AG-UI .NET — shared state

Goal: keep a structured object (a form, a document, a plan, a recipe) in sync between the client and the agent — the client provides the current state, the agent updates it, and the new state streams back next to the assistant's text.

State travels on the wire inside RunAgentInput.State (inbound) and as StateSnapshotEvent / StateDeltaEvent (outbound). In Microsoft.Extensions.AI terms, inbound state rides on the request's raw representation and outbound state rides on a ChatResponseUpdate's raw representation.

Client: seed the state and read it back

Send the starting state with the request, and watch for snapshots in the response:

using System.Text.Json;
using AGUI.Abstractions;
using Microsoft.Extensions.AI;

var initialState = JsonSerializer.SerializeToElement(new
{
    recipe = new { title = "", ingredients = Array.Empty<string>(), steps = Array.Empty<string>() }
});

var options = new ChatOptions
{
    RawRepresentationFactory = _ => new RunAgentInput { State = initialState },
};

var messages = new List<ChatMessage> { new(ChatRole.User, "Suggest an Italian pasta recipe") };

JsonElement? latestState = null;
await foreach (var update in client.GetStreamingResponseAsync(messages, options))
{
    if (update.RawRepresentation is StateSnapshotEvent snapshot)
    {
        latestState = snapshot.Snapshot;   // the full updated state object
    }
    Console.Write(update.Text);            // the assistant's summary streams as usual
}

State updates are content-less — they arrive on update.RawRepresentation, not as update.Text.

Server: read inbound state and emit the new state

Add a DelegatingChatClient to the pipeline that reads RunAgentInput.State, produces the new state, and yields it as a StateSnapshotEvent:

using System.Runtime.CompilerServices;
using System.Text.Json;
using AGUI.Abstractions;
using AGUI.Server;
using Microsoft.Extensions.AI;

internal sealed class RecipeStateChatClient(IChatClient inner, JsonSerializerOptions jso)
    : DelegatingChatClient(inner)
{
    public override async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
        IEnumerable<ChatMessage> messages,
        ChatOptions? options = null,
        [EnumeratorCancellation] CancellationToken ct = default)
    {
        if (options?.TryGetRunAgentInput(out var input) is true
            && input!.State is { ValueKind: JsonValueKind.Object } incoming)
        {
            var newState = await BuildStateAsync(messages, incoming, ct);   // your logic / an LLM call

            yield return new ChatResponseUpdate
            {
                RawRepresentation = new StateSnapshotEvent
                {
                    Snapshot = JsonSerializer.SerializeToElement(newState, jso.GetTypeInfo(typeof(AgentState))),
                },
            };
        }

        await foreach (var update in base.GetStreamingResponseAsync(messages, options, ct))
        {
            yield return update;   // the assistant's text summary
        }
    }
}

Register it after function invocation:

builder.Services.AddChatClient(/* provider IChatClient */)
    .UseFunctionInvocation()
    .Use((inner, sp) => new RecipeStateChatClient(
        inner, sp.GetRequiredService<IOptions<JsonOptions>>().Value.SerializerOptions));

TryGetRunAgentInput recovers the originating RunAgentInput from the ChatOptions the endpoint built — that is how the server-side chat client reaches State (and thread id, tools, …) without endpoint plumbing.

Snapshots vs deltas

• StateSnapshotEvent carries the entire state object. Simple and self-correcting; send it when the whole state changed or you want to resync.
• StateDeltaEvent carries a JSON Patch (RFC 6902) array of changes against the last known state. Use it for small, frequent edits to a large object to avoid resending everything. The client applies the patch to its copy.

AOT-safe state types

State objects are serialized through the source-generated context. Put each state type in a JsonSerializerContext, register it on the host (ConfigureHttpJsonOptions(o => o.SerializerOptions.TypeInfoResolverChain.Add(...))), and serialize through jso.GetTypeInfo(typeof(T)) rather than reflection.

Anti-patterns

• Aggregating the response instead of streaming it. StateSnapshotEvent / StateDeltaEvent arrive on content-less updates, which the non-streaming aggregation drops. Read state inside the await foreach over the streaming response.
• Sending a delta against state the client never received. A StateDeltaEvent patches the client's last known state; if the client missed the prior snapshot the patch fails to apply. Start a session (or resync) with a full StateSnapshotEvent, then send deltas.

Verify

1. The client sends a non-empty State and the run streams back at least one StateSnapshotEvent whose Snapshot reflects the agent's changes.
2. The assistant text and the state both arrive in the same run; reading update.RawRepresentation yields the StateSnapshotEvent while update.Text yields the summary.
3. If using deltas: applying the streamed JSON Patch to the prior state produces the same object a full snapshot would.