---

name: implementing-chat-streaming description: Provides SSE streaming patterns for the chat API and frontend. Use when implementing or modifying chat streaming, handling SSE events, or troubleshooting message flow between frontend and backend.

Chat Streaming Implementation

Backend: SSE Endpoint

Copyapp.MapPost("/api/chat/stream", async (
    ChatRequest request,
    AgentFrameworkService agentService,
    HttpContext httpContext,
    CancellationToken cancellationToken) =>
{
    httpContext.Response.Headers.Append("Content-Type", "text/event-stream");
    httpContext.Response.Headers.Append("Cache-Control", "no-cache");
    
    var conversationId = request.ConversationId 
        ?? await agentService.CreateConversationAsync(request.Message, cancellationToken);
    
    // Send conversation ID first
    await httpContext.Response.WriteAsync(
        $"data: {{\"type\":\"conversationId\",\"conversationId\":\"{conversationId}\"}}\n\n", 
        cancellationToken);
    await httpContext.Response.Body.FlushAsync(cancellationToken);
    
    // Stream chunks
    await foreach (var chunk in agentService.StreamMessageAsync(
        conversationId, request.Message, request.ImageDataUris, cancellationToken))
    {
        var json = JsonSerializer.Serialize(new { type = "chunk", content = chunk });
        await httpContext.Response.WriteAsync($"data: {json}\n\n", cancellationToken);
        await httpContext.Response.Body.FlushAsync(cancellationToken);
    }
    
    await httpContext.Response.WriteAsync("data: {\"type\":\"done\"}\n\n", cancellationToken);
})
.RequireAuthorization("RequireChatScope");

Backend: IAsyncEnumerable Service

Actual return type: IAsyncEnumerable<StreamChunk> (not raw strings)

Why direct SDK? Uses ProjectResponsesClient directly (not Agent Framework's ChatClientAgent.RunStreamingAsync()) because the IChatClient abstraction doesn't expose MCP approvals, file search quotes, or citation annotations. See .github/skills/researching-azure-ai-sdk/SKILL.md for full rationale.

Copypublic async IAsyncEnumerable<StreamChunk> StreamMessageAsync(
    string conversationId,
    string message,
    List<string>? imageDataUris = null,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    ObjectDisposedException.ThrowIf(_disposed, this);
    
    // Stream response - yields StreamChunk with text deltas OR annotations
    await foreach (var update in responsesClient.CreateResponseStreamingAsync(...))
    {
        if (update is StreamingResponseOutputTextDeltaUpdate deltaUpdate)
        {
            yield return StreamChunk.Text(deltaUpdate.Delta);
        }
        else if (update is StreamingResponseOutputItemDoneUpdate itemDoneUpdate)
        {
            var annotations = ExtractAnnotations(itemDoneUpdate.Item, fileSearchQuotes);
            if (annotations.Count > 0)
            {
                yield return StreamChunk.WithAnnotations(annotations);
            }
        }
    }
}

StreamChunk model (backend/WebApp.Api/Models/StreamChunk.cs):

• IsText / TextDelta - Text content
• HasAnnotations / Annotations - Citation metadata

Frontend: Action Flow

CopyCHAT_SEND_MESSAGE 
  → CHAT_ADD_ASSISTANT_MESSAGE 
  → CHAT_START_STREAM 
  → (repeat CHAT_STREAM_CHUNK) 
  → CHAT_STREAM_ANNOTATIONS (optional, for citations)
  → CHAT_STREAM_COMPLETE (with usage metrics)

If user cancels: CHAT_CANCEL_STREAM sets status to idle.

Frontend: ChatService Pattern

See: frontend/src/services/ChatService.ts

Key patterns:

• AbortController for cancellation
• EventSource or fetch with ReadableStream
• Parse SSE data: lines
• Dispatch actions for each event type

Image Validation

Backend limits (see AzureAIAgentService.cs):

• Max 5 images per request
• Max 5MB per image (decoded)
• Allowed: image/png, image/jpeg, image/gif, image/webp

Frontend limits (see frontend/src/utils/fileAttachments.ts):

• Same limits with user-friendly error messages
• Toast notifications for validation feedback

---

Project-Specific: Full Endpoint Implementation

Copyapp.MapPost("/api/chat/stream", async (
    ChatRequest request,
    AgentFrameworkService agentService,
    HttpContext httpContext,
    IHostEnvironment env,
    CancellationToken cancellationToken) =>
{
    httpContext.Response.Headers.Append("Content-Type", "text/event-stream");
    httpContext.Response.Headers.Append("Cache-Control", "no-cache");
    
    var conversationId = request.ConversationId 
        ?? await agentService.CreateConversationAsync(request.Message, cancellationToken);
    
    await httpContext.Response.WriteAsync(
        $"data: {{\"type\":\"conversationId\",\"conversationId\":\"{conversationId}\"}}\n\n", 
        cancellationToken);
    await httpContext.Response.Body.FlushAsync(cancellationToken);
    
    await foreach (var chunk in agentService.StreamMessageAsync(
        conversationId, request.Message, request.ImageDataUris, cancellationToken))
    {
        var json = System.Text.Json.JsonSerializer.Serialize(new { type = "chunk", content = chunk });
        await httpContext.Response.WriteAsync($"data: {json}\n\n", cancellationToken);
        await httpContext.Response.Body.FlushAsync(cancellationToken);
    }
    
    await httpContext.Response.WriteAsync("data: {\"type\":\"done\"}\n\n", cancellationToken);
})
.RequireAuthorization("RequireChatScope")
.WithName("StreamChatMessage");

Project-Specific: Service Implementation

See: backend/WebApp.Api/Services/AgentFrameworkService.cs

Key patterns in StreamMessageAsync:

• Disposal guard before processing
• Multi-modal message support (text + image data URIs)
• IAsyncEnumerable<StreamChunk> with [EnumeratorCancellation]
• StreamingResponseOutputTextDeltaUpdate for text content
• StreamingResponseOutputItemDoneUpdate for annotations
• Collects file search quotes via FileSearchCallResponseItem for citation context
• Usage captured from StreamingResponseCompletedUpdate

Project-Specific: Frontend State Flow

CopyCHAT_SEND_MESSAGE 
  → CHAT_ADD_ASSISTANT_MESSAGE 
  → CHAT_START_STREAM 
  → (repeat CHAT_STREAM_CHUNK) 
  → CHAT_STREAM_ANNOTATIONS (optional, for citations)
  → CHAT_STREAM_COMPLETE (with usage: promptTokens, completionTokens, totalTokens, duration)

Cancel: CHAT_CANCEL_STREAM sets status to idle and re-enables input.

Error: CHAT_ERROR with AppError containing message, optional retry action, timestamp.

SSE Event Types

Project-Specific: Dev Logging

Each state change prints (dev only):

Copy🔄 [HH:MM:SS] ACTION_TYPE
Action: { … }
Changes: { field: before → after }

Related Skills

• writing-csharp-code - Backend coding standards and AgentFrameworkService patterns
• writing-typescript-code - Frontend React patterns and ChatService implementation
• troubleshooting-authentication - Token acquisition for authenticated streaming