by_user_id=created_by_user_id,
+        sandbox_spec_id=sandbox_spec_id,
+        status=status,
+        session_api_key='test-api-key' if status == SandboxStatus.RUNNING else None,
+        created_at=created_at,
+    )
+
+
+@pytest.fixture
+def mock_sandbox_service():
+    """Fixture providing a mock sandbox service."""
+    return MockSandboxService()
+
+
+class TestCleanupOldSandboxes:
+    """Test cases for the pause_old_sandboxes method."""
+
+    @pytest.mark.asyncio
+    async def test_cleanup_with_no_sandboxes(self, mock_sandbox_service):
+        """Test cleanup when there are no sandboxes."""
+        # Setup: No sandboxes
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=[], next_page_id=None
+        )
+
+        # Execute
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=5)
+
+        # Verify
+        assert result == []
+        mock_sandbox_service.search_sandboxes_mock.assert_called_once_with(
+            page_id=None, limit=100
+        )
+        mock_sandbox_service.pause_sandbox_mock.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_cleanup_within_limit(self, mock_sandbox_service):
+        """Test cleanup when sandbox count is within the limit."""
+        # Setup: 3 running sandboxes, limit is 5
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info('sb1', SandboxStatus.RUNNING, now - timedelta(hours=3)),
+            create_sandbox_info('sb2', SandboxStatus.RUNNING, now - timedelta(hours=2)),
+            create_sandbox_info('sb3', SandboxStatus.RUNNING, now - timedelta(hours=1)),
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+
+        # Execute
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=5)
+
+        # Verify
+        assert result == []
+        mock_sandbox_service.pause_sandbox_mock.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_cleanup_exceeds_limit(self, mock_sandbox_service):
+        """Test cleanup when sandbox count exceeds the limit."""
+        # Setup: 5 running sandboxes, limit is 3
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info(
+                'sb1', SandboxStatus.RUNNING, now - timedelta(hours=5)
+            ),  # oldest
+            create_sandbox_info(
+                'sb2', SandboxStatus.RUNNING, now - timedelta(hours=4)
+            ),  # second oldest
+            create_sandbox_info(
+                'sb3', SandboxStatus.RUNNING, now - timedelta(hours=3)
+            ),  # should be stopped
+            create_sandbox_info(
+                'sb4', SandboxStatus.RUNNING, now - timedelta(hours=2)
+            ),  # should remain
+            create_sandbox_info(
+                'sb5', SandboxStatus.RUNNING, now - timedelta(hours=1)
+            ),  # newest
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+        mock_sandbox_service.pause_sandbox_mock.return_value = True
+
+        # Execute
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=2)
+
+        # Verify: Should pause the 2 oldest sandboxes
+        assert result == ['sb1', 'sb2', 'sb3']
+        assert mock_sandbox_service.pause_sandbox_mock.call_count == 3
+
+    @pytest.mark.asyncio
+    async def test_cleanup_filters_non_running_sandboxes(self, mock_sandbox_service):
+        """Test that cleanup only considers running sandboxes."""
+        # Setup: Mix of running and non-running sandboxes
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info('sb1', SandboxStatus.RUNNING, now - timedelta(hours=5)),
+            create_sandbox_info(
+                'sb2', SandboxStatus.PAUSED, now - timedelta(hours=4)
+            ),  # should be ignored
+            create_sandbox_info('sb3', SandboxStatus.RUNNING, now - timedelta(hours=3)),
+            create_sandbox_info(
+                'sb4', SandboxStatus.ERROR, now - timedelta(hours=2)
+            ),  # should be ignored
+            create_sandbox_info('sb5', SandboxStatus.RUNNING, now - timedelta(hours=1)),
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+        mock_sandbox_service.pause_sandbox_mock.return_value = True
+
+        # Execute: Limit is 2, but only 3 are running
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=2)
+
+        # Verify: Should stop only 1 sandbox (the oldest running one)
+        assert len(result) == 1
+        assert 'sb1' in result
+        mock_sandbox_service.pause_sandbox_mock.assert_called_once_with('sb1')
+
+    @pytest.mark.asyncio
+    async def test_cleanup_with_pagination(self, mock_sandbox_service):
+        """Test cleanup handles pagination correctly."""
+        # Setup: Multiple pages of sandboxes
+        now = datetime.now(timezone.utc)
+
+        # First page
+        page1_sandboxes = [
+            create_sandbox_info('sb1', SandboxStatus.RUNNING, now - timedelta(hours=3)),
+            create_sandbox_info('sb2', SandboxStatus.RUNNING, now - timedelta(hours=2)),
+        ]
+
+        # Second page
+        page2_sandboxes = [
+            create_sandbox_info('sb3', SandboxStatus.RUNNING, now - timedelta(hours=1)),
+        ]
+
+        def search_side_effect(page_id=None, limit=100):
+            if page_id is None:
+                return SandboxPage(items=page1_sandboxes, next_page_id='page2')
+            elif page_id == 'page2':
+                return SandboxPage(items=page2_sandboxes, next_page_id=None)
+
+        mock_sandbox_service.search_sandboxes_mock.side_effect = search_side_effect
+        mock_sandbox_service.pause_sandbox_mock.return_value = True
+
+        # Execute: Limit is 2, total is 3
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=2)
+
+        # Verify: Should stop the oldest sandbox
+        assert len(result) == 1
+        assert 'sb1' in result
+        assert mock_sandbox_service.search_sandboxes_mock.call_count == 2
+
+    @pytest.mark.asyncio
+    async def test_cleanup_handles_pause_failures(self, mock_sandbox_service):
+        """Test cleanup continues when some pause operations fail."""
