microsoft/autogen · error · InvalidOperationException

Unsupported message type

Error message

Unsupported message type

What it means

Thrown by SemanticKernelChatMessageContentConnector.ProcessMessageForSelf when a Message from the agent itself does not match any supported shape. The connector only accepts: (a) string content with null FunctionName/FunctionArguments, or (b) null content with both FunctionName and FunctionArguments set (which is then rejected separately). Any other combination falls into the final else and raises this InvalidOperationException.

Source

Thrown at dotnet/src/AutoGen.SemanticKernel/Middleware/SemanticKernelChatMessageContentConnector.cs:243

    [Obsolete("This method is deprecated, please use the specific method instead.")]
    private IEnumerable<ChatMessageContent> ProcessMessageForSelf(Message message)
    {
        if (message.Role == Role.System)
        {
            return [new ChatMessageContent(AuthorRole.System, message.Content)];
        }
        else if (message.Content is string && message.FunctionName is null && message.FunctionArguments is null)
        {
            return [new ChatMessageContent(AuthorRole.Assistant, message.Content)];
        }
        else if (message.Content is null && message.FunctionName is not null && message.FunctionArguments is not null)
        {
            throw new System.InvalidOperationException("Function call is not supported in the semantic kernel if it's from self.");
        }
        else
        {
            throw new System.InvalidOperationException("Unsupported message type");
        }
    }

    [Obsolete("This method is deprecated, please use the specific method instead.")]
    private IEnumerable<ChatMessageContent> ProcessMessageForOthers(Message message)
    {
        if (message.Role == Role.System)
        {
            return [new ChatMessageContent(AuthorRole.System, message.Content)];
        }
        else if (message.Content is string && message.FunctionName is null && message.FunctionArguments is null)
        {
            return [new ChatMessageContent(AuthorRole.User, message.Content)];
        }
        else if (message.Content is null && message.FunctionName is not null && message.FunctionArguments is not null)
        {
            throw new System.InvalidOperationException("Function call is not supported in the semantic kernel if it's from others.");
        }

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Ensure the message passed for 'self' history has plain string Content with FunctionName and FunctionArguments both null
2. If the message carries a function call, set Content to null AND populate both FunctionName and FunctionArguments as non-null strings (though that path throws separately in this connector)
3. Convert multimodal or structured content to a string (e.g. serialize/collapse to text) before sending to a SemanticKernel agent
4. Upgrade AutoGen.SemanticKernel to a newer 0.4x+ version where message processing was reworked to support function-call history

Example fix

// before
var msg = new Message(Role.Assistant, from: agentName)
{
    Content = resultJson,
    FunctionName = "get_weather", // FunctionArguments left null -> falls to 'Unsupported message type'
};

// after
var msg = new Message(Role.Assistant, content: "Called get_weather", from: agentName); // plain string, no function fields

bool IsSupportedSelfMessage(Message m) =>
    (m.Content is string && m.FunctionName is null && m.FunctionArguments is null)
    || (m.Content is null && m.FunctionName is not null && m.FunctionArguments is not null);

var safe = messages.Where(IsSupportedSelfMessage);

static bool IsSupportedSelfMessage(Message m) =>
    m.Content is string && m.FunctionName is null && m.FunctionArguments is null;

try { await skAgent.SendAsync(msg); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Unsupported message type"))
{
    _logger.LogWarning("Dropping unsupported self message {Type}", msg.Content?.GetType()); // drop and continue
}

When it happens

Trigger: Passing a Message to the SemanticKernel middleware where Content is not a string (e.g. multimodal content or an IList), or where FunctionName is set but FunctionArguments is null (or vice versa), or where both string content and a function call are present at once.

Common situations: Replaying a captured function-call message that was serialized and lost one field (arguments null after round-trip), mixing tool output with function name, or feeding a MultiModalMessage/ToolMessage content object into a SemanticKernel agent pipeline that expects plain chat text.

Related errors

• ResultsPerPrompt greater than 1 is not supported in this sem
• ResultsPerPrompt greater than 1 is not supported in this sem
• Please set OPENAI_API_KEY environment variable.
• Missing parameter type for {parameterName}
• Missing required parameter: {parameterName}

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/da824851a8ed837d. Report an issue: GitHub.