AG-UI .NET — reasoning (thinking) events

Goal: show a reasoning model's intermediate thoughts as they stream, kept visually separate from the final answer.

A reasoning model emits two kinds of text in the stream: its thinking and its answer. In Microsoft.Extensions.AI these arrive as different content types on each ChatResponseUpdate — TextReasoningContent for the thoughts, TextContent for the answer. update.Text only concatenates the answer parts, so to display thinking you inspect update.Contents.

Separate thinking from the answer

Iterate update.Contents and branch on the content type instead of reading update.Text:

using Microsoft.Extensions.AI;

var messages = new List<ChatMessage>
{
    new(ChatRole.User, "20 heads, 56 legs — how many chickens and rabbits? Show your reasoning."),
};

await foreach (var update in client.GetStreamingResponseAsync(messages))
{
    foreach (var content in update.Contents)
    {
        switch (content)
        {
            case TextReasoningContent { Text: { Length: > 0 } thinking }:
                RenderThinking(thinking);   // e.g. a dimmed/collapsible panel
                break;
            case TextContent { Text: { Length: > 0 } answer }:
                RenderAnswer(answer);       // the user-facing reply
                break;
        }
    }
}

Both stream incrementally and can interleave: thinking deltas arrive, then answer deltas. Render each as it comes rather than buffering, so the "thinking…" state is visible before the answer lands.

Anti-patterns

• Re-sending the reasoning text as conversation history. The thinking is scratch work for a single turn, not a message. Appending captured TextReasoningContent into the next request's messages bloats the prompt, and some providers reject prior reasoning as input. Carry forward only the assistant's answer.
• Treating reasoning as part of the answer. The thinking is a separate channel; concatenating it into the reply shows the user the model's scratch work as if it were the response. Keep the two render paths distinct.

Verify

1. For a prompt that asks the model to reason, the stream yields TextReasoningContent updates before the final TextContent answer.
2. The rendered answer contains only the response, with the reasoning shown (or hidden) through its own path — not mixed into the answer text.