microsoft/autogen · error · ArgumentException

Unexpected message type: {message?.GetType()}

Error message

Unexpected message type: {message?.GetType()}

What it means

AnthropicClientAgent.BuildMessages throws ArgumentException when an IMessage in the input is not IMessage<ChatMessage> — i.e. not a MessageEnvelope wrapping an Anthropic ChatMessage. The low-level AnthropicClientAgent speaks only its native ChatMessage envelope; AutoGen-native types like TextMessage, ImageMessage, or aggregate MessageEnvelope<string> fall into the default branch and are rejected.

Source

Thrown at dotnet/src/AutoGen.Anthropic/Agent/AnthropicClientAgent.cs:98

    }

    private List<ChatMessage> BuildMessages(IEnumerable<IMessage> messages)
    {
        List<ChatMessage> chatMessages = new();
        foreach (IMessage? message in messages)
        {
            switch (message)
            {
                case IMessage<ChatMessage> chatMessage when chatMessage.Content.Role == "system":
                    throw new InvalidOperationException(
                        "system message has already been set and only one system message is supported. \"system\" role for input messages in the Message");

                case IMessage<ChatMessage> chatMessage:
                    chatMessages.Add(chatMessage.Content);
                    break;

                default:
                    throw new ArgumentException($"Unexpected message type: {message?.GetType()}");
            }
        }

        // merge messages with the same role
        // fixing #2884
        var mergedMessages = chatMessages.Aggregate(new List<ChatMessage>(), (acc, message) =>
        {
            if (acc.Count > 0 && acc.Last().Role == message.Role)
            {
                acc.Last().Content.AddRange(message.Content);
            }
            else
            {
                acc.Add(message);
            }

            return acc;
        });

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Register the connector: new AnthropicClientAgent(...).RegisterMessageConnector() — the middleware converts AutoGen built-in message types to IMessage<ChatMessage> before they reach BuildMessages.
2. Or send only MessageEnvelope<ChatMessage> payloads to the un-wrapped agent.
3. Check you imported the right extension namespace (AutoGen.Anthropic.Extension) so RegisterMessageConnector resolves.
4. If you use group chats, register the connector on the agent before adding it to the group.

Example fix

// before
var agent = new AnthropicClientAgent(client, name: "assistant", systemMessage: "...");
await agent.SendAsync(new TextMessage(Role.User, "hello", from: "user")); // ArgumentException: Unexpected message type

// after
var agent = new AnthropicClientAgent(client, name: "assistant", systemMessage: "...")
    .RegisterMessageConnector();
await agent.SendAsync(new TextMessage(Role.User, "hello", from: "user")); // connector translates to ChatMessage

var unsupported = messages.Where(m => m is not IMessage<ChatMessage>).ToList();
if (unsupported.Count > 0)
{
    throw new InvalidOperationException($"These messages need the connector (call RegisterMessageConnector()): {string.Join(", ", unsupported.Select(m => m.GetType().Name))}");
}

static bool AllAnthropicChatMessages(IEnumerable<IMessage> messages) =>
    messages.All(m => m is IMessage<ChatMessage>);

try { await agent.SendAsync(messages); } catch (ArgumentException ex) when (ex.Message.Contains("Unexpected message type")) { throw new InvalidOperationException($"Forgot RegisterMessageConnector() on the Anthropic agent? {ex.Message}", ex); }

When it happens

Trigger: Calling AnthropicClientAgent.SendAsync with a TextMessage or MessageEnvelope<ToolCallMessage> without having called RegisterMessageConnector() on the agent; mixing middleware-processed and raw message types in one conversation; sending a Message[] array from a different provider integration.

Common situations: Using the raw agent instead of the wrapped one: forget .RegisterMessageConnector() which translates TextMessage/ToolCallMessage into IMessage<ChatMessage>; upgrading AutoGen where the connector extension moved namespaces; passing a DataMessage or WorkflowMessage to the client agent directly.

Related errors

• system message has already been set and only one system mess
• Failed to deserialize response
• Unknown content type
• Value was null.
• Unable to convert "{value}" to enum {typeToConvert}.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/a03e28002a68e9ee. Report an issue: GitHub.