> ## Documentation Index
> Fetch the complete documentation index at: https://deepl-c950b784-docs-language-table-from-v3.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Message Encoding

> Choose between JSON and MessagePack encoding for DeepL Voice API WebSocket messages, and avoid the frame type and map encoding pitfalls.

WebSocket messages in a Voice API session can be encoded in two formats, chosen with the `message_format` option when you [request the session](/api-reference/voice/request-session). Start with JSON for the best developer experience, and switch to MessagePack if you need better performance.

| **Format**                          | **Frame type** | **Binary data**        | **Trade-off**                                                 |
| :---------------------------------- | :------------- | :--------------------- | :------------------------------------------------------------ |
| JSON (default)                      | TEXT           | base64-encoded strings | Human-readable, easy to debug                                 |
| [MessagePack](https://msgpack.org/) | BINARY         | raw binary             | Roughly 25-30% less bandwidth, 2x-4x faster encoding/decoding |

<Warning>
  Send JSON messages as TEXT frames and MessagePack messages as BINARY frames. Sending the wrong frame type results in connection errors.
</Warning>

MessagePack messages must be encoded as maps with string keys, not as arrays. The structure must match the JSON schema exactly, with all field names preserved (for example, `{"source_media_chunk": {"data": <binary>}}`). Some MessagePack libraries default to array encoding for performance, so check your library's configuration.

The following example sends the same audio chunk in both encodings:

<CodeGroup>
  ```javascript JSON theme={null}
  // Raw binary audio data
  const audioData = getAudioChunk();

  // Base64 encode the audio data
  const base64Audio = btoa(audioData);

  const message = {
    source_media_chunk: {
      data: base64Audio
    }
  };

  // Send as TEXT frame
  websocket.send(JSON.stringify(message));
  ```

  ```javascript MessagePack theme={null}
  import { pack } from 'msgpackr';

  // Raw binary audio data
  const audioData = getAudioChunk();

  const message = {
    source_media_chunk: {
      data: audioData  // No base64 encoding needed
    }
  };

  // Send as BINARY frame
  websocket.send(pack(message));
  ```
</CodeGroup>

For the full list of message types exchanged during a session, see the [WebSocket Streaming reference](/api-reference/voice/websocket-streaming). For how those messages fit into the session lifecycle, see [Understanding Voice Sessions](/docs/voice/understanding-voice-sessions).