Defines model-callable tools and renders their calls in assistant-ui. The authoring model: a "use generative" file whose default export is defineToolkit({...}), compiled by withAui from @assistant-ui/next, aui() from @assistant-ui/vite, or withAui from @assistant-ui/metro, mounted with AuiConfig({ tools: Tools({ toolkit }) }), and exposed to the model with new AISDKToolkit({ toolkit }).tools({ frontend }). Covers the tool kinds (backend, "use client" frontend, humanTool(), providerTool(), externalTool(), stubTool(), defineMcpToolkit()), render and display, ToolCallMessagePartProps, approval gates, MCP servers, MCP Apps, WebMCP, and sub-agent messages. Reach for it when a tool is never called, a tool UI does not render, a frontend result never reaches the model, humanTool() throws at runtime, or an approval gate cannot be answered. For model-composed UI use generative-ui, for MCP servers users add in the browser use react-mcp, and for the styled ToolFallback and ToolGroup use elements.