microsoft/autogen · error · NotImplementedException

The method or operation is not implemented.

Error message

The method or operation is not implemented.

What it means

While flattening a MultiModalMessage's content parts into OpenAI ChatMessageContentParts, ProcessMultiModalMessage only knows TextMessage and ImageMessage. Any other IMessage inside the multimodal collection (e.g. a ToolCallMessage or AudioMessage) hits the default arm and throws NotImplementedException.

Source

Thrown at dotnet/src/AutoGen.OpenAI/Middleware/OpenAIChatRequestMessageConnector.cs:289

        }

        var imageContentItem = this.CreateChatMessageImageContentItemFromImageMessage(message);
        return [new UserChatMessage([imageContentItem]) { ParticipantName = message.From }];
    }

    private IEnumerable<ChatMessage> ProcessMultiModalMessage(IAgent agent, MultiModalMessage message)
    {
        if (agent.Name == message.From)
        {
            // image message from assistant is not supported
            throw new ArgumentException("MultiModalMessage is not supported when message.From is the same with agent");
        }

        IEnumerable<ChatMessageContentPart> items = message.Content.Select<IMessage, ChatMessageContentPart>(ci => ci switch
        {
            TextMessage text => ChatMessageContentPart.CreateTextPart(text.Content),
            ImageMessage image => this.CreateChatMessageImageContentItemFromImageMessage(image),
            _ => throw new NotImplementedException(),
        });

        return [new UserChatMessage(items) { ParticipantName = message.From }];
    }

    private ChatMessageContentPart CreateChatMessageImageContentItemFromImageMessage(ImageMessage message)
    {
        return message.Data is null && message.Url is not null
            ? ChatMessageContentPart.CreateImagePart(new Uri(message.Url))
            : ChatMessageContentPart.CreateImagePart(message.Data, message.Data?.MediaType);
    }

    private IEnumerable<ChatMessage> ProcessToolCallMessage(IAgent agent, ToolCallMessage message)
    {
        if (message.From is not null && message.From != agent.Name)
        {
            throw new ArgumentException("ToolCallMessage is not supported when message.From is not the same with agent");
        }

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Ensure MultiModalMessage.Content contains only TextMessage and ImageMessage instances.
2. Filter or project the content before constructing the message: drop tool-call messages or serialize them into a TextMessage.
3. Wrap custom payloads as text (e.g. JSON in a TextMessage) instead of embedding non-media IMessage types.

Example fix

// before
var items = new IMessage[] { new TextMessage(Role.User, "describe"), someToolCallMessage };
var mm = new MultiModalMessage(Role.User, items);

// after
var items = new IMessage[] { new TextMessage(Role.User, "describe"), imageMessage };
var mm = new MultiModalMessage(Role.User, items);

// Validate multimodal content before constructing the message
if (items.Any(i => i is not (TextMessage or ImageMessage)))
    throw new ArgumentException("MultiModalMessage.Content supports only TextMessage and ImageMessage.");

static bool IsValidMultimodalContent(IEnumerable<IMessage> items) =>
    items.All(i => i is TextMessage or ImageMessage);

catch (NotImplementedException) when (inMultimodalConversion)
{
    logger.LogError("Non text/image item in MultiModalMessage");
    throw;
}

When it happens

Trigger: Constructing MultiModalMessage with a content collection containing something other than TextMessage/ImageMessage, then sending it through the connector.

Common situations: Programmatically building multimodal content by mapping over a mixed message list; adding tool-call or custom message types into what was assumed to be a text+image list.

Related errors

• Invalid message type: {m.GetType().Name}
• Invalid Role
• MultiModalMessage is not supported when message.From is the
• Unsupported message type: {item.GetType().Name}
• Only one choice is supported in streaming response

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/1f0d3739e0350a13. Report an issue: GitHub.