Tool-Based Generative UI

Render tool calls as components

When your Crew or Flow calls a tool, you rarely want the raw arguments dumped into the chat. Tool-based generative UI maps each tool the agent calls to a React component you own. The agent decides when to call the tool; you decide what the user sees. Because CopilotKit streams the tool call to the frontend as the model generates it, the arguments fill in progressively. Your component can paint the moment the first field arrives and update as the rest stream in. This guide builds a haiku generator: the agent calls a generate_haiku tool, and the frontend renders each haiku as a card. It assumes you already have a Crew or Flow talking to a Next.js app. If not, start with the Frontend Overview for the full server, runtime, and provider setup.

Tool rendering works with both Crews and Flows. The example below uses a Flow, but the frontend wiring is identical either way.

Walkthrough

Define the tool on the backend

Declare the tool with a JSON schema and pass it to the model. The copilotkit_stream wrapper together with stream=True is what streams the tool call to the frontend as it is generated, one argument chunk at a time.

# haiku_flow.py
from crewai.flow.flow import Flow, start
from litellm import acompletion
from ag_ui_crewai.sdk import copilotkit_stream, CopilotKitState

GENERATE_HAIKU_TOOL = {
    "type": "function",
    "function": {
        "name": "generate_haiku",
        "description": "Generate a haiku in Japanese and its English translation",
        "parameters": {
            "type": "object",
            "properties": {
                "japanese": {
                    "type": "array",
                    "items": {"type": "string"},
                    "description": "Three lines in Japanese",
                },
                "english": {
                    "type": "array",
                    "items": {"type": "string"},
                    "description": "Three lines in English",
                },
            },
            "required": ["japanese", "english"],
        },
    },
}


class HaikuFlow(Flow[CopilotKitState]):
    @start()
    async def chat(self):
        system_prompt = "You help the user write haikus. Use the generate_haiku tool."

        response = await copilotkit_stream(
            await acompletion(
                model="openai/gpt-4o",
                messages=[
                    {"role": "system", "content": system_prompt},
                    *self.state.messages,
                ],
                tools=[GENERATE_HAIKU_TOOL],
                parallel_tool_calls=False,
                stream=True,
            )
        )

        message = response.choices[0].message
        self.state.messages.append(message)

        if message.tool_calls:
            self.state.messages.append({
                "tool_call_id": message.tool_calls[0].id,
                "role": "tool",
                "content": "Haiku generated.",
            })

The tool has no Python implementation. It exists only so the model emits a structured call the frontend can render. After the call, append a short tool result so the conversation stays well-formed for the next turn.

Serve the Flow over AG-UI

Expose the Flow from your FastAPI app on its own path:

# server.py
from fastapi import FastAPI
from ag_ui_crewai.endpoint import add_crewai_flow_fastapi_endpoint
from haiku_flow import HaikuFlow

app = FastAPI(title="CrewAI Agent Server")

add_crewai_flow_fastapi_endpoint(
    app=app,
    flow=HaikuFlow(),
    path="/haiku",
)

Register the agent with the CopilotKit runtime and point <CopilotKit> at it exactly as shown in the Frontend Overview. The rest of this guide assumes the agent is registered under the id haiku.

On the frontend, call useRenderTool with the same name the backend declared. useRenderTool is the hook for rendering a tool call: it takes a render function and nothing to execute, because this tool is pure display.

Use useRenderTool when the tool only draws UI. If the tool also needs to run something in the browser, use useFrontendTool instead, which pairs a handler with an optional render.

"use client";
import { useRenderTool } from "@copilotkit/react-core/v2";
import { z } from "zod";

useRenderTool({
  name: "generate_haiku",
  parameters: z.object({
    japanese: z.array(z.string()),
    english: z.array(z.string()),
  }),
  render: ({ args, status }) => {
    if (!args.japanese) return <></>; // still streaming
    return <HaikuCard japanese={args.japanese} english={args.english} />;
  },
});

The tool is scoped to the active agent by the <CopilotKit agent="haiku"> provider, so no agentId is needed here. A few things to note:

name must match the backend tool name exactly (generate_haiku). That match is how CopilotKit routes the call to this component.
render receives { args, status }. args fills in progressively as the model streams the call; early on it may be empty or partial. status moves through "inProgress" / "executing" to "complete" if you want to show a loading state while arguments stream.
Guard against partial args. Return an empty fragment until the fields you need exist. Here we wait for args.japanese before rendering the card.

Render the haiku

The render function delegates to an ordinary React component. Nothing about it is CopilotKit-specific: it takes props and returns markup.

function HaikuCard({
  japanese,
  english,
}: {
  japanese: string[];
  english: string[];
}) {
  return (
    <div className="haiku-card">
      {japanese.map((line, i) => (
        <div key={i} className="haiku-line">
          <span className="jp">{line}</span>
          <span className="en">{english?.[i]}</span>
        </div>
      ))}
    </div>
  );
}

Because english streams in alongside japanese, use optional access (english?.[i]) so the card renders cleanly while the translation is still arriving.

Run it

Start both processes and ask the assistant for a haiku. The card renders as the arguments stream in, filling out line by line.

uvicorn server:app --port 8000   # terminal 1
npm run dev                       # terminal 2

How progressive rendering works

The model does not emit the tool call all at once. It streams tokens, and CopilotKit re-invokes your render function every time a new chunk of arguments arrives:

The call begins. args is empty, so your guard returns an empty fragment.
args.japanese fills in line by line. The card appears and grows.
args.english fills in. Translations slot into place.
The call completes. args holds the final, fully-validated object.

This is why the partial-args guard matters: render runs against incomplete data by design. Read only the fields you have, and let the rest paint as they arrive.

Backend tools

The generate_haiku tool above has no Python implementation — it exists only so the model emits a structured call the frontend renders. But a real tool your Crew or Flow runs server-side renders the same way. When an Agent or Crew executes a tool during its run, the bridge surfaces that tool call along with its result. Register a useRenderTool for the tool’s name and read result in the render:

useRenderTool({
  name: "get_weather",
  parameters: z.object({ location: z.string() }),
  render: ({ args, result, status }) => {
    if (status !== "complete") return <WeatherSkeleton location={args.location} />;
    return <WeatherCard data={JSON.parse(result)} />;
  },
});

A backend tool must return a JSON string, not a Python dict. The bridge stringifies tool output, so a raw dict arrives as a Python repr the browser cannot JSON.parse. Return json.dumps(...) from the tool.

Agentic Generative UI

Render live agent state as it changes across a multi-step run.

Human-in-the-Loop

Pause the agent to collect user approval or input mid-run.

Frontend Actions

Let the agent call functions that run in the browser.

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Tool-Based Generative UI

Render tool calls as components

Walkthrough

How progressive rendering works

Backend tools

Agentic Generative UI

Human-in-the-Loop

Frontend Actions

​Render tool calls as components

​Walkthrough

​How progressive rendering works

​Backend tools

​Related

Agentic Generative UI

Human-in-the-Loop

Frontend Actions

Render tool calls as components

Walkthrough

How progressive rendering works

Backend tools

Related