microsoft/autogen · error · ValueError

Multiple system messages are not supported

Error message

Multiple system messages are not supported

What it means

Error "Multiple system messages are not supported" thrown in microsoft/autogen.

Source

Thrown at python/packages/autogen-ext/src/autogen_ext/models/anthropic/_anthropic_client.py:595

            if json_output is True:
                create_args["response_format"] = {"type": "json_object"}
            elif isinstance(json_output, type):
                raise ValueError("Structured output is currently not supported for Anthropic models")

        # Process system message separately
        system_message = None
        anthropic_messages: List[MessageParam] = []

        # Merge continuous system messages into a single message
        messages = self._merge_system_messages(messages)
        messages = self._rstrip_last_assistant_message(messages)

        for message in messages:
            if isinstance(message, SystemMessage):
                if system_message is not None:
                    # if that case, system message is must only one
                    raise ValueError("Multiple system messages are not supported")
                system_message = to_anthropic_type(message)
            else:
                anthropic_message = to_anthropic_type(message)
                if isinstance(anthropic_message, list):
                    anthropic_messages.extend(anthropic_message)
                elif isinstance(anthropic_message, str):
                    msg = MessageParam(
                        role="user" if isinstance(message, UserMessage) else "assistant", content=anthropic_message
                    )
                    anthropic_messages.append(msg)
                else:
                    anthropic_messages.append(anthropic_message)

        # Check for function calling support
        if self.model_info["function_calling"] is False and len(tools) > 0:
            raise ValueError("Model does not support function calling")

        # Set up the request

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Consolidate into a single system message

Example fix

Merge all system messages into one

When it happens

Trigger: Thrown at python/packages/autogen-ext/src/autogen_ext/models/anthropic/_anthropic_client.py:595 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/dd2bfa2416d2d428. Report an issue: GitHub.