Feat: Google Search Agent Tool

[AmaadMartin]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

Problem: The built-in GoogleSearchTool cannot be used together with other tools in the same agent due to model limitations in Gemini 1.x. In adk-python, a workaround was implemented to automatically wrap the GoogleSearchTool in a specialized sub-agent (GoogleSearchAgentTool) when multiple tools are present, allowing it to function alongside others. This capability was missing in adk-js.

Solution: Ported the GoogleSearchAgentTool feature from adk-python to adk-js. This includes:

• Automatic wrapping of GoogleSearchTool in GoogleSearchAgentTool when bypassMultiToolsLimit is true and multiple tools are present.
• Propagation of grounding metadata from the sub-agent to the parent agent's session state.
• Proper integration with the model override configuration.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required for all PRs that are not small documentation or typo fixes.

Unit Tests:

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

I ran the specific test file core/test/tools/google_search_agent_tool_test.ts and all 13 tests passed.
Coverage for core/src/tools/google_search_agent_tool.ts and core/src/tools/google_search_tool.ts achieved 100%.

Integration Tests (Serving as Manual E2E):
An integration test was created at tests/integration/tools/google_search_agent_tool_test.ts to simulate the full flow of automatic wrapping and grounding metadata propagation using mocked responses. This test serves as the verification for the end-to-end behavior, as manual testing with live models was not performed.

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.

[kalenkevich]

please do not use inline imports

[AmaadMartin]

removed the inline import

[ScottMansfield]

A couple comments otherwise LGTM

[ScottMansfield]

let's add specifically when this is needed and the bug to remove this when it's no longer needed.

[AmaadMartin]

Done. I've updated the JSDoc for GoogleSearchAgentTool to clarify that it's used as a workaround when bypassMultiToolsLimit is enabled and multiple tools are configured, due to Gemini models not supporting built-in search tools alongside custom tools. I also added a reference to the tracking bug b/448114567 to remove it when no longer needed.

[ScottMansfield]

Is this the expected API based on the Python implementation? How obvious is it to users if they misconfigure this?

[AmaadMartin]

Yes, this matches the Python implementation. In Python, GoogleSearchTool accepts bypass_multi_tools_limit: bool = False.
If users misconfigure this (i.e., they have multiple tools but don't set bypassMultiToolsLimit: true):

• TypeScript users will get a compile-time error if they misspell the option name, thanks to static typing.
• At runtime (both JS and TS):
  • If they are using a Gemini 1.x model, ADK will throw an explicit, local error before making the API call: "Google search tool can not be used with other
    tools in Gemini 1.x.".
  • If they are using a Gemini >=2.x model, ADK will pass both tools to the platform (since Gemini 2.x often supports this natively). However, if they hit a platform-side limitation (e.g., in certain enterprise configurations or stateful APIs), the Gemini API will return a standard 400 Bad Request error,typically stating: "Builtin tool 'google_search' cannot be combined with other tools." So it should be very obvious to users via compile-time checks, local ADK validation errors (for 1.x), or explicit API platform errors (for 2.x).