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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.

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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
  }
}
FieldTypeDescription
message_idstringSame across all chunks of one message — use this to group them
textstringA fragment of text to append to the message
attachmentsarrayAttachment(s) to append to the attachment list
response_suggestionsarraySuggestion(s) to append to the suggestions list
chunk_indexinteger1-based index. Process chunks in this order.
is_completebooleantrue on the final chunk — the message is now complete

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Reassembling chunks

To reconstruct the full message, concatenate text and append attachments / response_suggestions from each chunk in order:
ChunkReceivedAccumulated message
1text: "Hello ", attachments: [A1], suggestions: [S1]text: "Hello ", attachments: [A1], suggestions: [S1]
2text: "there, ", attachments: [], suggestions: []text: "Hello there, ", attachments: [A1], suggestions: [S1]
3text: "how can I help?", is_complete: truetext: "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.

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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];
  }
}

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Streaming flow

Last modified on May 19, 2026