check if transfer target is a sibling agent

[AriYusa]

Link to Issue or Description of Change

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

• Closes: disallow_transfer_to_peers=True can be bypassed #3850

Problem:
disallow_transfer_to_peers=True only affected the instruction prompt. Subagents could still transfer tasks directly to sibling agents by inferring their names from conversation history or the user’s prompt.

Solution:
Added a check in _get_agent_to_run. When disallow_transfer_to_peers=True, the function now raises an error if a transfer to a sibling agent is attempted. I placed the check here because _get_agent_to_run already contains a closely related validation for transfers to non-existent agents.

Testing Plan

Unit Tests:

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

Manual End-to-End (E2E) Tests:

Manually tested before and after fix with adk web by prompting subagents to transfer directly to sibling and getting newly added error after fix

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. (not applicable)

[gemini-code-assist]

Summary of Changes

Hello @AriYusa, 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 addresses an inconsistency in the disallow_transfer_to_peers setting, which previously failed to prevent direct task transfers between sibling subagents. By introducing a robust validation check in the _get_agent_to_run method, the change ensures that this setting is fully enforced, preventing unintended agent interactions and enhancing control over agent behavior. The update includes new unit tests to verify the fix and maintain system integrity.

Highlights

• Enforced Peer Transfer Restriction: Implemented a new check within the _get_agent_to_run method to prevent subagents from transferring tasks directly to sibling agents when the disallow_transfer_to_peers setting is enabled. This closes a loophole where the restriction previously only applied to instruction prompts.
• New Unit Tests: Added a new test file, test_base_llm_flow_transfer.py, with comprehensive unit tests to validate the correct behavior of the _get_agent_to_run method, specifically covering scenarios where transfers to sibling agents are both allowed and disallowed.

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[adk-bot]

Response from ADK Triaging Agent

Hello @AriYusa, thank you for creating this PR!

This is a great fix and the testing plan is very clear. To help reviewers better understand the result of the manual E2E test, could you please provide a screenshot of the error that is raised in adk web after the fix is applied?

This information will help reviewers to review your PR more efficiently. Thanks!

[gemini-code-assist]

Code Review

This pull request correctly adds a check to prevent task transfers to sibling agents when disallow_transfer_to_peers is enabled. The implementation is straightforward and includes relevant unit tests. I've added one suggestion to refine the logic to correctly handle self-transfers, which should not be considered a transfer to a peer.

[ryanaiagent]

Hi @thank you for your contribution! We appreciate you taking the time to submit this pull request.
Can you also fix the failing unit tests before we proceed with the review.