microsoft/semantic-kernel · error · ValueError

Only FunctionChoiceType.AUTO is supported.

Error message

Only FunctionChoiceType.AUTO is supported.

What it means

Raised by the Pydantic field_validator on BedrockAgentBase.function_choice_behavior. Amazon Bedrock agents always decide on their own whether to call functions; the FunctionChoiceBehavior in Semantic Kernel only controls whether the kernel auto-executes those requests. Therefore only FunctionChoiceType.AUTO is permitted — REQUIRED and NONE have no meaning for Bedrock and are rejected.

Source

Thrown at python/semantic_kernel/agents/bedrock/bedrock_agent_base.py:91

            "bedrock_client": bedrock_client or boto3.client("bedrock-agent"),
            **kwargs,
        }
        if function_choice_behavior:
            args["function_choice_behavior"] = function_choice_behavior

        super().__init__(**args)

    @field_validator("function_choice_behavior", mode="after")
    @classmethod
    def validate_function_choice_behavior(
        cls, function_choice_behavior: FunctionChoiceBehavior | None
    ) -> FunctionChoiceBehavior | None:
        """Validate the function choice behavior."""
        if function_choice_behavior and function_choice_behavior.type_ != FunctionChoiceType.AUTO:
            # Users cannot specify REQUIRED or NONE for the Bedrock agents.
            # Please note that the function choice behavior only control if the kernel will automatically
            # execute the functions the agent requests. It does not control the behavior of the agent.
            raise ValueError("Only FunctionChoiceType.AUTO is supported.")
        return function_choice_behavior

    def __repr__(self):
        """Return the string representation of the Bedrock Agent."""
        return f"{self.agent_model}"

    # region Agent Management

    async def prepare_agent_and_wait_until_prepared(self) -> None:
        """Prepare the agent for use."""
        if not self.agent_model.agent_id:
            raise ValueError("Agent does not exist. Please create the agent before preparing it.")

        try:
            await run_in_executor(
                None,
                partial(
                    self.bedrock_client.prepare_agent,

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Use FunctionChoiceBehavior.Auto() (the default) for BedrockAgent, or omit the argument entirely.
2. Understand that for Bedrock, function calling is always agent-driven; you only control auto-execution via AUTO.
3. Remove any Required/None behavior objects when switching to BedrockAgent.

Example fix

// before
agent = BedrockAgent(
    model,
    function_choice_behavior=FunctionChoiceBehavior.Required(),  # rejected
)

// after
agent = BedrockAgent(
    model,
    function_choice_behavior=FunctionChoiceBehavior.Auto(),  # or omit entirely
)

from semantic_kernel.connectors.ai.function_choice_behavior import FunctionChoiceBehavior, FunctionChoiceType

def assert_auto_only(fcb) -> None:
    if fcb is not None and fcb.type_ != FunctionChoiceType.AUTO:
        raise ValueError(
            "BedrockAgent only supports FunctionChoiceType.AUTO; "
            f"got {fcb.type_}. Use FunctionChoiceBehavior.Auto()."
        )

from semantic_kernel.connectors.ai.function_choice_behavior import FunctionChoiceBehavior, FunctionChoiceType

def is_bedrock_compatible_fcb(fcb) -> bool:
    return fcb is None or fcb.type_ == FunctionChoiceType.AUTO

try:
    agent = BedrockAgent(model, function_choice_behavior=fcb)
except ValueError as e:
    if "Only FunctionChoiceType.AUTO is supported" in str(e):
        agent = BedrockAgent(model, function_choice_behavior=FunctionChoiceBehavior.Auto())
    else:
        raise

When it happens

Trigger: Triggered during model validation when constructing BedrockAgent or BedrockAgentBase with function_choice_behavior=FunctionChoiceBehavior.Required() or FunctionChoiceBehavior.None().

Common situations: Porting config from a ChatCompletionAgent that used Required/None; explicitly setting function_choice_behavior assuming it controls agent behavior; copying a FunctionChoiceBehavior instance across agent types.

Related errors

• No function results were returned.
• The Bedrock agent requires a message to be invoked.
• The streaming configuration must be null for non-streaming r
• The streaming configuration must have StreamFinalResponse se
• Bedrock agents must be invoked with a user message

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/bba4f04754ea9c20. Report an issue: GitHub.