microsoft/autogen · error · TypeError

Expected return value of type {self._return_type.__name__},

Error message

Expected return value of type {self._return_type.__name__}, but got {type(return_value).__name__}

What it means

Error "Expected return value of type {self._return_type.__name__}, but got {type(return_value).__name__}" thrown in microsoft/autogen.

Source

Thrown at python/packages/autogen-core/src/autogen_core/tools/_base.py:257

            call_id (str | None): An optional identifier for the tool call, used for tracing.

        Returns:
            AsyncGenerator[StreamT | ReturnT, None]: A generator yielding results from the tool's :meth:`run_stream` method.
        """
        return_value: ReturnT | StreamT | None = None
        with trace_tool_span(
            tool_name=self._name,
            tool_description=self._description,
            tool_call_id=call_id,
        ):
            # Execute the tool's run_stream method
            async for result in self.run_stream(self._args_type.model_validate(args), cancellation_token):
                return_value = result
                yield result

        assert return_value is not None, "The tool must yield a final return value at the end of the stream."
        if not isinstance(return_value, self._return_type):
            raise TypeError(
                f"Expected return value of type {self._return_type.__name__}, but got {type(return_value).__name__}"
            )

        # Log the tool call event
        event = ToolCallEvent(
            tool_name=self.name,
            arguments=dict(args),  # Using the raw args passed to run_json
            result=self.return_value_as_string(return_value),
        )
        logger.info(event)


class BaseToolWithState(BaseTool[ArgsT, ReturnT], ABC, Generic[ArgsT, ReturnT, StateT], ComponentBase[BaseModel]):
    def __init__(
        self,
        args_type: Type[ArgsT],
        return_type: Type[ReturnT],
        state_type: Type[StateT],

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Return a value matching the tool's declared return type

Example fix

Cast/convert the return value to the declared type

When it happens

Trigger: Thrown at python/packages/autogen-core/src/autogen_core/tools/_base.py:257 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/53b8e466cf80c541. Report an issue: GitHub.