diff --git a/src/langbot/pkg/box/service.py b/src/langbot/pkg/box/service.py
index ea117b0cc..ca30eb930 100644
--- a/src/langbot/pkg/box/service.py
+++ b/src/langbot/pkg/box/service.py
@@ -82,7 +82,6 @@ class BoxService:
         return self._enabled
 
     async def initialize(self):
-        self._ensure_default_workspace()
         if not self._enabled:
             # Disabled by config: do NOT connect to a remote runtime, do NOT
             # fork a stdio subprocess. Every consumer of box_service should
@@ -99,6 +98,7 @@ class BoxService:
                 await self._runtime_connector.initialize()
             else:
                 await self.client.initialize()
+            self._ensure_default_workspace()
             self._available = True
             self._connector_error = ''
             self.ap.logger.info(
@@ -1152,6 +1152,9 @@ class BoxService:
         if self.default_workspace is None:
             return
 
+        if not self.shares_filesystem_with_box:
+            return
+
         if os.path.isdir(self.default_workspace):
             return
 
@@ -1176,7 +1179,7 @@ class BoxService:
             return
 
         host_path = os.path.realpath(spec.host_path)
-        if not os.path.isdir(host_path):
+        if self.shares_filesystem_with_box and not os.path.isdir(host_path):
             raise BoxValidationError('host_path must point to an existing directory on the host')
 
         if not self.allowed_mount_roots:
diff --git a/tests/unit_tests/box/test_box_service.py b/tests/unit_tests/box/test_box_service.py
index acf53264f..4d66ec8f8 100644
--- a/tests/unit_tests/box/test_box_service.py
+++ b/tests/unit_tests/box/test_box_service.py
@@ -256,6 +256,31 @@ class TestSharesFilesystemWithBox:
         assert service.shares_filesystem_with_box is False
 
 
+def test_separated_box_runtime_does_not_create_default_workspace_in_langbot(tmp_path):
+    logger = Mock()
+    runtime = BoxRuntime(logger=logger, backends=[FakeBackend(logger)], session_ttl_sec=300)
+    host_root = tmp_path / 'box'
+    service = BoxService(make_app(logger, host_root=str(host_root)), client=_InProcessBoxRuntimeClient(logger, runtime))
+    service._shares_filesystem_with_box_override = False
+
+    service._ensure_default_workspace()
+
+    assert not (host_root / 'default').exists()
+
+
+def test_separated_box_runtime_allows_box_owned_missing_host_path(tmp_path):
+    logger = Mock()
+    runtime = BoxRuntime(logger=logger, backends=[FakeBackend(logger)], session_ttl_sec=300)
+    host_root = tmp_path / 'box'
+    service = BoxService(make_app(logger, host_root=str(host_root)), client=_InProcessBoxRuntimeClient(logger, runtime))
+    service._shares_filesystem_with_box_override = False
+
+    spec = service.build_spec({'cmd': 'echo hi', 'session_id': 'missing-host-path'})
+
+    assert spec.host_path == str(host_root / 'default')
+    assert not (host_root / 'default').exists()
+
+
 @pytest.mark.asyncio
 async def test_box_service_get_sessions_delegates_to_client():
     client = Mock()
@@ -500,6 +525,7 @@ async def test_box_service_creates_default_workspace_on_initialize(tmp_path):
     app = make_app(logger, [str(allowed_root)])
     app.instance_config.data['box']['local']['default_workspace'] = str(default_workspace)
     service = BoxService(app, client=_InProcessBoxRuntimeClient(logger, runtime))
+    service._shares_filesystem_with_box_override = True
 
     await service.initialize()
 
@@ -514,6 +540,7 @@ async def test_box_service_derives_workspace_and_allowed_root_from_host_root(tmp
     shared_root = tmp_path / 'shared-box-root'
     app = make_app(logger, host_root=str(shared_root))
     service = BoxService(app, client=_InProcessBoxRuntimeClient(logger, runtime))
+    service._shares_filesystem_with_box_override = True
 
     await service.initialize()