CopilotKit

Tool Call Rendering

Render your agent's tool calls with custom UI components.

"""LangGraph agent for the CopilotKit Tool Rendering demos.Backs the three tool-rendering cells:  - tool-rendering-default-catchall  (no frontend renderers)  - tool-rendering-custom-catchall   (wildcard renderer on frontend)  - tool-rendering                   (per-tool + catch-all on frontend)All three share this backend — they differ only in how the frontendrenders the same tool calls. The `tool-rendering-reasoning-chain` cellhas its own agent (`tool_rendering_reasoning_chain_agent.py`) becauseit routes through the OpenAI Responses API for reasoning streaming."""from random import choice, randintfrom langchain.agents import create_agentfrom langchain.tools import toolfrom langchain_openai import ChatOpenAIfrom copilotkit import CopilotKitMiddleware# Multi-tool-per-question prompt.## This backend serves the tool-rendering demos, whose JOB is to show the# rendering patterns (per-tool, catch-all, default fallback). The agent# may call multiple tools per turn when the user asks for them. The# `roll_d20` tool accepts a deterministic `value` parameter so the# aimock fixtures can script the exact dice sequence the e2e tests# assert against.SYSTEM_PROMPT = (    "You are a travel & lifestyle concierge. Use the mock tools for "    "weather, flights, stock prices, or d20 rolls when the user asks; "    "otherwise reply in plain text. For flights, default origin to 'SFO' "    "if the user only names a destination. Call multiple tools in one "    "turn if asked. After tools return, summarize in one short sentence. "    "Never fabricate data a tool could provide.")@tooldef get_weather(location: str) -> dict:    """Get the current weather for a given location."""    return {        "city": location,        "temperature": 68,        "humidity": 55,        "wind_speed": 10,        "conditions": "Sunny",    }@tooldef search_flights(origin: str, destination: str) -> dict:    """Search mock flights from an origin airport to a destination airport."""    return {        "origin": origin,        "destination": destination,        "flights": [            {                "airline": "United",                "flight": "UA231",                "depart": "08:15",                "arrive": "16:45",                "price_usd": 348,            },            {                "airline": "Delta",                "flight": "DL412",                "depart": "11:20",                "arrive": "19:55",                "price_usd": 312,            },            {                "airline": "JetBlue",                "flight": "B6722",                "depart": "17:05",                "arrive": "01:30",                "price_usd": 289,            },        ],    }@tooldef get_stock_price(    ticker: str,    price_usd: float | None = None,    change_pct: float | None = None,) -> dict:    """Get a mock current price for a stock ticker.    The optional `price_usd` and `change_pct` arguments let the LLM (or    aimock fixture) script a deterministic ticker quote for testing —    when supplied, the tool echoes them back verbatim. When omitted (or    `None`), the tool returns mock random values. Mirrors the    deterministic-`value` pattern on `roll_d20`.    """    return {        "ticker": ticker.upper(),        "price_usd": (            round(float(price_usd), 2)            if price_usd is not None            else round(100 + randint(0, 400) + randint(0, 99) / 100, 2)        ),        "change_pct": (            round(float(change_pct), 2)            if change_pct is not None            else round(choice([-1, 1]) * (randint(0, 300) / 100), 2)        ),    }@tooldef roll_d20(value: int = 0) -> dict:    """Roll a 20-sided die.    The `value` argument lets the LLM (or aimock fixture) script a    deterministic roll for testing — the tool simply echoes it back as    the result. When called without `value` (or with 0), the tool    returns a random natural d20 roll.    """    rolled = value if isinstance(value, int) and 1 <= value <= 20 else randint(1, 20)    return {"sides": 20, "value": rolled, "result": rolled}model = ChatOpenAI(model="gpt-5.4")graph = create_agent(    model=model,    tools=[get_weather, search_flights, get_stock_price, roll_d20],    middleware=[CopilotKitMiddleware()],    system_prompt=SYSTEM_PROMPT,)

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.

Free course: See this pattern built end-to-end in Build Interactive Agents with Generative UI — a free DeepLearning.AI short course taught by CopilotKit's CEO covering the full Generative UI spectrum (Controlled, Declarative, and Open-Ended).

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.

page.tsx
  // Opt in to CopilotKit's built-in default tool-call card. Called with  // no config so the package-provided `DefaultToolCallRenderer` is used  // as the wildcard renderer — this is the "out-of-the-box" UI the cell  // is meant to showcase.  useDefaultRenderTool();

Here's what the built-in status card looks like for each tool call:

