fix: filter non-agent directoris from list_agents()

[markadelnawar]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: list_agents() returns non-agent directories in /list-apps endpoint #4647
• Related: Detailed List Apps #3429, feat(web): add list-apps-detailed endpoint #3430

2. Or, if no issue exists, describe the change:

Problem:
AgentLoader.list_agents() returns every non-hidden subdirectory in the agents directory, regardless of whether it contains a valid agent definition. This causes non-agent directories (e.g. tmp/, data/, utils/) to appear in the /list-apps API response. This affects both the ADK web UI agent selector and any production deployment depending on this API.

Solution:
Reuse the existing _determine_agent_language() method inside list_agents() to verify each candidate directory contains at least one recognized agent file (root_agent.yaml, agent.py, or __init__.py). Directories that fail this check are excluded from the result. This avoids introducing any new methods or abstractions and keeps the check lightweight (filesystem only, no agent imports).

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

27 passed in 2.85s:
pytest tests/unittests/cli/utils/test_agent_loader.py -v
======================= 27 passed, 14 warnings in 2.85s ========================

Added test_list_agents_excludes_non_agent_directories which creates a temp directory with three valid agent types (package with __init__.py, module with agent.py, YAML with root_agent.yaml) and three non-agent directories, and asserts only the valid agents are listed.

Screenshots / Video:

Before (non-agent directories listed)	After (only valid agents listed)

Manual End-to-End (E2E) Tests:

1. Create a project directory containing both valid agent subdirectories and non-agent subdirectories
2. Run adk web .
3. Open the web UI and verify only valid agents appear in the agent selector
4. See screenshots below for before/after comparison

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the agent discovery mechanism by ensuring that the list_agents function only returns directories that genuinely contain agent definitions. This change improves the accuracy of agent listings in the ADK web UI and API, preventing the display of irrelevant or invalid entries and enhancing the user experience.

Highlights

• Problem Addressed: The AgentLoader.list_agents() method previously returned all non-hidden subdirectories, including those that did not contain valid agent definitions, leading to incorrect listings in the ADK web UI and API.
• Solution Implemented: The list_agents() method now reuses the existing _determine_agent_language() method to verify if a directory contains a valid agent definition (e.g., root_agent.yaml, agent.py, or __init__.py). Directories failing this check are excluded.
• Testing: A new unit test, test_list_agents_excludes_non_agent_directories, was added to confirm that list_agents correctly filters out non-agent directories while including valid ones. Manual end-to-end tests were also performed and screenshots provided.

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Changelog

• src/google/adk/cli/utils/agent_loader.py
  • Refactored list_agents method from a list comprehension to an explicit loop.
  • Integrated a try-except ValueError block to call _determine_agent_language for each directory, filtering out those that do not represent valid agents.
• tests/unittests/cli/utils/test_agent_loader.py
  • Added a new unit test test_list_agents_excludes_non_agent_directories.
  • The test creates a temporary directory structure with various valid and invalid agent configurations.
  • Assertions confirm that list_agents correctly identifies and returns only the valid agent directories.

Activity

• The author identified a problem where AgentLoader.list_agents() was returning non-agent directories.
• A solution was proposed to reuse _determine_agent_language() for validation.
• Unit tests were added and updated to cover the new filtering logic.
• All unit tests passed locally.
• Manual end-to-end tests were conducted, and screenshots were provided to demonstrate the fix.
• The author performed a self-review and confirmed adherence to contribution guidelines.

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Code Review

This pull request correctly filters out non-agent directories from list_agents() by checking for valid agent definition files. The implementation is sound and is accompanied by a good unit test. I have a couple of suggestions to improve maintainability and strengthen the test assertions.