Mark Adel focused on backend reliability in the google/adk-python repository, addressing a bug in the agent discovery process. He enhanced the list_agents() API by filtering out non-agent directories, ensuring only valid agents appear in both the API response and the web UI selector. Leveraging Python for backend development and unit testing, Mark reused the existing _determine_agent_language() logic to verify agent validity based on recognized files. He expanded and updated unit tests to cover the new logic, with all tests passing locally, and performed manual end-to-end validation. This targeted fix improved the integrity and correctness of agent listing functionality.