.NET: [Bug]: AG UI session stops updating after first turn

[VaclavK]

Description

AG UI client is created
agent invoked
result is returned
session contains tool and input/out messages
another agent invoked with same session
session no longer updated

Code Sample

var clientAgent = new AGUIChatClient(httpClient, serverUrl, LoggerFactory).AsAIAgent();
var session = await clientAgent.CreateSessionAsync(cancellationToken: cancellationToken);
 var result = await clientAgent.RunAsync<WorkflowInstance>($"Start workflow for following conversation id {conversationId} with prompt:{message}", session, cancellationToken: cancellationToken);
while (true)
{
await clientAgent.RunAsync<WorkflowResult>(
                            $"monitor workflow",
                            session,
                            cancellationToken: cancellationToken);
}

Error Messages / Stack Traces

Package Versions

1.6.1

.NET Version

.net 10

Additional Context

what is perplexing to us is that the initial roundtrip is covered but not any subsequent one (the monitor workflow phase repeat prompts/runs)

backend uses inmemorysessionstore but no restarts in between (that would invalidate I suppose the wjhole session anyway)

[github-actions]

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

Agent analysis

Repro: ChatClientAgent.ResolveChatHistoryProvider() in dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs:979 returns null when chatOptions.ConversationId is non-null, disabling InMemoryChatHistoryProvider for AG-UI multi-turn sessions. Trigger: call RunAsync twice on an AGUIChatClient.AsAIAgent() with the same session — the first turn sets session.ConversationId from the AG-UI thread_id, causing subsequent turns to bypass history load/persist. Minimal repro: create AGUIChatClient, call AsAIAgent(), CreateSessionAsync(), RunAsync(msg1, session), RunAsync(msg2, session) and observe the second HTTP request only contains msg2 instead of full history.

• Failing test: dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUISessionMultiTurnBugTests.cs
• Files examined: dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs, dotnet/src/Microsoft.Agents.AI.AGUI/AGUIChatClient.cs, dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentSession.cs, dotnet/src/Microsoft.Agents.AI.Abstractions/InMemoryChatHistoryProvider.cs, dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs
• Tests run: AGUISessionMultiTurnBugTests.RunAsync_SecondTurn_ShouldIncludeFirstTurnMessagesInRequest (FAILED), AGUISessionMultiTurnBugTests.RunStreamingAsync_SecondTurn_ShouldIncludeFirstTurnMessagesInRequest (FAILED), AGUISessionMultiTurnBugTests.RunAsync_SecondTurn_ShouldPreserveThreadId (PASSED)
• Reported version: 1.6.1
• Current version: 1.6.1

[VaclavK]

as a further info this is the session dump after 2nd prompt finished - monitor workflow - you can see it missing

{
    "messages": [
        {
            "role": "user",
            "contents": [
                {
                    "$type": "text",
                    "text": "Start workflow for following conversation id 3affd058-de2b-4c6d-b553-f1de4903f175 with prompt:find teams user blah@blah.com"
                }
            ],
            "messageId": "40dbe01f87f545638b3aa38c5019b811"
        },
        {
            "createdAt": "2026-05-15T16:19:51.9355534\u002B00:00",
            "role": "assistant",
            "contents": [
                {
                    "$type": "functionCall",
                    "name": "workflow_start",
                    "arguments": {
                        "prompt": "find teams user blah@blah.com",
                        "sessionId": null,
                        "conversationId": "3affd058-de2b-4c6d-b553-f1de4903f175"
                    },
                    "informationalOnly": false,
                    "callId": "call_oohj7t5gkscS3o0Tam1jVGVf"
                }
            ],
            "messageId": "call_oohj7t5gkscS3o0Tam1jVGVf"
        },
        {
            "createdAt": "2026-05-15T16:19:56.4212568\u002B00:00",
            "role": "tool",
            "contents": [
                {
                    "$type": "functionResult",
                    "result": {
                        "workflowId": "d69a5467-62ff-47f8-9293-61b52cd71744",
                        "conversationId": "3affd058-de2b-4c6d-b553-f1de4903f175",
                        "sessionId": "0ee078b595e2446e998a6f8eb244c08e",
                        "failureRetrievingStatus": false
                    },
                    "callId": "call_oohj7t5gkscS3o0Tam1jVGVf"
                }
            ],
            "messageId": "b60d8b53619e4aebb64433d5604d40ae"
        },
        {
            "createdAt": "2026-05-15T16:19:57.7600975\u002B00:00",
            "role": "assistant",
            "contents": [
                {
                    "$type": "text",
                    "text": "{\u0022workflowId\u0022:\u0022d69a5467-62ff-47f8-9293-61b52cd71744\u0022,\u0022conversationId\u0022:\u00223affd058-de2b-4c6d-b553-f1de4903f175\u0022,\u0022sessionId\u0022:\u00220ee078b595e2446e998a6f8eb244c08e\u0022,\u0022failureRetrievingStatus\u0022:false}"
                }
            ],
            "messageId": "msg_086b7b9e95816689006a0747add8a08195b62233201f2e734f"
        }
    ]
}