# Streaming Event Object

Represents individual Server-Sent Events (SSE) streamed when `stream: true`. Each event shows incremental progress during response generation.

## Event Types

### Response Lifecycle Events

**`response.created`**

Emitted when response generation begins.

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**`event`** string - Always `response.created`

**`status`** string - Always `in_progress`

**`thread_id`** string - Thread identifier

**`is_summarized`** boolean - Whether conversation was summarized

**`assistant_id`** string optional - Assistant ID if thread has one

**`output_mode`** string optional - Output format mode (text, plain, blocks)

**`response.completed`**

Emitted when response generation completes successfully.

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**`event`** string - Always `response.completed`

**`status`** string - Always `completed`

**`thread_id`** string - Thread identifier

**`response_id`** string - Unique response identifier

**`is_summarized`** boolean - Whether conversation was summarized

**`response`** string optional - Complete response text (not present when using blocks mode)

**`response_blocks`** array optional - Structured response blocks (present in blocks mode or when tools are used)

**`assistant_id`** string optional - Assistant ID if thread has one

**`reasoning_content`** string optional - Complete reasoning content if reasoning was enabled

**`tools`** object optional - Tool information if requested via include parameter

**`response.error`**

Emitted when an error occurs during response generation.

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**`event`** string - Always `response.error`

**`status`** string - Always `failed`

**`message`** string - Error message

### Content Generation Events

**`response.content_delta`**

Emitted for each chunk of generated text.

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**`event`** string - Always `response.content_delta`

**`delta`** string - Text chunk

**`response.block`**

Emitted for each response block (tool calls, tool results) in blocks mode.

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**`event`** string - Always `response.block`

**`block`** object - Block object with type and content

Block types:

- **`text`** - Text content block
- **`tool_call`** - Tool invocation block
- **`tool_result`** - Tool execution result block


### Reasoning Events

**`reasoning.started`**

Emitted when model enters reasoning phase.

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**`event`** string - Always `reasoning.started`

**`reasoning.content`**

Emitted for each chunk of reasoning content.

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**`event`** string - Always `reasoning.content`

**`content`** string - Reasoning text chunk

**`reasoning.completed`**

Emitted when reasoning phase completes.

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**`event`** string - Always `reasoning.completed`

Response Created

```json
{
  "event": "response.created",
  "thread_id": "thread_xyz789",
  "status": "in_progress",
  "is_summarized": false,
  "output_mode": "text"
}
```

Text Delta

```json
{
  "event": "response.content_delta",
  "delta": "The capital of France"
}
```

Thinking Started

```json
{
  "event": "reasoning.started"
}
```

Thinking Delta

```json
{
  "event": "reasoning.content",
  "content": "The user is asking for weather information. I should call get_weather with city=Paris."
}
```

Thinking Completed

```json
{
  "event": "reasoning.completed"
}
```

Tool Calls Executing

```json
{
  "event": "response.block",
  "block": {
    "type": "tool_call",
    "tool_name": "get_weather",
    "arguments": {
      "city": "Paris"
    },
    "success": true,
    "execution_time_ms": 245,
    "id": "call_paris_weather",
    "result": "{\"temperature\":15,\"condition\":\"Partly Cloudy\"}"
  }
}
```

Tool Calls Completed

```json
{
  "event": "response.block",
  "block": {
    "type": "tool_call",
    "tool_name": "get_weather",
    "arguments": {
      "city": "Paris"
    },
    "success": true,
    "execution_time_ms": 245,
    "id": "call_paris_weather",
    "result": "{\"temperature\":15,\"condition\":\"Partly Cloudy\",\"humidity\":65}"
  }
}
```

Iteration Started

```json
{
  "event": "response.content_delta",
  "delta": "I'll check the current weather in Paris."
}
```

Response Done

```json
{
  "thread_id": "thread_xyz789",
  "event": "response.completed",
  "status": "completed",
  "response_id": "resp_abc123",
  "is_summarized": false,
  "response": "The capital of France is Paris."
}
```

Error Event

```json
{
  "event": "response.error",
  "status": "failed",
  "message": "Rate limit exceeded. Please try again later."
}
```

## Related

- [Create response](/docs/api-reference/responses/create) - Generate streaming responses
- [Response Object](/docs/api-reference/objects/response-object) - Non-streaming response format