microsoft/autogen · error · ArgumentException

Invalid aggregate message {reason}

Error message

Invalid aggregate message {reason}

What it means

MultiModalMessage.Validate() (run on construction) checks every inner message's From equals the aggregate's From; a mismatch produces ArgumentException('Invalid aggregate message The from property of the message {message} is different...'). MultiModalMessage aggregates TextMessage/ImageMessage parts that must all be attributed to the same sender so downstream consumers can treat the whole message as one speaker's turn.

Source

Thrown at dotnet/src/AutoGen.Core/Message/MultiModalMessage.cs:32

        this.Content = content;
        this.From = from;
        this.Validate();
    }

    public Role Role { get; set; }

    public IEnumerable<IMessage> Content { get; set; }

    public string? From { get; set; }

    private void Validate()
    {
        foreach (var message in this.Content)
        {
            if (message.From != this.From)
            {
                var reason = $"The from property of the message {message} is different from the from property of the aggregate message {this}";
                throw new ArgumentException($"Invalid aggregate message {reason}");
            }
        }

        // all message must be either text or image
        foreach (var message in this.Content)
        {
            if (message is not TextMessage && message is not ImageMessage)
            {
                var reason = $"The message {message} is not a text or image message";
                throw new ArgumentException($"Invalid aggregate message {reason}");
            }
        }
    }

    public override string ToString()
    {
        var stringBuilder = new System.Text.StringBuilder();
        stringBuilder.Append($"MultiModalMessage({this.Role}, {this.From})");

View on GitHub (pinned to 027ecf0a37)

Solutions

1. Set from on the MultiModalMessage and on every TextMessage/ImageMessage inside it to the same value.
2. Create inner messages with explicit from arguments rather than defaults.
3. Pre-validate: content.All(m => m.From == aggregateFrom) before construction and fail with a domain-specific error.

Example fix

// before
var parts = new IMessage[]
{
    new TextMessage(Role.User, "describe"), // From = null
    new ImageMessage(Role.User, uri, from: "user"),
};
var msg = new MultiModalMessage(Role.User, parts, from: "user"); // throws

// after
var parts = new IMessage[]
{
    new TextMessage(Role.User, "describe", from: "user"),
    new ImageMessage(Role.User, uri, from: "user"),
};
var msg = new MultiModalMessage(Role.User, parts, from: "user");

if (content.Any(m => m.From != aggregateFrom))
    throw new ArgumentException("All multimodal parts must have the same From as the aggregate");
var msg = new MultiModalMessage(role, content, from: aggregateFrom);

static bool IsFromConsistent(IEnumerable<IMessage> content, string? from) => content.All(m => m.From == from);

try { var msg = new MultiModalMessage(role, content, from); }
catch (ArgumentException ex) when (ex.Message.Contains("from property"))
{
    // normalize each part's From and retry construction
}

When it happens

Trigger: new MultiModalMessage(role, new IMessage[] { textWithFromNull, imageFromAgent }, from: "agent"); mixing messages produced by different agents into one multimodal message; inner messages created without the from parameter (defaults null) while the wrapper sets from.

Common situations: Assembling vision payloads (prompt text + image) where the text was built with a helper that omits from; merging messages collected from a group chat history into a single multimodal request; test fixtures constructing messages inconsistently.

Related errors

• The from property of the message {message} is different from
• Invalid DataUri format, expected data:[<mediatype>][;base64]
• Data cannot be empty
• Role mismatch
• All agents must have a name.

AI-assisted analysis of microsoft/autogen@027ecf0a37 (2026-08-15). Data as JSON: /api/errors/f8b8be8ef7f2b375. Report an issue: GitHub.