Bug: Sub-agent routing succeeds but execution fails when delegated agent uses GoogleSearchTool

[vishvas-chauhan]

🔴 Required Information

Describe the Bug:

When using Google ADK multi-agent routing where a root router agent delegates to a specialist sub-agent, routing works correctly (transfer_to_agent is emitted), but execution fails when the selected sub-agent uses GoogleSearchTool.

This occurs when another sub-agent uses custom/MCP tools. The failure appears related to tool compatibility validation between heterogeneous tool types (custom/MCP tools vs built-in Google search).

I would like to confirm whether this scenario is expected to work or if it is a known limitation of the framework.

---

Steps to Reproduce:

1. Install ADK

pip install google-adk

2. Run toolbox server providing MCP tools

./toolbox --tools-file tools.yml --port 5000

3. Define agents

from google.adk.agents import LlmAgent, Agent
from google.adk.tools.google_search_tool import GoogleSearchTool
from toolbox_sync import ToolboxSyncClient

_toolset = None

def _load_tools():
    global _toolset
    if _toolset is None:
        client = ToolboxSyncClient("http://127.0.0.1:5000")
        _toolset = client.load_toolset("rm-demo-toolset")
    return _toolset

rm_agent = LlmAgent(
    name="rm_agent",
    model="gemini-2.5-flash",
    tools=_load_tools(),
    instruction="Handle customer queries"
)

etf_agent_2 = LlmAgent(
    name="etf_agent_2",
    model="gemini-2.5-flash",
    tools=[GoogleSearchTool()],
    instruction="Answer ETF questions using web search"
)

router_root_agent = Agent(
    name="root_router",
    model="gemini-2.5-flash",
    sub_agents=[rm_agent, etf_agent_2],
    instruction="""
Route queries:
- Customers → rm_agent
- ETFs → etf_agent_2
Call only one agent.
"""
)

4. Execute runner with ETF query

tell me about this etf VanEck Gold Miners UCITS ETF

5. Observe failure

---

Expected Behavior:

• Router selects etf_agent_2
• ETF agent executes normally
• Google search tool runs
• Final response returned

---

Observed Behavior:

Routing succeeds:

Tool Call: transfer_to_agent {'agent_name': 'etf_agent_2'}
Tool Result: {'result': None}

Execution fails:

400 INVALID_ARGUMENT
Multiple tools are supported only when they are all search tools.

---

Environment Details:

• ADK Library Version (pip show google-adk): 1.24.0
• Desktop OS: Linux (Google Colab)
  Kernel: Linux 4c63b968cc70 6.6.105+ x86_64
• Python Version (python -V): 3.12.12

---

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.5-flash

---

🟡 Optional Information

Regression:

Not tested on earlier versions.

---

Logs:

Tool Call: transfer_to_agent {'agent_name': 'etf_agent_2'}
Tool Result: {'result': None}

Sub-agent error:
<class 'google.genai.errors.ClientError'>
400 INVALID_ARGUMENT
Multiple tools are supported only when they are all search tools.

---

Screenshots / Video:

N/A

---

Additional Context:

This scenario works when both agents are executed using ParallelAgent, but that executes unnecessary agents.
The issue occurs specifically when hierarchical routing (sub_agents) is used and execution is delegated to a sub-agent that uses GoogleSearchTool.

---

Minimal Reproduction Code:

(Provided above in Steps to Reproduce)

---

How often has this issue occurred?:

• Always (100%)

[llalitkumarrr]

Hello @vishvas-chauhan ,

We have investigated this issue and observed an error when calling google_search within a sub-agent, even with a setup of one tool in the sub-agent and only one sub-agent.

As a workaround, please try wrapping your sub-agent (which uses the google_search tool) within an AgentTool and then provide this AgentTool as tools to your root_agent.

Please let us know if this helps resolve your problem.

[vishvas-chauhan]

Hello @llalitkumarrr -> It worked, if may ask, ADK pickups my all agents logic even if i have not added them in Runner agent
For example, my router agent is router_root_agent,
router_root_agent = Agent(
name="root_router",
model=GEMINI_MODEL,

sub_agents=[rm_customer_agent, etf_agent_2], #etf_agent_2 etf_tool_agent

instruction="""

You are a supervisor router.

Choose the best specialist agent from two below:

• rm_customer_agent → customers, meetings, transactions
• etf_tool_agent → ETFs, funds, ISIN, tickers

Call ONLY the relevant agent.
Do not answer directly unless trivial.
"""
)

and my runner agent is rm_customer_agent
_runner = Runner(
agent=rm_customer_agent,

My logs always generated with transfer_to_agent , How it logs my router_root_agent logics, which i never intended to read.