+        # Setup: 4 running sandboxes, limit is 2
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info('sb1', SandboxStatus.RUNNING, now - timedelta(hours=4)),
+            create_sandbox_info('sb2', SandboxStatus.RUNNING, now - timedelta(hours=3)),
+            create_sandbox_info('sb3', SandboxStatus.RUNNING, now - timedelta(hours=2)),
+            create_sandbox_info('sb4', SandboxStatus.RUNNING, now - timedelta(hours=1)),
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+
+        # Setup: First pause fails, second succeeds
+        def pause_side_effect(sandbox_id):
+            if sandbox_id == 'sb1':
+                return False  # Simulate failure
+            return True
+
+        mock_sandbox_service.pause_sandbox_mock.side_effect = pause_side_effect
+
+        # Execute
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=2)
+
+        # Verify: Should only include successfully paused sandbox
+        assert len(result) == 1
+        assert 'sb2' in result
+        assert mock_sandbox_service.pause_sandbox_mock.call_count == 2
+
+    @pytest.mark.asyncio
+    async def test_cleanup_handles_pause_exceptions(self, mock_sandbox_service):
+        """Test cleanup continues when pause operations raise exceptions."""
+        # Setup: 3 running sandboxes, limit is 1
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info('sb1', SandboxStatus.RUNNING, now - timedelta(hours=3)),
+            create_sandbox_info('sb2', SandboxStatus.RUNNING, now - timedelta(hours=2)),
+            create_sandbox_info('sb3', SandboxStatus.RUNNING, now - timedelta(hours=1)),
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+
+        # Setup: First pause raises exception, second succeeds
+        def pause_side_effect(sandbox_id):
+            if sandbox_id == 'sb1':
+                raise Exception('Delete failed')
+            return True
+
+        mock_sandbox_service.pause_sandbox_mock.side_effect = pause_side_effect
+
+        # Execute
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=1)
+
+        # Verify: Should only include successfully paused sandbox
+        assert len(result) == 1
+        assert 'sb2' in result
+        assert mock_sandbox_service.pause_sandbox_mock.call_count == 2
+
+    @pytest.mark.asyncio
+    async def test_cleanup_invalid_max_num_sandboxes(self, mock_sandbox_service):
+        """Test cleanup raises ValueError for invalid max_num_sandboxes."""
+        # Test zero
+        with pytest.raises(
+            ValueError, match='max_num_sandboxes must be greater than 0'
+        ):
+            await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=0)
+
+        # Test negative
+        with pytest.raises(
+            ValueError, match='max_num_sandboxes must be greater than 0'
+        ):
+            await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=-1)
+
+    @pytest.mark.asyncio
+    async def test_cleanup_sorts_by_creation_time(self, mock_sandbox_service):
+        """Test that cleanup properly sorts sandboxes by creation time."""
+        # Setup: Sandboxes in random order by creation time
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info('sb_newest', SandboxStatus.RUNNING, now),  # newest
+            create_sandbox_info(
+                'sb_oldest', SandboxStatus.RUNNING, now - timedelta(hours=5)
+            ),  # oldest
+            create_sandbox_info(
+                'sb_middle', SandboxStatus.RUNNING, now - timedelta(hours=2)
+            ),  # middle
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+        mock_sandbox_service.pause_sandbox_mock.return_value = True
+
+        # Execute: Keep only 1 sandbox
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=1)
+
+        # Verify: Should stop the 2 oldest sandboxes in order
+        assert len(result) == 2
+        assert 'sb_oldest' in result
+        assert 'sb_middle' in result
+
+        # Verify pause was called in the correct order (oldest first)
+        calls = mock_sandbox_service.pause_sandbox_mock.call_args_list
+        assert calls[0][0][0] == 'sb_oldest'
+        assert calls[1][0][0] == 'sb_middle'
+
+    @pytest.mark.asyncio
+    async def test_cleanup_exact_limit(self, mock_sandbox_service):
+        """Test cleanup when sandbox count exactly equals the limit."""
+        # Setup: Exactly 3 running sandboxes, limit is 3
+        now = datetime.now(timezone.utc)
+        sandboxes = [
+            create_sandbox_info('sb1', SandboxStatus.RUNNING, now - timedelta(hours=3)),
+            create_sandbox_info('sb2', SandboxStatus.RUNNING, now - timedelta(hours=2)),
+            create_sandbox_info('sb3', SandboxStatus.RUNNING, now - timedelta(hours=1)),
+        ]
+
+        mock_sandbox_service.search_sandboxes_mock.return_value = SandboxPage(
+            items=sandboxes, next_page_id=None
+        )
+
+        # Execute
+        result = await mock_sandbox_service.pause_old_sandboxes(max_num_sandboxes=3)
+
+        # Verify: No sandboxes should be stopped
+        assert result == []
+        mock_sandbox_service.pause_sandbox_mock.assert_not_called()