feat(agents): wrap single_turn non-LlmAgent sub-agents as inline tools

Relax the parent `LlmAgent` auto-wrapping loop so any sub-agent declaring
`mode='single_turn'` is exposed as an inline `_SingleTurnAgentTool`, not just
`LlmAgent` sub-agents. This lets a `ManagedAgent` with `mode='single_turn'` be
called like a tool while staying excluded from LLM-transfer targets. `LlmAgent`
sub-agents still default to `mode='chat'` (unchanged), and sub-agents that do
not declare `mode` are never wrapped.

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 945960582

Author: haranrk

src/google/adk/agents/llm_agent.py

@@ -1119,18 +1119,19 @@ def model_post_init(self, __context: Any) -> None:
 
     if self.sub_agents:
       for sub_agent in self.sub_agents:
-        if isinstance(sub_agent, LlmAgent):
-          mode = getattr(sub_agent, 'mode', None)
-          if mode is None:
-            try:
-              sub_agent.mode = 'chat'
-              mode = 'chat'
-            except (AttributeError, TypeError):
-              continue
-          if mode == 'single_turn':
-            self.tools.append(_SingleTurnAgentTool(sub_agent))
-          elif mode == 'task':
-            self.tools.append(_TaskAgentTool(sub_agent))
+        # `mode` is defined by whichever agent classes declare the field; any
+        # agent that defines `mode` participates here. A sub-agent that does not
+        # declare `mode` returns None and is never wrapped (it stays an
+        # LLM-transfer target).
+        mode = getattr(sub_agent, 'mode', None)
+        # LlmAgent sub-agents default to chat mode (unchanged behavior).
+        if isinstance(sub_agent, LlmAgent) and mode is None:
+          sub_agent.mode = 'chat'
+          mode = 'chat'
+        if mode == 'single_turn':
+          self.tools.append(_SingleTurnAgentTool(sub_agent))
+        elif mode == 'task':
+          self.tools.append(_TaskAgentTool(sub_agent))
 
   @classmethod
   @experimental(FeatureName.AGENT_CONFIG)

tests/unittests/agents/test_llm_agent_single_turn_subagents.py

@@ -0,0 +1,49 @@
+# 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.
+
+from google.adk.agents import LlmAgent
+
+
+def test_single_turn_managed_agent_is_wrapped_as_tool():
+  from google.adk.agents import ManagedAgent
+  from google.adk.tools.agent_tool import _SingleTurnAgentTool
+
+  managed = ManagedAgent(name='m', agent_id='a', mode='single_turn')
+  coordinator = LlmAgent(name='c', sub_agents=[managed])
+
+  assert any(
+      isinstance(t, _SingleTurnAgentTool) and t.agent is managed
+      for t in coordinator.tools
+  )
+
+
+def test_managed_agent_without_mode_is_not_wrapped():
+  from google.adk.agents import ManagedAgent
+  from google.adk.tools.agent_tool import _SingleTurnAgentTool
+
+  managed = ManagedAgent(name='m', agent_id='a')  # mode defaults to None
+  coordinator = LlmAgent(name='c', sub_agents=[managed])
+
+  assert not any(isinstance(t, _SingleTurnAgentTool) for t in coordinator.tools)
+
+
+def test_single_turn_managed_agent_excluded_from_transfer_targets():
+  from google.adk.agents import ManagedAgent
+  from google.adk.flows.llm_flows.agent_transfer import _get_transfer_targets
+
+  managed = ManagedAgent(name='m', agent_id='a', mode='single_turn')
+  coordinator = LlmAgent(name='c', sub_agents=[managed])
+
+  targets = _get_transfer_targets(coordinator)
+  assert managed not in targets