.NET: Foundry hosting rejects Harness local chat history marker when AllowStoredOutputEnabled is false

[rogerbarreto]

Description

Microsoft.Agents.AI.Foundry.Hosting fails requests from HarnessAgent (and any ChatClientAgent using per-service-call local chat history persistence) with agent_stored_output_not_disabled when FoundryResponsesOptions.AllowStoredOutputEnabled is left at its default false.

Root cause

• HarnessAgent defaults to InMemoryChatHistoryProvider, which keeps history in AgentSession.StateBag.
• After each service call, PerServiceCallChatHistoryPersistingChatClient writes the internal sentinel LocalHistoryConversationId ("_agent_local_chat_history") into ChatClientAgentSession.ConversationId. The sentinel means "history is persisted locally", not "the service stored this conversation".
• AgentFrameworkResponseHandler.CheckNotAllowedStoreUsage treats every non-null ConversationId as service storage, so the local sentinel triggers the stored-output rejection.

Impact

Hosted Harness agents only work if developers opt in to AllowStoredOutputEnabled = true, even though the host still sends store=false and no conversation is stored by the service. The public foundry-samples Harness samples currently carry that opt-in as a workaround.

Expected behavior

• The local history sentinel is allowed when AllowStoredOutputEnabled is false.
• A real service-stored conversation ID is still rejected.
• The host continues to request store=false by default.

Related

• .NET: Should we exclude cases where ConversationId="_agent_local_chat_history"? #8133 describes the same sentinel being misread by CompactionProvider; that component is not covered by this issue.