Streaming | PolyAI Platform

When streaming_enabled is true on session creation, agent responses arrive incrementally as EVENT_TYPE_POLY_AGENT_MESSAGE_CHUNK events instead of a single EVENT_TYPE_POLY_AGENT_MESSAGE. This lets you display the response as it’s generated.

Chunk format

{
  "type": "EVENT_TYPE_POLY_AGENT_MESSAGE_CHUNK",
  "payload": {
    "message_id": "msg_abc123",
    "text": "I'd be happy to ",
    "attachments": [],
    "response_suggestions": [],
    "chunk_index": 1,
    "is_complete": false
  }
}

Field	Type	Description
`message_id`	string	Same across all chunks of one message — use this to group them
`text`	string	A fragment of text to append to the message
`attachments`	array	Attachment(s) to append to the attachment list
`response_suggestions`	array	Suggestion(s) to append to the suggestions list
`chunk_index`	integer	1-based index. Process chunks in this order.
`is_complete`	boolean	`true` on the final chunk — the message is now complete

Reassembling chunks

To reconstruct the full message, concatenate text and append attachments / response_suggestions from each chunk in order:

Chunk	Received	Accumulated message
1	`text: "Hello "`, `attachments: [A1]`, `suggestions: [S1]`	`text: "Hello "`, `attachments: [A1]`, `suggestions: [S1]`
2	`text: "there, "`, `attachments: []`, `suggestions: []`	`text: "Hello there, "`, `attachments: [A1]`, `suggestions: [S1]`
3	`text: "how can I help?"`, `is_complete: true`	`text: "Hello there, how can I help?"`, `attachments: [A1]`, `suggestions: [S1]`

The final chunk (is_complete: true) may have empty text. It signals that the message is complete and may carry attachments or response suggestions that were only available after the full response was generated.

Example: handling chunks in JavaScript

const streamingMessages = {};
function handleChunk(payload) {
  const { message_id, text, attachments, response_suggestions, chunk_index, is_complete } = payload;
  if (!streamingMessages[message_id]) {
    streamingMessages[message_id] = { text: "", attachments: [], suggestions: [] };
  }
  const msg = streamingMessages[message_id];
  msg.text += text;
  msg.attachments.push(...attachments);
  msg.suggestions.push(...response_suggestions);
  updateMessageInUI(message_id, msg);
  if (is_complete) {
    finalizeMessageInUI(message_id, msg);
    delete streamingMessages[message_id];
  }
}

Chunk format

Reassembling chunks

Example: handling chunks in JavaScript

Streaming flow