> Fetch clean Markdown by appending `.md` to any page URL under https://signalwire.com/docs or requesting it with the HTTP header `Accept: text/markdown`. The root index at https://signalwire.com/docs/llms.txt lists the available documentation indexes.

# setOnRequestCallback

> Set a per-request callback for dynamic SWML generation.

[swmlbuilder]: /docs/server-sdks/reference/typescript/agents/swml-builder

Set a callback invoked per-request to dynamically build SWML. When set, the
static SwmlBuilder is ignored and the callback's returned
[`SwmlBuilder`][swmlbuilder] is used instead.

## **Parameters**

**`cb`** `OnRequestCallback` — required

Callback function that receives `(queryParams, bodyParams, headers)` and
returns a `SwmlBuilder` or `Promise<SwmlBuilder>`.

---

> **Note**
>
> On GET requests, `bodyParams` is always an empty object (`{}`). The body is only
> parsed on POST requests.

## **Returns**

`this` -- Returns the service for method chaining.

## **Example**

```typescript {5}
import { SWMLService, SwmlBuilder } from '@signalwire/sdk';

const service = new SWMLService({ name: 'dynamic-ivr' });

service.setOnRequestCallback(async (queryParams, bodyParams, headers) => {
  const builder = new SwmlBuilder();
  builder.addVerb('answer', {});

  if (queryParams.lang === 'es') {
    builder.addVerb('play', { url: 'https://example.com/greeting-es.mp3' });
  } else {
    builder.addVerb('play', { url: 'https://example.com/greeting-en.mp3' });
  }

  builder.addVerb('hangup', {});
  return builder;
});

await service.run();
```

> Set a per-request callback for dynamic SWML generation.