Using LoopAgent causes Attribute Error in agent_transfer code

[avlaskin]

🔴 Required Information

Describe the Bug:
Any agent with LoopAgent will cause Attribute Error with ADK 2.1.0 when run using
runner API and root_agent is LlmAgent.

Steps to Reproduce:
Please provide a numbered list of steps to reproduce the behavior:

1. Install ADK==2.1.0
2. Run agent where root_agent has on of the subagents - LoopAgent.
3. See the error:

  File "/home/alex/env3/lib/python3.13/site-packages/google/adk/flows/llm_flows/agent_transfer.py", line 196, in _get_transfer_targets
    and peer_agent.mode not in ('single_turn', 'task')
        ^^^^^^^^^^^^^^^
  File "/home/alex/env3/lib/python3.13/site-packages/pydantic/main.py", line 1042, in __getattr__
    raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}')
AttributeError: 'LoopAgent' object has no attribute 'mode'

Expected Behavior:
LoopAgent should work like in version 1.34.1.

Observed Behavior:
What actually happened? Include error messages or crash stack traces here.

Environment Details:

• ADK Library Version (pip show google-adk): 2.1.0
• Desktop OS:** Ubuntu/Debian
• Python Version (python -V): 3.13

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-3.5-flash

Fix
Do hasattr check in the line 192 similar to the code above that.

  if not agent.disallow_transfer_to_peers:
    result.extend([
        peer_agent
        for peer_agent in agent.parent_agent.sub_agents
        if peer_agent.name != agent.name
        and (not hasattr(peer_agent, 'mode')    #  <-- Add guard here
             or peer_agent.mode not in ('single_turn', 'task'))
    ])

How often has this issue occurred?:

• Always (100%)

[sanketpatil06]

Hi @avlaskin , Thanks for reporting this.
Could you please provide a minimal reproducible code snippet? This will allow us to investigate the issue accurately.