AWS Bedrock LLM Plugin raises ValidationException when tool_choice=None

[timothyaufdermauer]

Bug Description

When the chat context contains tool calls, attempting to get an LLM response with tool_choice=None raises the ValidationException shown below. This will happen any time an agent handoff occurs with an agent object and string response both returned by the tool. Agent handoff sets draining=True and the string result triggers an LLM call on the old agent. When constructing the request for that LLM call, draining is True, which forces tool_choice=None.

I suspect this will also happen if session.generate_reply(tool_choice=None) is called when there are tool calls in chat context history already.

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/livekit/plugins/aws/llm.py", line 227, in _run
    response = await client.converse_stream(**self._opts)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/aiobotocore/context.py", line 36, in wrapper
    return await func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/aiobotocore/client.py", line 424, in _make_api_call
    raise error_class(parsed_response, operation_name)
botocore.errorfactory.ValidationException: An error occurred (ValidationException) when calling the ConverseStream operation: The toolConfig field must be defined when using toolUse and toolResult content blocks.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/livekit/agents/llm/fallback_adapter.py", line 176, in _try_generate
    async for chunk in stream:
  File "/usr/local/lib/python3.11/site-packages/livekit/agents/llm/llm.py", line 381, in __anext__
    raise exc  # noqa: B904
    ^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/livekit/agents/llm/llm.py", line 194, in _traceable_main_task
    await self._main_task()
  File "/usr/local/lib/python3.11/site-packages/livekit/agents/llm/llm.py", line 219, in _main_task
    return await self._run()
           ^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/livekit/plugins/aws/llm.py", line 246, in _run
    raise APIConnectionError(
livekit.agents._exceptions.APIConnectionError: aws bedrock llm: error generating content: An error occurred (ValidationException) when calling the ConverseStream operation: The toolConfig field must be defined when using toolUse and toolResult content blocks.

Expected Behavior

• Using tool_choice=None on a conversation with tool calls in the history succeeds.
• Returning an agent object and string result from a tool allows the old agent to generate its reply successfully

Reproduction Steps

from livekit import agents
from livekit.agents import AgentServer, AgentSession, Agent, room_io, function_tool, RunContext
from livekit.plugins import aws

class AssistantOne(Agent):
    def __init__(self) -> None:
        super().__init__(instructions="You are a helpful voice AI assistant.")

    @function_tool()
    async def handoff(
            self,
            context: RunContext,
    ):
        """Handoff to next agent
        """

        return AssistantTwo(), "Transferring the user to SomeAgent"

class AssistantTwo(Agent):
    def __init__(self) -> None:
        super().__init__(instructions="You are a helpful voice AI assistant.")

server = AgentServer()

@server.rtc_session(agent_name="my-agent")
async def my_agent(ctx: agents.JobContext):
    session = AgentSession(
        llm=aws.LLM(model="us.anthropic.claude-sonnet-4-6"),
    )
    session.output.set_audio_enabled(False)

    await session.start(
        room=ctx.room,
        agent=AssistantOne(),
        room_options=room_io.RoomOptions(),
    )

    await session.generate_reply(
        instructions="hand off to the next agent immediately."
    )


if __name__ == "__main__":
    agents.cli.run_app(server)

Operating System

Windows 11

Models Used

Claude Sonnet 4.6 in bedrock

Package Versions

livekit-agents: 1.5.6
livekit-plugins-aws: 1.5.6

Session/Room/Call IDs

No response

Proposed Solution

Additional Context

No response

Screenshots and Recordings

No response

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.