Expose server-side (backend) tools an AG-UI agent can call with the AG-UI .NET SDK — C# functions that run on the server, where the server executes the call and feeds the result back to the model. USE FOR: defining an AIFunction with AIFunctionFactory.Create and registering it on the server's IChatClient via ConfigureOptions + UseFunctionInvocation; making the model call your backend function during a run; AOT-safe tool arguments/results (registering a JsonSerializerContext for complex tool parameter types); parallel/concurrent backend tool calls (AllowConcurrentInvocation); TerminateOnUnknownCalls behavior. DO NOT USE FOR: tools that run in the client/frontend (use agui-dotnet-client-tools); pausing a tool for human approval or input (interrupts / human-in-the-loop); plain streaming chat with no tools (use agui-dotnet-streaming-chat); shared state, generative UI, multimodal, or protobuf.