AG-UI .NET — protobuf transport

Goal: carry the AG-UI event stream as compact protobuf binary instead of Server-Sent Events, with automatic fallback to SSE when one side doesn't support it.

Transport is negotiated by content type: the client advertises the formats it accepts (in preference order) on the Accept header, and the server replies in the first format it also supports. Nothing above the transport changes — AGUIChatClient, messages, and your endpoint logic are identical to SSE.

Install

dotnet add package AGUI.Protobuf   # ProtobufEventStreamFormatter

Client apps also need AGUI.Client (for AGUIEventStreamHandler); server apps also need AGUI.Formatting (for SseEventStreamFormatter) and AGUI.Server.

Client: prefer protobuf, fall back to SSE

Wrap your HttpClient in an AGUIEventStreamHandler configured with the formatters in preference order. The handler advertises them on Accept and decodes whatever the server returns:

using AGUI.Client;
using AGUI.Formatting;
using AGUI.Protobuf;
using Microsoft.Extensions.AI;

var handler = new AGUIEventStreamHandler(
    [new ProtobufEventStreamFormatter(), new SseEventStreamFormatter()])
{
    InnerHandler = new HttpClientHandler(),
};

using var httpClient = new HttpClient(handler);
IChatClient client = new AGUIChatClient(new(httpClient, "http://localhost:5013"));

Protobuf is listed first, so the client prefers it; if the server only speaks SSE, the handler transparently decodes SSE instead. Switching transports is purely a matter of reordering (or trimming) this formatter list.

Server: answer protobuf when asked

Register both formatters and pick one per request from the Accept header, defaulting to SSE:

using System.Linq;
using AGUI.Abstractions;
using AGUI.Formatting;
using AGUI.Protobuf;
using AGUI.Server;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.AI;
using Microsoft.Extensions.Options;
using JsonOptions = Microsoft.AspNetCore.Http.Json.JsonOptions;

builder.Services.AddSingleton<IAGUIEventStreamFormatter, SseEventStreamFormatter>();
builder.Services.AddSingleton<IAGUIEventStreamFormatter, ProtobufEventStreamFormatter>();

var app = builder.Build();

app.MapPost("/", async (
    [FromBody] RunAgentInput input,
    IChatClient chatClient,
    IEnumerable<IAGUIEventStreamFormatter> formatters,
    IOptions<JsonOptions> jsonOptions,
    HttpContext http,
    CancellationToken ct) =>
{
    var accept = http.Request.Headers.Accept.ToString();
    var formatter =
        formatters.FirstOrDefault(f => f is ProtobufEventStreamFormatter
                                       && accept.Contains(f.MediaType, StringComparison.OrdinalIgnoreCase))
        ?? formatters.First(f => f is SseEventStreamFormatter);

    var ctx = input.ToChatRequestContext(jsonOptions.Value.SerializerOptions);
    var updates = chatClient.GetStreamingResponseAsync(ctx.Messages, ctx.ChatOptions, ct);
    var events = updates.AsAGUIEventStreamAsync(ctx, ct);

    http.Response.ContentType = formatter.MediaType;
    http.Response.Headers.CacheControl = "no-cache";
    await formatter.WriteAsync(events, http.Response.Body, ct);
});

Protobuf is selected only when the client explicitly accepts its media type, so a default SSE client keeps working unchanged.

Anti-patterns

• Registering the protobuf formatter on only one side. Negotiation needs both ends to agree. If the server registers protobuf but the client's handler lists only SSE (or vice versa), every connection silently falls back to SSE and the binary path is never exercised — verify the chosen Content-Type, don't assume.
• Hand-parsing the protobuf body. The wire framing must match @ag-ui/proto byte-for-byte. Always read and write through ProtobufEventStreamFormatter; don't decode the stream yourself.

Verify

1. With both sides configured, the response Content-Type is the protobuf media type (ProtobufEventStreamFormatter.MediaType), and the body is binary, not data: text frames.
2. Point the same client at an SSE-only server (drop the server's protobuf registration): it keeps working, now over SSE — confirming fallback.
3. The decoded event stream and the assistant output are identical to the SSE transport.