.NET: [Bug]: TextContent.AdditionalProperties dropped by AsAGUIEventStreamAsync for TEXT_MESSAGE_START/TEXT_MESSAGE_CONTENT events

[joelfransson]

Description

When A2A agents send TextPart with metadata (e.g., textContentType: "Markdown"), Part.ToAIContent() correctly stores the metadata in TextContent.AdditionalProperties. However, AsAGUIEventStreamAsync creates TextMessageStartEvent and TextMessageContentEvent with only MessageId, Role, and Delta — AdditionalProperties is silently dropped.

This means the AG-UI client has no way to know how to render the text (Markdown, code, plain text, etc.) from the standard event stream.

The AG-UI protocol supports this

The @ag-ui/core Zod schemas use "passthrough" mode, meaning extra properties on any event are preserved and valid. Adding a metadata field to TEXT_MESSAGE_START / TEXT_MESSAGE_CONTENT events is fully compatible with the AG-UI protocol specification.

Reproduction

1. Create an A2A agent that returns text with metadata:

   {
     "parts": [{
       "type": "text",
       "text": "Hello world",
       "metadata": { "textContentType": "Markdown" }
     }]
   }

2. Use MapAGUI to expose the orchestrator as an AG-UI endpoint.

3. Observe the SSE output:

   data: {"type":"TEXT_MESSAGE_START","messageId":"msg-1","role":"assistant"}
   

   The textContentType metadata is missing.

Expected behavior

AdditionalProperties from TextContent should be forwarded as extra properties on the SSE event JSON, since the AG-UI protocol's passthrough schemas allow it:

data: {"type":"TEXT_MESSAGE_START","messageId":"msg-1","role":"assistant","textContentType":"Markdown"}

Or alternatively, as a nested metadata field:

data: {"type":"TEXT_MESSAGE_START","messageId":"msg-1","role":"assistant","metadata":{"textContentType":"Markdown"}}

Root cause

In ChatResponseUpdateAGUIExtensions.AsAGUIEventStreamAsync, the TextMessageStartEvent is constructed with only two properties:

new TextMessageStartEvent
{
    MessageId = chatResponse.MessageId,
    Role = chatResponse.Role.Value
};

TextContent.AdditionalProperties from the ChatResponseUpdate.Contents is never read or forwarded.

Similarly, TextMessageContentEvent only copies MessageId and Delta:

new TextMessageContentEvent
{
    MessageId = chatResponse.MessageId,
    Delta = textContent.Text
};

Code Sample

Error Messages / Stack Traces

Package Versions

Microsoft.Agents.AI.Hosting.AGUI.AspNetCore: 1.0.0-preview.260121.1 and 1.0.0-preview.260311.1

.NET Version

No response

Additional Context

No response