microsoft/autogen · error · InvalidOperationException

The message is not a valid message

Error message

The message is not a valid message

What it means

PrintMessageMiddleware's streaming branch keeps the last IMessage seen in the update stream (recentUpdate) and returns it; if the stream completes without yielding any IMessage instance, recentUpdate is null and it throws InvalidOperationException('The message is not a valid message'). In effect, the middleware was asked to process a streaming reply that contained zero actual messages.

Source

Thrown at dotnet/src/AutoGen.Core/Middleware/PrintMessageMiddleware.cs:37

    public async Task<IMessage> InvokeAsync(MiddlewareContext context, IAgent agent, CancellationToken cancellationToken = default)
    {
        if (agent is IStreamingAgent streamingAgent)
        {
            IMessage? recentUpdate = null;
            await foreach (var message in this.InvokeAsync(context, streamingAgent, cancellationToken))
            {
                if (message is IMessage imessage)
                {
                    recentUpdate = imessage;
                }
            }
            Console.WriteLine();
            if (recentUpdate is not null && recentUpdate is not TextMessage)
            {
                Console.WriteLine(recentUpdate.FormatMessage());
            }

            return recentUpdate ?? throw new InvalidOperationException("The message is not a valid message");
        }
        else
        {
            var reply = await agent.GenerateReplyAsync(context.Messages, context.Options, cancellationToken);

            var formattedMessages = reply.FormatMessage();

            Console.WriteLine(formattedMessages);

            return reply;
        }
    }

    public async IAsyncEnumerable<IMessage> InvokeAsync(MiddlewareContext context, IStreamingAgent agent, [EnumeratorCancellation] CancellationToken cancellationToken = default)
    {
        IMessage? recentUpdate = null;
        await foreach (var message in agent.GenerateStreamingReplyAsync(context.Messages, context.Options, cancellationToken))
        {

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Ensure the agent/stream produces at least one IMessage before routing through PrintMessageMiddleware in streaming mode.
2. Check for empty input upstream and short-circuit with a sensible default (return an empty TextMessage instead of forwarding nothing).
3. Reorder middleware so PrintMessageMiddleware is not downstream of a filter that can drop all messages.
4. Wrap in try/catch for InvalidOperationException and treat it as an empty-reply condition (log and skip).

Example fix

// before
await foreach (var unused in agent.GenerateStreamingReplyAsync(messages)) { } // zero chunks
var reply = await printMiddleware.InvokeAsync(agent, messages, options); // throws

// after
var chunks = new List<IMessage>();
await foreach (var chunk in agent.GenerateStreamingReplyAsync(messages)) chunks.Add(chunk);
if (chunks.Count == 0) return new TextMessage(Role.Assistant, string.Empty, from: agent.Name);
var reply = await printMiddleware.InvokeAsync(agent, chunks, options);

var chunks = await CollectChunksAsync(agent, messages);
if (chunks.Count == 0)
    return new TextMessage(Role.Assistant, string.Empty, from: agent.Name); // avoid empty-stream middleware call

try { return await printMiddleware.InvokeAsync(agent, messages, options); }
catch (InvalidOperationException ex) when (ex.Message.Contains("not a valid message"))
{
    return new TextMessage(Role.Assistant, string.Empty, from: agent.Name); // empty-reply sentinel
}

When it happens

Trigger: Invoking an agent through PrintMessageMiddleware in streaming mode where the update collection is empty or contains only non-IMessage items; a stream that errors/completes early before any chunk arrives; downstream middleware filtering out every message before print runs.

Common situations: Empty completions from the provider (no content chunks); unit tests passing empty message sequences; middleware pipeline ordering where an earlier middleware swallows all messages; cancellation racing the first chunk.

Related errors

• The first message is ToolCallMessage, but the update message
• FunctionMap is not available
• Invalid streaming message type {reply.GetType().Name}
• No user message found.
• No user message found.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/f77d6243141c65d1. Report an issue: GitHub.