> For the complete documentation index, see [llms.txt](https://docs.layerlens.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.layerlens.ai/6.-build-wire-your-code/streaming.md).

# Streaming

Streaming evaluation results and trace progress — SSE and WebSocket patterns.

Async evaluations and trace evaluations queue and run in the background. The default consumption pattern is **polling** (`wait_for_completion()` in the SDK). For responsive UIs, Stratix supports **server-sent events (SSE)**.

## When to use polling

* CI / batch jobs — runs to completion, then exits
* Single-shot scripts
* Anywhere the caller is comfortable blocking for seconds-to-minutes

## When to use streaming

* Dashboards showing live progress to a user
* Long-running evaluations (large datasets, GEPA optimizations)
* Multi-eval runs where you want incremental results as each completes

## SSE endpoint

```
GET /api/v1/evaluations/{id}/stream
X-API-Key: ll_...
Accept: text/event-stream
```

Events emitted:

| Event          | Payload                                                     |
| -------------- | ----------------------------------------------------------- |
| `status`       | Current status (`queued`, `running`, `completed`, `failed`) |
| `progress`     | `{rows_complete: N, rows_total: M}`                         |
| `row_complete` | Row-level result (one per scored example)                   |
| `final`        | Final result object on completion                           |
| `error`        | Error object on failure                                     |

## Streaming from your backend to the browser

Your backend consumes the SSE from Stratix; pipe events to the browser via your own SSE or WebSocket connection. See [calling Stratix from a backend](https://github.com/LayerLens/gitbook-full/blob/main/06-build/guides/calling-stratix-from-a-backend.md) for the proxy shape.

## SDK helpers

The Python SDK exposes async iteration:

```python
async for event in client.evaluations.stream(eval_id):
 if event.type == "row_complete":
 print(event.row.score)
 elif event.type == "final":
 print("done:", event.result.accuracy)
 break
```

## Limits

* One concurrent stream per eval per consumer
* Streams close after 60 minutes of inactivity (use polling for very long jobs)
* Server-side reconnect is supported; client should resume with `Last-Event-ID` header

## See also

* [Async vs sync workflow](/6.-build-wire-your-code/async-vs-sync-workflow.md)
* [Calling Stratix from a backend](https://github.com/LayerLens/gitbook-full/blob/main/06-build/guides/calling-stratix-from-a-backend.md)
* [SDK: async patterns](https://github.com/LayerLens/gitbook-full/blob/main/13-reference/sdk-python/async-patterns.md)