ParallelAgent works with LLMAgent but not with custom agents — is additional setup needed?

[shewalesantosh06-sant]

I'm working with the ADK Agents and have encountered an issue when using ParallelAgent with custom agents.

Context:
I have created 3 custom agents (not using LLMAgent) and grouped them under a ParallelAgent.

When running, I notice that these agents are executed sequentially(one after the other), not in parallel as expected.

However, if I replace the custom agents with LLMAgent instances, the parallel execution works as intended — the LLM calls are dispatched concurrently.

Questions:
Why does ParallelAgent support true parallel execution with LLMAgent but not with custom agents?

Is there any additional configuration or specific interface/behavior required for custom agents to be compatible with ParallelAgent and actually run in parallel?

parallel agent code :

parallel_extraction_agent = ParallelAgent(
name="parallel_extraction_agent",
description="This agent orchestrates multiple sub agents to extract specific information from the medical record",
sub_agents=[accommodations_extraction_agent, info_agent, synopsis_agent],
)