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

# Streaming replies

> Pipe an existing Vercel AI SDK UI message stream into Relay in one request.

Relay accepts the output your agent already produces. Send one request with
`stream=true` and pipe a Vercel AI SDK `UIMessageStream v1` into its body.
Relay consumes the whole stream and commits one canonical message when it
finishes.

There is no Relay draft to open, append, edit, or clean up.

## Pipe an AI SDK response

Use the `conversation_id` from `message.received` and derive the idempotency key
from that event's `event_id`.

```ts theme={null}
const result = streamText({ model, messages });
const uiResponse = result.toUIMessageStreamResponse();

const relayResponse = await fetch(
  `${process.env.RELAY_API_URL}/v1/messages?stream=true&conversation_id=${event.data.message.conversation_id}`,
  {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.RELAY_AGENT_TOKEN}`,
      "Idempotency-Key": `reply-${event.event_id}`,
      "Content-Type": "text/event-stream",
      "x-vercel-ai-ui-message-stream": "v1",
    },
    body: uiResponse.body,
    duplex: "half",
  },
);

if (!relayResponse.ok) throw new Error(await relayResponse.text());
```

This is a direct HTTPS integration, not a Relay SDK. Your backend still owns
the model, tool loop, prompts, memory, and hosting.

## Wire request

The same contract can be exercised without a framework:

```bash theme={null}
curl -sS -X POST \
  "$RELAY_API_URL/v1/messages?stream=true&conversation_id=cnv_01JZC7K4RQ" \
  -H "Authorization: Bearer $RELAY_AGENT_TOKEN" \
  -H "Idempotency-Key: reply-evt_01JZE9M2XW" \
  -H "Content-Type: text/event-stream" \
  -H "x-vercel-ai-ui-message-stream: v1" \
  --data-binary @reply.sse
```

`reply.sse` uses the AI SDK's existing framing:

```text theme={null}
data: {"type":"start","messageId":"ui-message-42"}

data: {"type":"text-start","id":"answer"}

data: {"type":"text-delta","id":"answer","delta":"Tomorrow at "}

data: {"type":"text-delta","id":"answer","delta":"2:00 PM works."}

data: {"type":"text-end","id":"answer"}

data: {"type":"finish","finishReason":"stop"}

data: [DONE]

```

Relay returns the normal `202` message response after the canonical commit.

## What Relay preserves

| AI SDK part                         | Relay presentation                                          |
| ----------------------------------- | ----------------------------------------------------------- |
| Text parts                          | Canonical `text` parts                                      |
| Tool input and output               | A transcript `tool_call` data part; Relay never executes it |
| URL sources                         | `link_preview` parts                                        |
| Files and document sources          | Transcript artifact parts when they have usable metadata    |
| Reasoning and transient custom data | Remain private to the agent backend                         |

The external agent owns the full loop. Relay is the messaging channel and
presentation surface.

## Completion and recovery

* `finish` is semantic completion. `[DONE]` only closes the transport; it does
  not commit a message by itself.
* `abort`, `error`, malformed ordering, or an early disconnect writes no
  transcript row.
* A successful stream produces one durable `message.created` event and one
  notification.
* Retry the whole request with the same `Idempotency-Key`. The same completed
  stream returns the original message; different content returns
  `409 idempotency_conflict`.
* A disconnected app recovers the final message through normal cursor sync
  and history.

Use a separate [typing indicator](/guides/typing-indicators) before the output
stream begins when the agent has a long planning or tool phase.

## Next steps

* [Sending finalized messages](/guides/sending-messages)
* [Delivery model](/guides/delivery-model)
* [API overview](/api-reference/overview)