diff --git a/synapse/handlers/sliding_sync/extensions.py b/synapse/handlers/sliding_sync/extensions.py
index 5ce185ff80..b616a3eec6 100644
--- a/synapse/handlers/sliding_sync/extensions.py
+++ b/synapse/handlers/sliding_sync/extensions.py
@@ -179,10 +179,7 @@ class SlidingSyncExtensionHandler:
             )
 
         threads_coro = None
-        if (
-            sync_config.extensions.threads is not None
-            and self._enable_threads_ext
-        ):
+        if sync_config.extensions.threads is not None and self._enable_threads_ext:
             threads_coro = self.get_threads_extension_response(
                 sync_config=sync_config,
                 threads_request=sync_config.extensions.threads,
diff --git a/tests/rest/client/sliding_sync/test_extension_threads.py b/tests/rest/client/sliding_sync/test_extension_threads.py
new file mode 100644
index 0000000000..049cf8e96b
--- /dev/null
+++ b/tests/rest/client/sliding_sync/test_extension_threads.py
@@ -0,0 +1,109 @@
+#
+# This file is licensed under the Affero General Public License (AGPL) version 3.
+#
+# Copyright (C) 2025 New Vector, Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# See the GNU Affero General Public License for more details:
+# <https://www.gnu.org/licenses/agpl-3.0.html>.
+#
+import logging
+
+from twisted.test.proto_helpers import MemoryReactor
+
+import synapse.rest.admin
+from synapse.rest.client import login, room, sync
+from synapse.server import HomeServer
+from synapse.types import JsonDict
+from synapse.util.clock import Clock
+
+from tests.rest.client.sliding_sync.test_sliding_sync import SlidingSyncBase
+
+logger = logging.getLogger(__name__)
+
+
+# The name of the extension. Currently unstable-prefixed.
+EXT_NAME = "io.element.msc4360.threads"
+
+
+class SlidingSyncThreadsExtensionTestCase(SlidingSyncBase):
+    """
+    Test the threads extension in the Sliding Sync API.
+    """
+
+    maxDiff = None
+
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+        sync.register_servlets,
+        # TODO:
+        # threads.register_servlets,
+    ]
+
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+        config["experimental_features"] = {"msc4360_enabled": True}
+        return config
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.store = hs.get_datastores().main
+        self.storage_controllers = hs.get_storage_controllers()
+        super().prepare(reactor, clock, hs)
+
+    def test_no_data_initial_sync(self) -> None:
+        """
+        Test enabling threads extension during initial sync with no data.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                EXT_NAME: {
+                    "enabled": True,
+                }
+            },
+        }
+
+        # Sync
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        # Assert
+        self.assertNotIn(EXT_NAME, response_body["extensions"])
+
+    def test_no_data_incremental_sync(self) -> None:
+        """
+        Test enabling threads extension during incremental sync with no data.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        initial_sync_body: JsonDict = {
+            "lists": {},
+        }
+
+        # Initial sync
+        response_body, sync_pos = self.do_sync(initial_sync_body, tok=user1_tok)
+
+        # Incremental sync with extension enabled
+        sync_body = {
+            "lists": {},
+            "extensions": {
+                EXT_NAME: {
+                    "enabled": True,
+                }
+            },
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok, since=sync_pos)
+
+        # Assert
+        self.assertNotIn(
+            EXT_NAME,
+            response_body["extensions"],
+            response_body,
+        )