Expose client-side (frontend) tools to an AG-UI agent with the AG-UI .NET SDK — C# functions that run in the CLIENT app (read local state, GPS, UI, device APIs), where the client executes the call and returns the result so the run continues. USE FOR: declaring an AIFunction in the client and passing it via ChatOptions.Tools to AGUIChatClient; having the client execute a tool the model requested and feed the result back automatically; understanding why no UseFunctionInvocation is needed on the client (AGUIChatClient already invokes functions); what the server must do (UseFunctionInvocation + TerminateOnUnknownCalls) so it forwards an unknown/client tool instead of erroring. DO NOT USE FOR: tools that run on the server/backend (use agui-dotnet-server-tools); pausing for human approval/input before acting (interrupts / human-in-the-loop); plain chat (use agui-dotnet-streaming-chat); shared state, generative UI, multimodal, or protobuf.