Represents a streamed chunk of a chat completion response returned by the model, based on the provided input.

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Object Fields

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id
string
A unique identifier for the chat completion. Each chunk has the same ID.
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choices
array
A list of chat completion choices. Can be more than one if n is greater than 1.Each choice object contains:
  • index (integer): The index of the choice in the list
  • delta (object): The delta content generated by the model
    • role (string): The role of the message author (only in first chunk)
    • content (string): The content fragment of the message
  • finish_reason (string | null): The reason the model stopped generating, null if still streaming
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created
integer
The Unix timestamp (in seconds) of when the chat completion was created. Each chunk has the same timestamp.
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model
string
The model that generated the completion.
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system_fingerprint
string
This fingerprint represents the backend configuration that the model runs with.
​
object
string
The object type, which is always chat.completion.chunk.

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Example

{
  "id": "chatcmpl-abc123",
  "object": "chat.completion.chunk",
  "created": 1677652288,
  "model": "gpt-4o",
  "choices": [
    {
      "index": 0,
      "delta": {
        "content": "Hello"
      },
      "finish_reason": null
    }
  ],
  "system_fingerprint": "fp_44709d6fcb"
}

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Streaming Format

When streaming is enabled (stream: true), the response is sent as Server-Sent Events (SSE). Each event contains a chunk object prefixed with data: : 
data: {"id":"chatcmpl-abc123","object":"chat.completion.chunk","created":1677652288,"model":"gpt-4o","choices":[{"index":0,"delta":{"role":"assistant","content":""},"finish_reason":null}]}

data: {"id":"chatcmpl-abc123","object":"chat.completion.chunk","created":1677652288,"model":"gpt-4o","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]}

data: {"id":"chatcmpl-abc123","object":"chat.completion.chunk","created":1677652288,"model":"gpt-4o","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}

data: [DONE]