.NET: [Bug]: Agent chat completions endpoint does not terminate SSE stream with data: [DONE]\n\n

[baonguyenrb21]

Description

• The agent chat completions endpoint does not reliably write a data: [DONE]\n\n event at the end of the SSE stream. The stream closes without the terminator event that OpenAI-compatible clients expect.
• The SSE stream should terminate with data: [DONE]\n\n, consistent with the OpenAI SSE specification. Clients that rely on the [DONE] signal to detect stream completion will hang or timeout.
• Steps to reproduce the issue:
  • Register an agent chat completions endpoint
  • Send a request that produces a streaming SSE response
  • Observe that the stream closes without a final data: [DONE]\n\n event
• We implemented an opt-in middleware that wraps the response body and appends data: [DONE]\n\n if the endpoint didn't already write it.

Code Sample

Error Messages / Stack Traces

Package Versions

Microsoft.Agents.AI 1.17.0; Microsoft.Agents.AI.Hosting.OpenAI 1.17.0-alpha.260804.1; Microsoft.Agents.AI.Hosting.AGUI.AspNetCore 1.17.0-preview.260804.1

.NET Version

NET 10.0

Additional Context

This requires consumers to implement custom middleware or post-processing to detect stream end, which is fragile. The chat completions endpoint should terminate its SSE stream with data: [DONE]\n\n per the OpenAI SSE convention.

[github-actions]

🤖 Automated triage reproduction notes (agent-authored — trust but verify)

Agent analysis

Repro: AIAgentChatCompletionsProcessor.StreamingResponse.ExecuteAsync writes only items from GetStreamingChunksAsync; when a request has stream: true and agent enumeration completes normally, the response closes after the final JSON chunk without [DONE]. Minimal repro: run OpenAIChatCompletionsConformanceTests.StreamingRequestResponseAsync with an assertion that the normalized response ends in data: [DONE]\n.

• Failing test: dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIChatCompletionsConformanceTests.cs
• Files examined: dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/AIAgentChatCompletionsProcessor.cs, dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/EndpointRouteBuilderExtensions.ChatCompletions.cs, dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/OpenAIChatCompletionsConformanceTests.cs, dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/ConformanceTraces/ChatCompletions/streaming/response.txt
• Tests run: OpenAIChatCompletionsConformanceTests.StreamingRequestResponseAsync
• Reported version: 1.17.0-alpha.260804.1