Represents a streamed chunk of a chat completion response returned by Claude models.

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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.
​
choices
array
A list of chat completion choices.Each choice object contains:
  • index (integer): The index of the choice
  • delta (object): The delta content generated by the model
    • role (string): The role (only in first chunk)
    • content (string): The content fragment
  • finish_reason (string | null): null if still streaming
​
created
integer
The Unix timestamp (in seconds). Each chunk has the same timestamp.
​
model
string
The model that generated the completion.
​
object
string
The object type, which is always chat.completion.chunk.

​
Example

{
  "id": "chatcmpl-claude-123",
  "object": "chat.completion.chunk",
  "created": 1677652288,
  "model": "claude-sonnet-4-20250514",
  "choices": [
    {
      "index": 0,
      "delta": {
        "content": "Hello"
      },
      "finish_reason": null
    }
  ]
}

​
Streaming Format

data: {"id":"chatcmpl-claude-123","object":"chat.completion.chunk","created":1677652288,"model":"claude-sonnet-4-20250514","choices":[{"index":0,"delta":{"role":"assistant","content":""},"finish_reason":null}]}

data: {"id":"chatcmpl-claude-123","object":"chat.completion.chunk","created":1677652288,"model":"claude-sonnet-4-20250514","choices":[{"index":0,"delta":{"content":"Hello"},"finish_reason":null}]}

data: {"id":"chatcmpl-claude-123","object":"chat.completion.chunk","created":1677652288,"model":"claude-sonnet-4-20250514","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}

data: [DONE]