fix: Default subagents to chat mode in build_node

* Dynamically attached subagents bypass LlmAgent.model_post_init normalization and retain mode=None.
* Previously, build_node defaulted all mode=None agents to single_turn, preventing agent transfer and causing duplicate output errors.
* Now, build_node checks for agent.parent_agent to identify subagents and defaults them to chat mode, preserving single_turn only for standalone nodes.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 929471803

Author: DeanChensj

src/google/adk/workflow/utils/_workflow_graph_utils.py

@@ -92,7 +92,13 @@ def build_node(
       agent.parent_agent = node_like.parent_agent
 
       if agent.mode is None:
-        agent.mode = 'single_turn'
+        # Sub-agents dynamically attached to a parent agent default to 'chat'
+        # mode to enable agent transfer.
+        # Standalone agents in a workflow graph default to 'single_turn'.
+        if agent.parent_agent is not None:
+          agent.mode = 'chat'
+        else:
+          agent.mode = 'single_turn'
 
       if agent.mode in ('task', 'chat'):
         agent.wait_for_output = True

tests/unittests/workflow/utils/test_workflow_graph_utils.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+from google.adk.agents.llm_agent import LlmAgent
 from google.adk.tools.base_tool import BaseTool
 from google.adk.workflow._base_node import BaseNode
 from google.adk.workflow._base_node import START
@@ -117,3 +118,19 @@ def test_raises_value_error_for_invalid_type(self):
     """build_node raises ValueError for invalid types."""
     with pytest.raises(ValueError, match="Invalid node type"):
       build_node(123)
+
+  def test_llm_agent_mode_defaults(self):
+    """build_node sets correct default mode for LlmAgent."""
+    root_agent = LlmAgent(name="root", instruction="test")
+    sub_agent = LlmAgent(name="sub", description="test")
+    # Dynamic subagent attachment without model_post_init normalization
+    sub_agent.parent_agent = root_agent
+
+    # Subagent with parent_agent should default to chat mode
+    built_sub = build_node(sub_agent)
+    assert built_sub.mode == "chat"
+
+    # Standalone agent without parent_agent should default to single_turn
+    standalone = LlmAgent(name="standalone", instruction="test")
+    built_standalone = build_node(standalone)
+    assert built_standalone.mode == "single_turn"