> ## Documentation Index
> Fetch the complete documentation index at: https://docs.aion.to/llms.txt
> Use this file to discover all available pages before exploring further.

# Streaming Delta Semantics

> How streaming artifacts are emitted and reconstructed.

Streaming responses are emitted as `artifact-update` events. The main artifact used for delta
streaming is typically `aion:stream-delta`.

## Core Fields

| Field                             | Meaning                                                     |
| --------------------------------- | ----------------------------------------------------------- |
| `append`                          | `true` appends chunk content, `false` replaces content.     |
| `lastChunk`                       | Reserved field, currently `false` during streaming.         |
| `artifact.metadata.status`        | `active` while streaming, `finalized` when complete.        |
| `artifact.metadata.status_reason` | `chunk_streaming`, `complete_message`, or interrupt reason. |

## Typical Sequence

1. First chunk with `append: false`.
2. Intermediate chunks with `append: true`.
3. Finalized event, often with complete assembled parts.

## Example Chunk

```json theme={null}
{
  "kind": "artifact-update",
  "append": true,
  "artifact": {
    "artifactId": "aion:stream-delta",
    "metadata": {
      "status": "active",
      "status_reason": "chunk_streaming"
    },
    "parts": [
      {
        "kind": "text",
        "text": " World!"
      }
    ]
  }
}
```

## Reconstruction Rule

A finalized artifact may later be reconstructed within the same task after an interruption or
resume flow. Clients should treat the latest finalized artifact as source of truth.