diff --git a/src/clusterfuzz/_internal/base/tasks/__init__.py b/src/clusterfuzz/_internal/base/tasks/__init__.py
index 17eed20855..4f70ad1a35 100644
--- a/src/clusterfuzz/_internal/base/tasks/__init__.py
+++ b/src/clusterfuzz/_internal/base/tasks/__init__.py
@@ -586,40 +586,43 @@ def lease(self, _event=None):  # pylint: disable=arguments-differ
 
 
 def _filter_task_for_os_mismatch(message, queue) -> bool:
-  """Filters a Pub/Sub message if its OS version does not match the bot's OS.
+  """Filters a Pub/Sub message if its OS version mismatches the bot's OS.
 
-  This function checks the `base_os_version` attribute in the incoming message
-  against the bot's `BASE_OS_VERSION` environment variable. This handles cases
-  where a message is misrouted or received from a legacy subscription without
-  OS-specific filters.
+  This function compares the `base_os_version` attribute from the message
+  against the bot's `BASE_OS_VERSION` environment variable.
 
-  If an OS version mismatch is detected, the function logs a warning and
-  acknowledges (`ack()`) the message. Acknowledging the message permanently
-  removes it from the current subscription, effectively skipping it for this
-  bot. This assumes the message was also correctly delivered to another,
-  properly filtered subscription for processing.
+  A task is skipped (and the message is acknowledged) if the message specifies
+  an OS version, and the bot's OS is different. This includes the legacy
+  scenario where a bot does not have `BASE_OS_VERSION` set (evaluating to
+  `None`), preventing it from processing tasks meant for newer OS versions.
+
+  If the message does not specify an OS version, it can be processed by any
+  bot. If the versions match, it is also processed.
 
   Args:
-    message: The `pubsub.Message` object to check.
-    queue: The name of the queue from which the message was pulled.
+    message (pubsub.Message): The message object to check.
+    queue (str): The name of the queue from which the message was pulled.
 
   Returns:
-    True if the message had a mismatch and was acknowledged; False otherwise.
+    bool: True if the message had a mismatch and was acknowledged, otherwise
+    False.
   """
   base_os_version = environment.get_value('BASE_OS_VERSION')
   message_base_os_version = message.attributes.get('base_os_version')
 
-  if not (message_base_os_version and base_os_version and
-          message_base_os_version != base_os_version):
+  if not message_base_os_version:
     return False
 
-  logs.warning(
-      'Skipping task for different OS.',
-      queue=queue,
-      message_os_version=message_base_os_version,
-      base_os_version=base_os_version)
-  message.ack()
-  return True
+  if message_base_os_version != base_os_version:
+    logs.warning(
+        'Skipping task for different OS.',
+        queue=queue,
+        message_os_version=message_base_os_version,
+        base_os_version=base_os_version)
+    message.ack()
+    return True
+
+  return False
 
 
 def get_task_from_message(message, queue=None, can_defer=True,
diff --git a/src/clusterfuzz/_internal/tests/core/base/tasks/tasks_test.py b/src/clusterfuzz/_internal/tests/core/base/tasks/tasks_test.py
index fd1d58ca4b..0324657154 100644
--- a/src/clusterfuzz/_internal/tests/core/base/tasks/tasks_test.py
+++ b/src/clusterfuzz/_internal/tests/core/base/tasks/tasks_test.py
@@ -280,6 +280,8 @@ def test_no_message(self):
 
   def test_success(self):
     """Test successful task creation from a message."""
+    self.mock_env_get.return_value = None
+    self.mock_message.attributes = {}
     self.assertEqual(
         tasks.get_task_from_message(self.mock_message), self.mock_task)
 
@@ -297,6 +299,8 @@ def test_defer(self):
   def test_set_queue(self):
     """Tests the set_queue method of a task."""
     mock_queue = mock.Mock()
+    self.mock_env_get.return_value = None
+    self.mock_message.attributes = {}
     task = tasks.get_task_from_message(self.mock_message, queue=mock_queue)
     task.set_queue.assert_called_with(mock_queue)
 
@@ -336,15 +340,21 @@ def test_bot_has_os_message_does_not(self):
     self.assertEqual(result, self.mock_task)
     self.mock_message.ack.assert_not_called()
 
-  def test_bot_has_no_os_message_does(self):
-    """Test that a message is processed if the message has an OS but the bot does not."""
+  @mock.patch('clusterfuzz._internal.metrics.logs.warning')
+  def test_bot_has_no_os_message_does(self, mock_log_warning):
+    """Test that a message is skipped if it has an OS but the bot does not."""
     self.mock_env_get.return_value = None
     self.mock_message.attributes = {'base_os_version': 'ubuntu-24-04'}
 
     result = tasks.get_task_from_message(self.mock_message)
 
-    self.assertEqual(result, self.mock_task)
-    self.mock_message.ack.assert_not_called()
+    self.assertIsNone(result)
+    self.mock_message.ack.assert_called_once()
+    mock_log_warning.assert_called_with(
+        'Skipping task for different OS.',
+        queue=None,
+        message_os_version='ubuntu-24-04',
+        base_os_version=None)
 
 
 @test_utils.with_cloud_emulators('datastore')