Tool Rendering
Render your agent's tool calls with custom UI components.
What is this?
Tools are how an LLM invokes predefined, typically-deterministic functions. Tool rendering lets you decide how each of those tool calls appears in the chat. Instead of showing raw JSON, you register a React component that draws a branded card for the call — arguments, live status, and the eventual result. This is the Generative UI variant CopilotKit calls tool rendering.
When should I use this?
Render tool calls when you want to:
- Show users exactly what tools the agent is invoking and with what arguments
- Display live progress indicators while a tool executes
- Render rich, polished results once a tool completes
- Give tool-heavy agents a transparent, on-brand chat experience
Default tool rendering (zero-config)
The simplest entry point: call useDefaultRenderTool() with no arguments.
CopilotKit registers its built-in DefaultToolCallRenderer as the *
wildcard — every tool call renders as a tidy status card (tool name, live
Running → Done pill, collapsible arguments/result) without you writing
any UI.
Without this hook the runtime has no * renderer and tool calls are
invisible — the user only sees the assistant's final text summary.
ms-agent-dotnet::tool-rendering-default-catchall. Known demos are bundled from manifest demos[i]; check the cell id and framework slug.Custom catch-all
Once you want on-brand chrome, pass a render function to
useDefaultRenderTool. It's a convenience wrapper around
useRenderTool({ name: "*", ... }) — one wildcard renderer handles every
tool call, named or not:
ms-agent-dotnet::tool-rendering-custom-catchall. Known demos are bundled from manifest demos[i]; check the cell id and framework slug.Per-tool renderers
The most expressive path is one renderer per tool name. The primary
tool-rendering cell wires two: get_weather draws a branded
WeatherCard, search_flights draws a FlightListCard. Each renderer
receives the tool's parsed arguments, a live status, and — once the agent
returns — the result:
render-weather-tool not found in ms-agent-dotnet::tool-rendering. Tag the relevant source lines with // @region[render-weather-tool] / // @endregion[render-weather-tool].render-flight-tool not found in ms-agent-dotnet::tool-rendering. Tag the relevant source lines with // @region[render-flight-tool] / // @endregion[render-flight-tool].The name you pass to useRenderTool must match the tool name the agent
exposes — that's how the runtime routes the call to your component.
Per-tool renderers compose with a catch-all: named renderers claim the
"interesting" tools and a wildcard handles everything else. In the primary
cell, the same CustomCatchallRenderer from above catches get_stock_price
and roll_dice:
catchall-renderer not found in ms-agent-dotnet::tool-rendering. Tag the relevant source lines with // @region[catchall-renderer] / // @endregion[catchall-renderer].The backend tool definition
The frontend renderer only sees what the agent sends down. Here's the
matching Python definition for get_weather — a standard LangChain tool,
no CopilotKit-specific plumbing required:
weather-tool-backend not found in ms-agent-dotnet::tool-rendering. Tag the relevant source lines with // @region[weather-tool-backend] / // @endregion[weather-tool-backend].