---
title: Streaming Updates from Tools
description: Show progress updates and stream step output to users during long-running tool executions.
type: guide
summary: Write custom data parts from step functions to show progress updates during long-running tool calls.
prerequisites:
  - /docs/ai
  - /docs/foundations/streaming
related:
  - /docs/ai/defining-tools
  - /docs/ai/resumable-streams
  - /docs/api-reference/workflow/get-writable
---

After [building a durable AI agent](/docs/ai), we already get UI message chunks for displaying tool invocations and return values. However, for long-running steps, we may want to show progress updates, or stream step output to the user while it's being generated.

Workflow SDK enables this by letting step functions write custom chunks to the same stream the agent uses. These chunks appear as data parts in your messages, which you can render however you like.

As an example, we'll extend out Flight Booking Agent to use emit more granular progress updates while searching for flights.

<Steps>

<Step>

### Define Your Data Part Type

First, define a TypeScript type for your custom data part. This ensures type safety across your tool and client code:

```typescript title="schemas/chat.ts" lineNumbers
export interface FoundFlightDataPart {
  type: "data-found-flight"; // [!code highlight]
  id: string;
  data: {
    flightNumber: string;
    from: string;
    to: string;
  };
}
```

The `type` field must be a string starting with `data-` followed by your custom identifier. The `id` field should match the `toolCallId` so the client can associate the data with the correct tool invocation. Learn more about [data parts](https://ai-sdk.dev/docs/ai-sdk-ui/streaming-data#data-parts-persistent) in the AI SDK documentation.

</Step>

<Step>

### Emit Updates from Your Tool

Use [`getWritable()`](/docs/api-reference/workflow/get-writable) inside a step function to get a handle to the stream. This is the same stream that the LLM and other tools calls are writing to, so we can inject out own data packets directly.

{/* @skip-typecheck: incomplete code sample */}
```typescript title="workflows/chat/steps/tools.ts" lineNumbers
import { getWritable } from "workflow"; // [!code highlight]
import type { UIMessageChunk } from "ai";

export async function searchFlights(
  { from, to, date }: { from: string; to: string; date: string },
  { toolCallId }: { toolCallId: string } // [!code highlight]
) {
  "use step";

  const writable = getWritable<UIMessageChunk>(); // [!code highlight]
  const writer = writable.getWriter(); // [!code highlight]

  // ... existing logic to generate flights ...

  for (const flight of generatedFlights) { // [!code highlight]

    // Simulate the time it takes to find each flight
    await new Promise((resolve) => setTimeout(resolve, 1000)); // [!code highlight]

    await writer.write({ // [!code highlight]
      id: `${toolCallId}-${flight.flightNumber}`, // [!code highlight]
      type: "data-found-flight", // [!code highlight]
      data: flight, // [!code highlight]
    }); // [!code highlight]
  } // [!code highlight]

  writer.releaseLock(); // [!code highlight]

  return {
    message: `Found ${generatedFlights.length} flights from ${from} to ${to} on ${date}`,
    flights: generatedFlights.sort((a, b) => a.price - b.price), // Sort by price
  };
}
```

Key points:

- Call `getWritable<UIMessageChunk>()` to get the stream
- Use `getWriter()` to acquire a writer
- Write objects with `type`, `id`, and `data` fields
- Always call `releaseLock()` when done writing (learn more about [streaming](/docs/foundations/streaming))

</Step>

<Step>
### Handle Data Parts in the Client

Update your chat component to detect and render the custom data parts. Data parts are stored in the message's `parts` array alongside text and tool invocation parts:

{/* @skip-typecheck: incomplete code sample */}
```typescript title="app/page.tsx" lineNumbers
{message.parts.map((part, partIndex) => {
  // Render text parts
  if (part.type === "text") {
    return (
      <Response key={`${message.id}-text-${partIndex}`}>
        {part.text}
      </Response>
    );
  }

  // Render streaming flight data parts // [!code highlight]
  if (part.type === "data-found-flight") { // [!code highlight]
    const flight = part.data as { // [!code highlight]
      flightNumber: string; // [!code highlight]
      airline: string; // [!code highlight]
      from: string; // [!code highlight]
      to: string; // [!code highlight]
    }; // [!code highlight]
    return ( // [!code highlight]
      <div key={`${part.id}-${flight.flightNumber}`} className="p-3 bg-muted rounded-md"> // [!code highlight]
        <div className="font-medium">{flight.airline} - {flight.flightNumber}</div> // [!code highlight]
        <div className="text-muted-foreground">{flight.from} → {flight.to}</div> // [!code highlight]
      </div> // [!code highlight]
    ); // [!code highlight]
  } // [!code highlight]

  // ... other rendering logic ...
})}
```

The pattern is:

1. Data parts have a `type` field starting with `data-`
2. Match the type to your custom identifier (e.g., `data-found-flight`)
3. Use the data part's payload to display progress or intermediate results

</Step>

</Steps>

Now, when you run the agent to search for flights, you'll see the flight results pop up one after another. This will be most useful if you have tool calls that take minutes to complete, and you need to show granular progress updates to the user.

## Related Documentation

- [Building Durable AI Agents](/docs/ai) - Complete guide to durable agents
- [`getWritable()` API Reference](/docs/api-reference/workflow/get-writable) - Stream API details
- [Streaming](/docs/foundations/streaming) - Understanding workflow streams