"use client";// Tool Rendering — DEFAULT CATCH-ALL variant (simplest).//// This cell is the simplest point in the three-way progression. The// backend exposes a handful of mock tools (get_weather, search_flights,// get_stock_price, roll_dice) and the frontend ONLY opts into// CopilotKit's built-in default tool-call card — no per-tool renderers,// no custom wildcard UI.//// `useDefaultRenderTool()` (called with no config) registers the built-// in `DefaultToolCallRenderer` under the `*` wildcard. That renderer// shows the tool name, a live status pill (Running → Done), and a// collapsible "Arguments / Result" section that fills in as the call// progresses. Without this hook the runtime has NO `*` renderer, so// `useRenderToolCall` falls through to `null` and tool calls are// invisible — the user only sees the assistant's final text summary.import React from "react";import {  CopilotKit,  CopilotChat,  useDefaultRenderTool,} from "@copilotkit/react-core/v2";import { useSuggestions } from "./suggestions";export default function ToolRenderingDefaultCatchallDemo() {  return (    <CopilotKit      runtimeUrl="/api/copilotkit"      agent="tool-rendering-default-catchall"    >      <div className="flex justify-center items-center h-screen w-full">        <div className="h-full w-full max-w-4xl">          <Chat />        </div>      </div>    </CopilotKit>  );}function Chat() {  // Opt in to CopilotKit's built-in default tool-call card. Called with  // no config so the package-provided `DefaultToolCallRenderer` is used  // as the wildcard renderer — this is the "out-of-the-box" UI the cell  // is meant to showcase.  useDefaultRenderTool();  useSuggestions();  return (    <CopilotChat      agentId="tool-rendering-default-catchall"      className="h-full rounded-2xl"    />  );}

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:

page.tsx
  // `useDefaultRenderTool` is a convenience wrapper around  // `useRenderTool({ name: "*", ... })` — a single wildcard renderer  // that handles every tool call not claimed by a named renderer.  useDefaultRenderTool(    {      render: ({ name, parameters, status, result }) => (        <CustomCatchallRenderer          name={name}          parameters={parameters}          status={status as CatchallToolStatus}          result={result}        />      ),    },    [],  );

Here's the branded catch-all in action, where every tool call gets the same on-brand card:

"use client";// Tool Rendering — CUSTOM CATCH-ALL variant (middle of the progression).//// Same backend tools as `tool-rendering-default-catchall`, but this// cell opts out of CopilotKit's built-in default tool-call UI by// registering a SINGLE custom wildcard renderer via// `useDefaultRenderTool`. The same branded card now paints every tool// call — no per-tool renderers yet.import React from "react";import {  CopilotKit,  CopilotChat,  useDefaultRenderTool,} from "@copilotkit/react-core/v2";import {  CustomCatchallRenderer,  type CatchallToolStatus,} from "./custom-catchall-renderer";import { useSuggestions } from "./suggestions";export default function ToolRenderingCustomCatchallDemo() {  return (    <CopilotKit      runtimeUrl="/api/copilotkit"      agent="tool-rendering-custom-catchall"    >      <div className="flex justify-center items-center h-screen w-full">        <div className="h-full w-full max-w-4xl">          <Chat />        </div>      </div>    </CopilotKit>  );}function Chat() {  // `useDefaultRenderTool` is a convenience wrapper around  // `useRenderTool({ name: "*", ... })` — a single wildcard renderer  // that handles every tool call not claimed by a named renderer.  useDefaultRenderTool(    {      render: ({ name, parameters, status, result }) => (        <CustomCatchallRenderer          name={name}          parameters={parameters}          status={status as CatchallToolStatus}          result={result}        />      ),    },    [],  );  useSuggestions();  return (    <CopilotChat      agentId="tool-rendering-custom-catchall"      className="h-full rounded-2xl"    />  );}

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:

The frontend pattern is the same for every backend. This shared, docs-only example includes every component, type, and helper that its renderers use:

components/weather-card.tsx
export interface WeatherCardProps {
  loading: boolean;
  location: string;
  temperature?: number;
  humidity?: number;
  windSpeed?: number;
  conditions?: string;
}

export function WeatherCard({
  loading,
  location,
  temperature,
  humidity,
  windSpeed,
  conditions,
}: WeatherCardProps) {
  return (
    <article className="rounded-xl border p-4">
      <h3 className="font-semibold">{location || "Weather"}</h3>
      {loading ? (
        <p>Fetching weather...</p>
      ) : (
        <dl>
          <div>
            <dt>Conditions</dt>
            <dd>{conditions ?? "--"}</dd>
          </div>
          <div>
            <dt>Temperature</dt>
            <dd>{temperature ?? "--"}&deg;F</dd>
          </div>
          <div>
            <dt>Humidity</dt>
            <dd>{humidity ?? "--"}%</dd>
          </div>
          <div>
            <dt>Wind</dt>
            <dd>{windSpeed ?? "--"} mph</dd>
          </div>
        </dl>
      )}
    </article>
  );
}
components/flight-list-card.tsx
export interface Flight {
  airline?: string;
  flight?: string;
  depart?: string;
  arrive?: string;
  price_usd?: number;
}

export interface FlightListCardProps {
  loading: boolean;
  origin: string;
  destination: string;
  flights: Flight[];
}

