microsoft/autogen · error · InvalidOperationException

Unsupported message type: {m.GetType()}

Error message

Unsupported message type: {m.GetType()}

What it means

Thrown by GeminiMessageConnector.ProcessMessage when strictMode is enabled (GeminiMessageConnector strictMode: true) and an incoming IMessage does not match any of the supported message types (TextMessage, ImageMessage, MultiModalMessage, ToolCallMessage, ToolCallResultMessage, ToolCallAggregateMessage, or an IMessage<Content> envelope). Strict mode exists to surface mapping gaps instead of silently passing untranslatable messages to Gemini.

Source

Thrown at dotnet/src/AutoGen.Gemini/Middleware/GeminiMessageConnector.cs:220

    private IEnumerable<IMessage> ProcessMessage(IEnumerable<IMessage> messages, IAgent agent)
    {
        return messages.SelectMany(m =>
        {
            if (m is Core.IMessage<Content> messageEnvelope)
            {
                return [m];
            }
            else
            {
                return m switch
                {
                    TextMessage textMessage => ProcessTextMessage(textMessage, agent),
                    ImageMessage imageMessage => ProcessImageMessage(imageMessage, agent),
                    MultiModalMessage multiModalMessage => ProcessMultiModalMessage(multiModalMessage, agent),
                    ToolCallMessage toolCallMessage => ProcessToolCallMessage(toolCallMessage, agent),
                    ToolCallResultMessage toolCallResultMessage => ProcessToolCallResultMessage(toolCallResultMessage, agent),
                    ToolCallAggregateMessage toolCallAggregateMessage => ProcessToolCallAggregateMessage(toolCallAggregateMessage, agent),
                    _ when strictMode => throw new InvalidOperationException($"Unsupported message type: {m.GetType()}"),
                    _ => [m],
                };
            }
        });
    }

    private IEnumerable<IMessage> ProcessToolCallAggregateMessage(ToolCallAggregateMessage toolCallAggregateMessage, IAgent agent)
    {
        var parseAsUser = ShouldParseAsUser(toolCallAggregateMessage, agent);
        if (parseAsUser)
        {
            var content = toolCallAggregateMessage.GetContent();

            if (content is string str)
            {
                var textMessage = new TextMessage(Role.User, str, toolCallAggregateMessage.From);

                return ProcessTextMessage(textMessage, agent);

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Convert foreign/custom messages to TextMessage (or MultiModalMessage) before they reach the Gemini agent.
2. Use the non-strict constructor (strictMode defaults to false) if passing unknown messages through is acceptable.
3. Wrap the payload as MessageEnvelope.Create(content) with Gemini Content if you already have Gemini-shaped data.
4. Log m.GetType() from the exception to find which producer emitted the unsupported type.

Example fix

// before
var connector = new GeminiMessageConnector(strictMode: true);
await geminiAgent.InitiateChatAsync(otherAgent, nil, new MyCustomMessage("hi"));

// after
var text = new TextMessage(Role.User, "hi", from: otherAgent.Name);
await geminiAgent.SendAsync(text);

static bool IsGeminiSupportedMessage(IMessage m) => m is TextMessage or ImageMessage or MultiModalMessage or ToolCallMessage or ToolCallResultMessage or ToolCallAggregateMessage or Core.IMessage<Content>;

When it happens

Trigger: Constructing GeminiMessageConnector with strictMode:true and sending a message whose runtime type is outside the switch — e.g. a custom IMessage subclass, MessageEnvelope wrapping an unexpected payload, or another vendor's message type from a multi-agent pipeline.

Common situations: Heterogeneous agent graphs (Gemini agent talking to an LLM/OpenAI agent whose message type leaks through); custom middleware injecting proprietary message classes; strictMode turned on to harden a pipeline that previously relied on the lenient pass-through.

Related errors

• Unsupported message type: {message.GetType()}
• The response should contain exactly one candidate.
• ToolCallMessage is not supported as user role in Gemini.
• Image message is not supported as model role in Gemini.
• System role is not supported in Gemini.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/247aad4e85e791c2. Report an issue: GitHub.