Streaming tools example in the documentation does not work

[alvarossl]

Describe the bug
The streaming tool example in https://google.github.io/adk-docs/streaming/streaming-tools/ returns errors on tool invokation.

To Reproduce

Steps to reproduce the behavior:
Run adk create streaming-tool-example
Take the code from the documentation and replace in agent.py file (adding right imports, API key etc)
Run adk web

Error 1 - running the example as is

ValueError: Failed to parse the parameter return_value: AsyncGenerator[str, NoneType] of function monitor_stock_price for automatic function calling.Automatic function calling works best with simpler function signature schema, consider manually parsing your function declaration for function monitor_stock_price.

Error 2 - removing type hinting from the monitor_stock_price function

async def monitor_stock_price(stock_symbol: str) -> AsyncGenerator[str, None]:
with
async def monitor_stock_price(stock_symbol: str):

Running adk web and requesting the agent eg "provide stock price for GOOGL" returns the following error:

Unable to serialize unknown type: <class 'async_generator'>.
TypeError: cannot pickle 'async_generator'

Desktop (please complete the following information):

• OS: macOS
• Python version(python -V): Python 3.11.14
• ADK version(pip show google-adk): 1.21.0 , 1.18.0 (tested both)

Model Information:

• Are you using LiteLLM: No
• gemini-2.0-flash-exp

[alvarossl]

More detail on error 2:

2025-12-18 11:53:36,801 - ERROR - adk_web_server.py:1509 - Error in event_generator: Unable to serialize unknown type: <class 'async_generator'>
Traceback (most recent call last):
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/google/adk/cli/adk_web_server.py", line 1499, in event_generator
async for event in agen:
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/google/adk/runners.py", line 505, in run_async
async for event in agen:
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/google/adk/runners.py", line 493, in _run_with_trace
async for event in agen:
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/google/adk/runners.py", line 769, in _exec_with_plugin
await self.session_service.append_event(
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/google/adk/cli/utils/local_storage.py", line 200, in append_event
return await service.append_event(session, event)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/google/adk/sessions/sqlite_session_service.py", line 392, in append_event
event.model_dump_json(exclude_none=True),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/anaconda3/envs/agent-conda/lib/python3.11/site-packages/pydantic/main.py", line 528, in model_dump_json
return self.pydantic_serializer.to_json(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.PydanticSerializationError: Unable to serialize unknown type: <class 'async_generator'>

[llalitkumarrr]

Hello @alvarossl ,

Thank you for bringing this to our attention. We have reproduced the issue and observed the same error. We are currently investigating it in detail. In the meantime, could you please check if Bidi Streamer meets your requirements?

[morganroux]

Hello @llalitkumarrr
Jumping right from the ADK community call, did your team already start working on this ?
I can have a look as a first issue.

[morganroux]

After investigating, I found the root cause: adk web doesn't call the /run_live endpoint when using text input. As a result, streaming tools are not processed correctly.

Quick workaround: Using the microphone button and speaking to the agent (instead of typing) triggers the correct /run_live endpoint, which properly showcases the example.

Proposed solutions:

1. Documentation update: Clarify that this example currently only works with the live-audio option, and direct users to the bidi-demo sample for reference.

2. Feature enhancement: I can dig into implementing text-based live streaming in adk web, similar to how it's done in the bidi-demo, to enable the /run_live endpoint for text input.

What do you think @llalitkumarrr?

[llalitkumarrr]

Hello @morganroux ,

Thank you for taking time to dig deep into this, you are welcome to raise PR with your proposed solution. Our team will review it and merge once all checks are cleared.