# Streaming Events

Source: https://aiand-docs-staging.pages.dev/reference/streaming-events/

When you request a streaming response, ai& streams the model's SSE events unchanged, apart from two small repairs on `/v1/responses` described below. If the request carries the opt-in header `X-Aiand-Metrics: true`, ai& appends a single **trailer event** named `metrics` carrying token counts, cost, and timing.

## OpenAI shape

```
data: {"id":"chatcmpl-...","object":"chat.completion.chunk","choices":[{"delta":{"content":"Hel"}}]}

data: {"id":"chatcmpl-...","object":"chat.completion.chunk","choices":[{"delta":{"content":"lo"}}]}

data: {"id":"chatcmpl-...","object":"chat.completion.chunk","choices":[{"delta":{},"finish_reason":"stop"}],"usage":{"prompt_tokens":7,"completion_tokens":2,"total_tokens":9}}

data: [DONE]

event: metrics
data: {"tokens":{"input":7,"output":2,"total":9,"cached":0},"cost":0.000018,"currency":"usd","ttft_ms":120,"inference_ms":850}
```

The `metrics` event is emitted **after** `[DONE]`. Robust clients should keep the connection open until the stream closes naturally.

## The trailer event

| Field | Description |
|-------|-------------|
| `tokens.input` | Input tokens counted toward billing. |
| `tokens.output` | Output tokens counted toward billing. |
| `tokens.total` | Sum of input and output tokens. |
| `tokens.cached` | Input tokens billed at the model's discounted cached rate. `0` when there is no cache hit. See [Prompt caching](/models/pricing/#prompt-caching). |
| `cost` | Final cost of the request, in your billing currency. |
| `currency` | The currency `cost` is denominated in (`usd` or `jpy`). |
| `ttft_ms` | Time to first token, in milliseconds: from sending the request to the model until its first generated text, reasoning or tool call. Metadata-only chunks do not count. `null` when the response generated no output. |
| `inference_ms` | Time the upstream model spent producing the response, in milliseconds. |

<Aside type="caution">
  Do not parse `metrics` as a chat-completion chunk — it carries the `event:` line so clients that ignore unknown event names will skip it safely.
</Aside>

## Which endpoints emit it

The trailer is emitted on `/v1/chat/completions` and `/v1/responses` streams. Anthropic-shaped `/v1/messages` streams do not carry it — read the native `usage` block, or query [Request Logs](/analytics/logs/) for cost.

## Repairs on `/v1/responses`

Some models emit two small defects that break strict clients such as Codex, so ai& repairs them on `/v1/responses`:

- A message announced in `response.output_item.added` without `content` gets `"content": []`.
- In a tool call's final arguments, whole numbers written as floats become integers: `{"yield_time_ms":10000.0}` becomes `{"yield_time_ms":10000}`. This applies to `response.output_item.done`, `response.function_call_arguments.done`, the `output` of `response.completed` and `response.incomplete`, and non-streaming responses. Streamed argument deltas are left as they are, and numbers inside strings are never touched.

Both keep the same meaning in JSON, so clients that parse JSON see no difference.

## Why a trailer event?

Apart from those repairs, ai& never modifies the response body emitted by the model. Tokens, cost, and timing are delivered as a named SSE event after the terminal message, so the byte stream above the trailer is identical to what the source API would produce — the official OpenAI and Anthropic SDKs work without modification.