.NET: Is Singleton the intended lifetime for agents, tools and AIContextProviders, or just the default?

[Paulo Pozeti (ppozeti)]

AddAIAgent defaults to ServiceLifetime.Singleton, WithAITool(AITool) always registers a keyed singleton, and the WithAITool factory overload throws if a tool is shorter-lived than its agent. I cannot find anywhere in the docs or docs/decisions that explains whether this is the intended lifetime model or only the historical default.

Searching for related discussions/issues I found:

• The AIAgent and AIContextProvider remarks say instances are shared across many conversations and must keep per-session state in AgentSession. That suggests a stateless-definition model where Singleton is the natural choice.
• The accepted answer in Best practice for handling per-user chat history with DI in agent-based REST APIs #4930 says "the agent should be a singleton, not per-request", citing only the parameter default.
• .NET: Allow specifying the lifetime of agents when registering via AddAIAgent in Hosting #4457 / .NET: Add ServiceLifetime support for Hosting DI registration #4476 added the ServiceLifetime parameter because "we cannot assume that all agent dependencies (e.g. function tools) will always be singletons".
• .NET: [Bug]: Protocol hosting ignores AIAgent lifetimes and lacks consistent request-time selection #7733 is fixing the hosting endpoints so scoped and transient agents resolve per request.

To me is clear that agents/tools being singleton isn't part of the design at all, and it's up for us, the consumers, to handle our scopes however we like. But some people might look at everything using Singleton and think that this is a framework design pattern, not a choice.

The question is: "agents, tools and context providers are stateless and shared, state lives in the session" the design intent, or is Singleton a convenience default carried over from AddChatClient? Can we make it more explicit in the docs?