# Streaming Events

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

When you request a streaming response, ai& streams the model's SSE events unchanged. 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` | Cached (repeated-prefix) 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. |
| `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 and legacy `/v1/completions` streams do not carry it — read the native `usage` block, or query [Request Logs](/analytics/logs/) for cost.

## Why a trailer event?

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.