export function FlightListCard({
  loading,
  origin,
  destination,
  flights,
}: FlightListCardProps) {
  return (
    <article className="rounded-xl border p-4">
      <h3 className="font-semibold">
        {origin || "?"} → {destination || "?"}
      </h3>
      {loading ? (
        <p>Searching...</p>
      ) : (
        <ul>
          {flights.map((flight, index) => (
            <li key={`${flight.flight ?? "flight"}-${index}`}>
              {flight.airline ?? "--"} {flight.flight ?? ""}:{" "}
              {flight.depart ?? "?"} → {flight.arrive ?? "?"}
              {flight.price_usd !== undefined ? ` ($${flight.price_usd})` : ""}
            </li>
          ))}
        </ul>
      )}
    </article>
  );
}
lib/parse-json-result.ts
export function parseJsonResult<T>(result: unknown): T {
  if (!result) return {} as T;

  try {
    return (typeof result === "string" ? JSON.parse(result) : result) as T;
  } catch {
    return {} as T;
  }
}
app/tool-renderers.tsx
"use client";

import { useRenderTool } from "@copilotkit/react-core/v2";
import { z } from "zod";
import { WeatherCard } from "../components/weather-card";
import { FlightListCard, type Flight } from "../components/flight-list-card";
import { parseJsonResult } from "../lib/parse-json-result";

interface WeatherResult {
  city?: string;
  temperature?: number;
  humidity?: number;
  wind_speed?: number;
  conditions?: string;
}

interface FlightSearchResult {
  origin?: string;
  destination?: string;
  flights?: Flight[];
}

export function ToolRenderers() {
  useRenderTool(
    {
      name: "get_weather",
      parameters: z.object({ location: z.string() }),
      render: ({ parameters, result, status }) => {
        const parsed = parseJsonResult<WeatherResult>(result);
        return (
          <WeatherCard
            loading={status !== "complete"}
            location={parameters?.location ?? parsed.city ?? ""}
            temperature={parsed.temperature}
            humidity={parsed.humidity}
            windSpeed={parsed.wind_speed}
            conditions={parsed.conditions}
          />
        );
      },
    },
    [],
  );

  useRenderTool(
    {
      name: "search_flights",
      parameters: z.object({
        origin: z.string(),
        destination: z.string(),
      }),
      render: ({ parameters, result, status }) => {
        const parsed = parseJsonResult<FlightSearchResult>(result);
        return (
          <FlightListCard
            loading={status !== "complete"}
            origin={parameters?.origin ?? parsed.origin ?? ""}
            destination={parameters?.destination ?? parsed.destination ?? ""}
            flights={parsed.flights ?? []}
          />
        );
      },
    },
    [],
  );

  return null;
}

Mount the renderers anywhere beneath the same CopilotKit provider as your chat. The renderer component returns no layout of its own; it registers the two named renderers for tool calls in the chat:

app/page.tsx
"use client";

import { CopilotChat, CopilotKit } from "@copilotkit/react-core/v2";
import { ToolRenderers } from "./tool-renderers";

export default function Page() {
  return (
    <CopilotKit runtimeUrl="/api/copilotkit" agent="tool-rendering">
      <ToolRenderers />
      <CopilotChat agentId="tool-rendering" />
    </CopilotKit>
  );
}

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:

page.tsx
  // Wildcard catch-all for anything that doesn't match a per-tool  // renderer above.  useDefaultRenderTool(    {      render: ({ name, parameters, status, result }) => (        <CustomCatchallRenderer          name={name}          parameters={parameters}          status={status as CatchallToolStatus}          result={result}        />      ),    },    [],  );

The backend tool definition#

The frontend renderer only sees what the agent sends down. Here's the matching backend definition for get_weather: expose a tool named get_weather, return structured data, and let the frontend renderer with the same name paint the card.

tool_rendering_agent.py
from random import choice, randintfrom langchain.agents import create_agentfrom langchain.tools import toolfrom langchain_openai import ChatOpenAIfrom copilotkit import CopilotKitMiddleware# Multi-tool-per-question prompt.## This backend serves the tool-rendering demos, whose JOB is to show the# rendering patterns (per-tool, catch-all, default fallback). The agent# may call multiple tools per turn when the user asks for them. The# `roll_d20` tool accepts a deterministic `value` parameter so the# aimock fixtures can script the exact dice sequence the e2e tests# assert against.SYSTEM_PROMPT = (    "You are a travel & lifestyle concierge. Use the mock tools for "    "weather, flights, stock prices, or d20 rolls when the user asks; "    "otherwise reply in plain text. For flights, default origin to 'SFO' "    "if the user only names a destination. Call multiple tools in one "    "turn if asked. After tools return, summarize in one short sentence. "    "Never fabricate data a tool could provide.")@tooldef get_weather(location: str) -> dict:    """Get the current weather for a given location."""    return {        "city": location,        "temperature": 68,        "humidity": 55,        "wind_speed": 10,        "conditions": "Sunny",    }

On this page

What is this?When should I use this?Default tool rendering (zero-config)Custom catch-allPer-tool renderersThe backend tool definition