fix(agent_transfer): add hasattr guard for peer agents without mode attribute

src/google/adk/flows/llm_flows/agent_transfer.py

@@ -193,7 +193,10 @@ def _get_transfer_targets(agent: LlmAgent) -> list[BaseAgent]:
         peer_agent
         for peer_agent in agent.parent_agent.sub_agents
         if peer_agent.name != agent.name
-        and peer_agent.mode not in ('single_turn', 'task')
+        and (
+            not hasattr(peer_agent, 'mode')
+            or peer_agent.mode not in ('single_turn', 'task')
+        )
     ])
 
   return result

tests/unittests/flows/llm_flows/test_get_transfer_targets.py

@@ -0,0 +1,63 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for _get_transfer_targets in agent_transfer module."""
+
+from google.adk.agents.llm_agent import Agent
+from google.adk.agents.loop_agent import LoopAgent
+from google.adk.flows.llm_flows.agent_transfer import _get_transfer_targets
+
+from ... import testing_utils
+
+
+def test_loop_agent_peer_does_not_raise_attribute_error():
+  """LoopAgent has no 'mode' attribute; _get_transfer_targets must not crash.
+
+  Regression test for https://github.com/google/adk-python/issues/5863.
+  """
+  mock_model = testing_utils.MockModel.create(responses=['response'])
+
+  loop_peer = LoopAgent(name='loop_peer', sub_agents=[])
+  llm_agent = Agent(name='llm_agent', model=mock_model)
+
+  root = Agent(
+      name='root',
+      model=mock_model,
+      sub_agents=[llm_agent, loop_peer],
+  )
+
+  # Before the fix this raised:
+  #   AttributeError: 'LoopAgent' object has no attribute 'mode'
+  targets = _get_transfer_targets(llm_agent)
+
+  target_names = [t.name for t in targets]
+  # loop_peer should be included as a valid peer transfer target
+  assert 'loop_peer' in target_names
+
+
+def test_loop_agent_sub_agent_does_not_raise_attribute_error():
+  """LoopAgent as a sub-agent of the current agent should also not crash."""
+  mock_model = testing_utils.MockModel.create(responses=['response'])
+
+  loop_sub = LoopAgent(name='loop_sub', sub_agents=[])
+  llm_agent = Agent(
+      name='llm_agent',
+      model=mock_model,
+      sub_agents=[loop_sub],
+  )
+
+  targets = _get_transfer_targets(llm_agent)
+
+  target_names = [t.name for t in targets]
+  assert 'loop_sub' in target_names