Tool Hallucination when using ContextWindowCompression and BIDI Live

[GrahamGGreig]

Running into an issue when using ContextWindowCompression whereby I see agents hallucinate tools they do not have access to. This is a transient issue and can be hard to reproduce. I have only ever seen this when using context window compression and it does not happen otherwise (Instead I hit a context overflow error).

I have two agents defined as such (note: I'm excluding some context for brevity):

def get_general_responder_agent(live_api: bool = False):
    return LlmAgent(
        name=NAME,
        model=select_model(live_api),
        description=get_description(),
        instruction=get_instructions(),
        planner=_get_planner(),
        tools=[get_context_tool()],
        generate_content_config=_get_generate_content_config(),
    )

def get_persona_responder_agent(live_api: bool = False):
    return LlmAgent(
        name=NAME,
        model=select_model(live_api),
        description=get_description(),
        instruction=get_instructions(),
        tools=[get_persona_tool()],
        generate_content_config=_get_generate_content_config(),
    )

def get_context_tool():
    return FunctionTool(func=set_context) # function definition left out for brevity

def get_persona_tool():
    return FunctionTool(func=set_persona_instructions) # function definition left out for brevity

def configure_root_agent(live_api: bool = False) -> LlmAgent:
    # agents
    general_responder = get_general_responder_agent(live_api)
    general_responder = configure_user_facing_agent(general_responder)

    persona_responder = get_persona_responder_agent(live_api)
    persona_responder = configure_user_facing_agent(persona_responder)

    # routing
    general_responder.sub_agents = [persona_responder]
    persona_responder.parent_agent = general_responder

    general_responder.before_agent_callback = initialize_state_variables

    return general_responder


def initialize_state_variables(
    callback_context: CallbackContext,
) -> Optional[types.Content]:
    callback_context = initialize_language_selector_state_variables(callback_context)
    callback_context = initialize_persona_state_variables(callback_context)
    callback_context = initialize_context_state_variables(callback_context)


def configure_user_facing_agent(agent: LlmAgent) -> LlmAgent:
    agent.tools.extend(get_default_tools())
    agent.instruction += language_instructions()
    agent.instruction += general_instructions()

    return agent

def get_default_tools():
    return [
        get_general_user_assistance_tool(),
        get_language_selector_tool(),
    ]

and using run config:

    run_config = RunConfig(
        streaming_mode=StreamingMode.BIDI,
        response_modalities=[types.Modality.AUDIO],
        speech_config=types.SpeechConfig(
            voice_config=types.VoiceConfig(
                prebuilt_voice_config=types.PrebuiltVoiceConfig(voice_name=VOICE_NAME)
            )
        ),
        output_audio_transcription=types.AudioTranscriptionConfig(),
        input_audio_transcription=types.AudioTranscriptionConfig(),
        session_resumption=types.SessionResumptionConfig(),
        save_live_blob=False,
        context_window_compression=types.ContextWindowCompressionConfig(
            trigger_tokens=60000, sliding_window=types.SlidingWindow() # Our application currently draws in a lot of contexts. I imagine this would need to be smaller to trigger on a simpler application.
        )
    )

Using the model: gemini-live-2.5-flash through Vertex AI. Using ADK 1.22.0. The error I can transiently get is:

Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/base_llm_flow.py", line 199, in run_live
    async for item in agen:
      yield item
  File "/usr/local/lib/python3.13/site-packages/google/adk/agents/base_agent.py", line 327, in run_live
    async for event in agen:
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/agents/llm_agent.py", line 492, in _run_live_impl
    async for event in agen:
      self.__maybe_save_output_to_state(event)
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/base_llm_flow.py", line 199, in run_live
    async for item in agen:
      yield item
  File "/usr/local/lib/python3.13/site-packages/google/adk/agents/base_agent.py", line 327, in run_live
    async for event in agen:
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/agents/llm_agent.py", line 492, in _run_live_impl
    async for event in agen:
      self.__maybe_save_output_to_state(event)
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/base_llm_flow.py", line 199, in run_live
    async for item in agen:
      yield item
  File "/usr/local/lib/python3.13/site-packages/google/adk/agents/base_agent.py", line 327, in run_live
    async for event in agen:
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/agents/llm_agent.py", line 492, in _run_live_impl
    async for event in agen:
      self.__maybe_save_output_to_state(event)
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/base_llm_flow.py", line 152, in run_live
    async for event in agen:
    ...<59 lines>...
        return
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/base_llm_flow.py", line 329, in _receive_from_model
    async for event in agen:
    ...<19 lines>...
      yield event
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/base_llm_flow.py", line 643, in _postprocess_live
    function_response_event = await functions.handle_function_calls_live(
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        invocation_context, model_response_event, llm_request.tools_dict
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/functions.py", line 476, in handle_function_calls_live
    function_response_events = await asyncio.gather(*tasks)
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/functions.py", line 509, in _execute_single_function_call_live
    tool, tool_context = _get_tool_and_context(
                         ~~~~~~~~~~~~~~~~~~~~~^
        invocation_context, function_call, tools_dict
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/functions.py", line 754, in _get_tool_and_context
    tool = _get_tool(function_call, tools_dict)
  File "/usr/local/lib/python3.13/site-packages/google/adk/flows/llm_flows/functions.py", line 729, in _get_tool
    raise ValueError(error_msg)
ValueError: Tool 'set_context' not found.
Available tools: transfer_to_agent, set_persona_instructions, general_user_assistance_tool, Language_Logging_Tool

Possible causes:
  1. LLM hallucinated the function name - review agent instruction clarity
  2. Tool not registered - verify agent.tools list
  3. Name mismatch - check for typos

Suggested fixes:
  - Review agent instruction to ensure tool usage is clear
  - Verify tool is included in agent.tools list
  - Check for typos in function name
2026-01-16 21:57:33,696 - ERROR - Exception in agent_to_client_sse: Tool 'set_context' not found.

To Reproduce
Run an application with multiple agents where they each have different tools. Interact with the agents using the live api model (gemini-live-2.5-flash through Vertex AI). Have a long enough conversation that context is reset. Ask questions that require the tool use of the opposite agent. See if it hallucinates the tool instead of performing an agent transfer.

Expected behavior
To transfer to the correct agent that has the tool instead of hallucinating the tool.

Desktop (please complete the following information):

• OS: Linux container
• Python version(python -V): 3.13.7
• ADK version(pip show google-adk): 1.22.1

Model Information:

• Are you using LiteLLM: No
• Which model is being used(e.g. gemini-2.5-pro): gemini-live-2.5-flash through VertexAI

[klateefa]

@GrahamGGreig Thanks for reporting
This issue originates from the ADK runtime rather than model behavior or user configuration.

When ContextWindowCompression (SlidingWindow) is applied, the model may carry forward references to tools that were valid earlier in the session. However, after an agent transition, the active agent’s tools_dict is rebuilt and no longer includes those tools. As a result, the model can emit a function call that is no longer executable in the current agent context, causing _get_tool to fail.
The fix is to enforce tool ownership at execution time. If a requested tool is not available on the active agent, the flow should redirect via agent transfer instead of raising an error,this restores correct multi-agent routing semantics and ensures safe use of context compression in BIDI live mode.
Thank you.