SSE Format

Expected Format

The backend should emit events as data: {JSON}\n\n:

data: {"type": "text_delta", "delta": "Hello"}
data: {"type": "text_delta", "delta": " world"}
data: {"type": "tool_call", "tool_name": "search", "argument": "..."}

The stream can optionally end with:

data: [DONE]

Event Types

text_delta

The primary event type. Each event contains a chunk of the assistant’s response:

{"type": "text_delta", "delta": "Hello"}

By default, useChat appends all delta values as text parts in the assistant message’s parts array.

tool_call

Emitted when the AI invokes a tool:

{"type": "tool_call", "tool_name": "search_web", "argument": "{\"query\": \"...\" }"}

These are not handled by default. Use the onEvent callback to process them — for example, by calling helpers.appendPart() to add a tool_call part to the message.

HTTP Request

useChat sends a POST request with:

• Content-Type: application/json
• Body: { "message": "user text", ...body } (where body is the extra fields you passed to useChat)

The response must be a streaming response with Content-Type: text/event-stream.

How Streaming Works

Under the hood, useChat uses native fetch with ReadableStream:

1. A POST request is sent to the api URL
2. The response body is read as a stream
3. Each data: line is parsed as JSON
4. Events are dispatched to the event handler (default or custom onEvent)
5. The stream ends when the connection closes or data: [DONE] is received