microsoft/autogen · error · ValueError

The model does not support function calling, which is needed

Error message

The model does not support function calling, which is needed for handoffs.

What it means

AssistantAgent requires function-calling capability when handoffs are configured, because handoffs are implemented as tools the model must be able to call. If model_client.model_info['function_calling'] is False while handoffs is not None, construction fails with this ValueError.

Source

Thrown at python/packages/autogen-agentchat/src/autogen_agentchat/agents/_assistant_agent.py:796

                elif callable(tool):
                    if hasattr(tool, "__doc__") and tool.__doc__ is not None:
                        description = tool.__doc__
                    else:
                        description = ""
                    self._tools.append(FunctionTool(tool, description=description))
                else:
                    raise ValueError(f"Unsupported tool type: {type(tool)}")
        # Check if tool names are unique.
        tool_names = [tool.name for tool in self._tools]
        if len(tool_names) != len(set(tool_names)):
            raise ValueError(f"Tool names must be unique: {tool_names}")

        # Handoff tools.
        self._handoff_tools: List[BaseTool[Any, Any]] = []
        self._handoffs: Dict[str, HandoffBase] = {}
        if handoffs is not None:
            if model_client.model_info["function_calling"] is False:
                raise ValueError("The model does not support function calling, which is needed for handoffs.")
            for handoff in handoffs:
                if isinstance(handoff, str):
                    handoff = HandoffBase(target=handoff)
                if isinstance(handoff, HandoffBase):
                    self._handoff_tools.append(handoff.handoff_tool)
                    self._handoffs[handoff.name] = handoff
                else:
                    raise ValueError(f"Unsupported handoff type: {type(handoff)}")
        # Check if handoff tool names are unique.
        handoff_tool_names = [tool.name for tool in self._handoff_tools]
        if len(handoff_tool_names) != len(set(handoff_tool_names)):
            raise ValueError(f"Handoff names must be unique: {handoff_tool_names}")
        # Create sets for faster lookup
        tool_names_set = set(tool_names)
        handoff_tool_names_set = set(handoff_tool_names)

        # Check if there's any overlap between handoff tool names and tool names
        overlap = tool_names_set.intersection(handoff_tool_names_set)

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Use a model that supports function calling for agents with handoffs.
2. Correct model_info to function_calling=True if the model does support it.
3. Remove the handoffs parameter if tool calling is unavailable.

Example fix

# before
client = OpenAIChatCompletionClient(model="legacy", model_info={"function_calling": False, ...})
agent = AssistantAgent(name="a", model_client=client, handoffs=["b"])

# after
client = OpenAIChatCompletionClient(model="gpt-4o", model_info={"function_calling": True, ...})
agent = AssistantAgent(name="a", model_client=client, handoffs=["b"])

if handoffs and not model_client.model_info.get("function_calling", False):
    raise ValueError("handoffs require a function-calling capable model")

def supports_handoffs(client) -> bool:
    return bool(client.model_info.get("function_calling", False))

try:
    agent = AssistantAgent(name="a", model_client=client, handoffs=handoffs)
except ValueError as e:
    if "needed for handoffs" in str(e):
        agent = AssistantAgent(name="a", model_client=client)  # single-agent fallback
    else:
        raise

When it happens

Trigger: Passing handoffs=[...] together with a model client whose model_info declares function_calling=False (local models, legacy models, or hand-written model_info).

Common situations: Trying to build multi-agent handoff patterns on non-tool-calling models, mis-declared model_info for capable models, or downgrading the model without removing the handoffs configuration.

Related errors

• The model does not support function calling.
• Unsupported handoff type: {type(handoff)}
• Handoff names must be unique: {handoff_tool_names}
• Handoff names must be unique from tool names
• Expected Memory, List[Memory], or None, got {type(memory)}

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/e9628c77df00d99c. Report an issue: GitHub.