AG-UI .NET SDK 1.0: AGUI.Server and AGUI.Client Move the Protocol Out of Agent Framework

On September 25, 2026 the .NET team announced a first-class .NET SDK for the AG-UI protocol. The packages hit NuGet as 1.0.0 on September 17: AGUI.Abstractions, AGUI.Formatting, AGUI.Protobuf, AGUI.Server, and AGUI.Client, MIT licensed, living in ag-ui-protocol/ag-ui under sdks/dotnet. Until now, AG-UI support in .NET meant taking a dependency on Microsoft Agent Framework. That coupling is gone.

What AG-UI is, in one paragraph

AG-UI is a wire protocol between an agent backend and a frontend. The client POSTs a RunAgentInput (messages, tools, state), and the server answers with a stream of typed events: RUN_STARTED, TEXT_MESSAGE_CONTENT, tool call events, STATE_DELTA, RUN_FINISHED. Server-Sent Events is the default transport, with an optional protobuf codec for part of the event set. Frontends like CopilotKit already speak it, so a .NET backend that emits correct AG-UI events plugs straight into them.

Any IChatClient becomes an AG-UI endpoint

AGUI.Server targets net8.0, net9.0, and net10.0, and it only needs a Microsoft.Extensions.AI IChatClient. No agent abstraction required:

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSingleton(CreateChatClient());
builder.Services.Configure<JsonOptions>(options =>
    options.SerializerOptions.TypeInfoResolverChain.Insert(
        0, AGUIJsonUtilities.DefaultTypeInfoResolver));

var app = builder.Build();

app.MapPost("/", (RunAgentInput input, IChatClient chatClient,
    IOptions<JsonOptions> jsonOptions, CancellationToken ct) =>
{
    var context = input.ToChatRequestContext(jsonOptions.Value.SerializerOptions);

    var events = chatClient
        .GetStreamingResponseAsync(context.Messages, context.ChatOptions, ct)
        .AsAGUIEventStreamAsync(context, ct);

    return TypedResults.ServerSentEvents(events);
});

await app.RunAsync();

The interesting work happens in AsAGUIEventStreamAsync. It wraps the run in RUN_STARTED and RUN_FINISHED, closes an open text or reasoning block before switching to another message or tool call, and folds multiple interrupts into a single terminal RUN_FINISHED. Those are exactly the ordering rules a hand-rolled SSE mapper tends to get wrong, and a frontend that receives a TEXT_MESSAGE_CONTENT for a block that was never started usually fails silently.

The client side reaches .NET Framework 4.7.2

AGUI.Client also targets netstandard2.0 and net472. Its AGUIChatClient implements IChatClient, so a remote AG-UI agent looks like any other model to your code:

using AGUI.Client;

using var httpClient = new HttpClient();
IChatClient agent = new AGUIChatClient(
    new AGUIChatClientOptions(httpClient, "http://localhost:5001"));

await foreach (var update in agent.GetStreamingResponseAsync("Summarize ticket 4211"))
    Console.Write(update.Text);

That is useful for a legacy WinForms or WPF app on .NET Framework that needs to call an agent hosted elsewhere without migrating first.

Breaking renames for Agent Framework users

If you already used Microsoft.Agents.AI.Hosting.AGUI.AspNetCore (currently 1.22.0-preview.260918.1), the hosting package now builds on the new SDK and the names changed:

builder.Services.AddAGUIServer();
var app = builder.Build();

AIAgent agent = chatClient.AsAIAgent(
    name: "AGUIAssistant",
    instructions: "You are a helpful assistant.");

app.MapAGUIServer("/", agent);

Combined with last week’s AsIChatClient in Agent Framework 1.22, the pieces now compose in both directions: an AIAgent can be an IChatClient, an IChatClient can be an AG-UI endpoint, and an AG-UI endpoint can be an IChatClient again. If you only need a streaming chat backend for a CopilotKit-style frontend, AGUI.Server plus your existing chat client is now the smallest dependency that does it correctly.