microsoft/autogen · error · ArgumentException

Unsupported message type {m.GetType()}

Error message

Unsupported message type {m.GetType()}

What it means

OpenAIChatCompletionService.ProcessMessages translates each incoming OpenAIMessage into a TextMessage/MultiModalMessage, but each case arm has a 'when' guard requiring non-null string content (or non-empty content array). Any message whose content is null, an unexpected type, or any unknown OpenAIMessage subclass falls through to ArgumentException 'Unsupported message type {type}'.

Source

Thrown at dotnet/src/AutoGen.WebAPI/OpenAI/Service/OpenAIChatCompletionService.cs:126

            doneMessage.Choices = [doneChoice];

            yield return doneMessage;
        }
        else
        {
            yield return await this.GetChatCompletionAsync(request);
        }
    }

    private IEnumerable<IMessage> ProcessMessages(IEnumerable<OpenAIMessage> messages)
    {
        return messages.Select<OpenAIMessage, IMessage>(m => m switch
        {
            OpenAISystemMessage systemMessage when systemMessage.Content is string content => new TextMessage(Role.System, content, this.agent.Name),
            OpenAIUserMessage userMessage when userMessage.Content is string content => new TextMessage(Role.User, content, this.agent.Name),
            OpenAIAssistantMessage assistantMessage when assistantMessage.Content is string content => new TextMessage(Role.Assistant, content, this.agent.Name),
            OpenAIUserMultiModalMessage userMultiModalMessage when userMultiModalMessage.Content is { Length: > 0 } => this.CreateMultiModaMessageFromOpenAIUserMultiModalMessage(userMultiModalMessage),
            _ => throw new ArgumentException($"Unsupported message type {m.GetType()}")
        });
    }

    private GenerateReplyOptions ProcessReplyOptions(OpenAIChatCompletionOption request)
    {
        return new GenerateReplyOptions()
        {
            Temperature = request.Temperature,
            MaxToken = request.MaxTokens,
            StopSequence = request.Stop,
        };
    }

    private MultiModalMessage CreateMultiModaMessageFromOpenAIUserMultiModalMessage(OpenAIUserMultiModalMessage message)
    {
        if (message.Content is null)
        {
            throw new ArgumentNullException(nameof(message.Content));

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Send only system/user/assistant messages with non-null string content, plus user multimodal messages with a non-empty content array
2. Strip tool messages and null-content turns from the history before calling the endpoint
3. For assistant pre-fill, send content:"" instead of null
4. Catch ArgumentException in the controller and return 400 with the rejected message type name

Example fix

// before
{"role":"assistant","content":null}

// after
{"role":"assistant","content":""}

var rejected = request.Messages.Where(m =>
    !(m.Content is string)
    && !(m is OpenAIUserMultiModalMessage mm && mm.Content is { Length: > 0 })).ToList();
if (rejected.Count > 0)
    return BadRequest($"Messages with null/unsupported content: {string.Join(",", rejected.Select(m => m.GetType().Name))}");

static bool IsAcceptableOpenAIMessage(OpenAIMessage m) => m switch
{
    OpenAISystemMessage { Content: string } => true,
    OpenAIUserMessage { Content: string } => true,
    OpenAIAssistantMessage { Content: string } => true,
    OpenAIUserMultiModalMessage { Content: { Length: > 0 } } => true,
    _ => false
};

catch (ArgumentException ex) when (ex.Message.Contains("Unsupported message type"))
{
    return BadRequest(new { error = "unsupported_message", detail = ex.Message });
}

When it happens

Trigger: POSTing a user/assistant/system message with content:null, a tool message (no case exists for OpenAIToolMessage in this method), or a user message whose content array is empty.

Common situations: Clients replaying tool-calling transcripts (tool role has no mapping here), assistant pre-fill messages with null content, or empty content arrays from edge-case UIs.

Related errors

• Invalid message type
• Invalid message type
• Exception of type 'System.Text.Json.JsonException' was throw
• Unsupported reply content type
• Value cannot be null. (Parameter 'Content')

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/77c1579ae4dc4d3e. Report an issue: GitHub.