microsoft/autogen · error · RuntimeError

McpWorkbench is not started. Call start() first.

Error message

McpWorkbench is not started. Call start() first.

What it means

Error "McpWorkbench is not started. Call start() first." thrown in microsoft/autogen.

Source

Thrown at python/packages/autogen-ext/src/autogen_ext/tools/mcp/_workbench.py:482

                UserWarning,
                stacklevel=2,
            )
            return  # Already initialized, no need to start again

        if isinstance(self._server_params, (StdioServerParams, SseServerParams, StreamableHttpServerParams)):
            self._actor = McpSessionActor(self._server_params, host=self._host)
            await self._actor.initialize()
            self._actor_loop = asyncio.get_event_loop()
        else:
            raise ValueError(f"Unsupported server params type: {type(self._server_params)}")

    async def stop(self) -> None:
        if self._actor:
            # Close the actor
            await self._actor.close()
            self._actor = None
        else:
            raise RuntimeError("McpWorkbench is not started. Call start() first.")

    async def reset(self) -> None:
        pass

    async def save_state(self) -> Mapping[str, Any]:
        return McpWorkbenchState().model_dump()

    async def load_state(self, state: Mapping[str, Any]) -> None:
        pass

    def _to_config(self) -> McpWorkbenchConfig:
        host_config = self._host.dump_component() if self._host else None
        return McpWorkbenchConfig(
            server_params=self._server_params, tool_overrides=self._tool_overrides, host=host_config
        )

    @classmethod
    def _from_config(cls, config: McpWorkbenchConfig) -> Self:

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Call start() first

Example fix

await workbench.start()

When it happens

Trigger: Thrown at python/packages/autogen-ext/src/autogen_ext/tools/mcp/_workbench.py:482 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/528d8f42d64ec9a9. Report an issue: GitHub.