# Streaming events

> The event sequence each endpoint sends when stream is true.

Streams are server-sent events with `content-type: text/event-stream`. Events are relayed as they arrive.

## Sequences

**Chat Completions**

```text
data: {"id":"gen-...","choices":[{"index":0,"delta":{"role":"assistant","content":""}}]}

data: {"id":"gen-...","choices":[{"index":0,"delta":{"content":"Hello"}}]}

data: {"id":"gen-...","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}

data: {"id":"gen-...","choices":[],"usage":{"prompt_tokens":14,"completion_tokens":7,"total_tokens":21}}

data: [DONE]
```

**Responses**

```text
event: response.created
event: response.in_progress
event: response.output_item.added
event: response.content_part.added
event: response.output_text.delta
event: response.output_text.done
event: response.content_part.done
event: response.output_item.done
event: response.completed
```

**Messages**

```text
event: message_start
event: content_block_start
event: content_block_delta
event: content_block_stop
event: message_delta
event: message_stop
```

## Chat Completions

Each chunk has `choices[].delta`. The last content chunk carries `finish_reason`. The last chunk before `data: [DONE]` carries `usage`. With `stream_options.include_usage`, that chunk has an empty `choices` array, as in OpenAI's API. Without it, the chunk keeps one choice with an empty delta.

Comment lines that begin with `:` may appear between events. Parsers skip them.

## Responses

The event types are `response.created`, `response.in_progress`, `response.output_item.added`, `response.content_part.added`, `response.output_text.delta`, `response.output_text.done`, `response.content_part.done`, `response.function_call_arguments.delta`, `response.function_call_arguments.done`, `response.output_item.done`, `response.completed`, `response.incomplete`, `response.failed` and `error`. The stream ends with one of three final events, each carrying the final response object, with its usage when the provider reports it: `response.completed` when the answer finished, `response.incomplete` when it stopped early (the reason is in `incomplete_details`), or `response.failed`.

## Messages

`message_start` opens with the input token count. Each content block sends `content_block_start`, one or more `content_block_delta` events, and `content_block_stop`. Delta types are `text_delta`, `input_json_delta`, `thinking_delta` and `signature_delta`. `message_delta` carries `stop_reason` and cumulative `usage.output_tokens`, then `message_stop` ends the stream.

`ping` events can appear anywhere, and an `error` event can appear mid-stream. Ignore event types you do not know.

## Images

`POST /v1/images` streams only on models that support it. The events are `image_generation.partial_image` as the image forms, then `image_generation.completed` with the final image and usage, then `data: [DONE]`. An `error` event can replace either.

## Errors

A failure after the stream starts keeps HTTP `200` and arrives as an event: a top-level `error` with `finish_reason: "error"` on Chat Completions, `response.failed` or `error` on Responses, and an `error` event on Messages.