# Resumable Streaming

The `AIChatAgent` class provides **automatic resumable streaming** out of the box. When a client disconnects and reconnects during an active stream, the response automatically resumes from where it left off.

This is client reconnect recovery, not Durable Object eviction recovery. If the Worker process or Durable Object is evicted while the model call is in flight, enable `chatRecovery` so the turn runs inside a recoverable fiber. `Think` enables `chatRecovery` by default; plain `AIChatAgent` subclasses opt in with `override chatRecovery = true`.

## How It Works

When you use `AIChatAgent` with `useAgentChat`:

1. **During streaming**: All chunks are automatically persisted to SQLite
2. **On disconnect**: The stream continues server-side, buffering chunks
3. **On reconnect**: Client requests a resume, receives all buffered chunks, and continues streaming

No extra code is needed -- it just works. Generic client stream abort/cleanup is local-only by default, so browser navigation or React cleanup does not stop the server turn. An explicit `stop()` still cancels the server turn.

## Example

### Server

```typescript
import { AIChatAgent } from "@cloudflare/ai-chat";
import { createWorkersAI } from "workers-ai-provider";
import { streamText, convertToModelMessages } from "ai";

export class ChatAgent extends AIChatAgent {
  async onChatMessage() {
    const workersai = createWorkersAI({ binding: this.env.AI });

    const result = streamText({
      model: workersai("@cf/moonshotai/kimi-k2.7-code"),
      messages: await convertToModelMessages(this.messages)
    });

    // Automatic resumable streaming - no extra code needed
    return result.toUIMessageStreamResponse();
  }
}
```

### Client

```tsx
import { useAgent } from "agents/react";
import { useAgentChat } from "@cloudflare/ai-chat/react";

function Chat() {
  const agent = useAgent({
    agent: "ChatAgent",
    name: "my-chat"
  });

  const { messages, sendMessage, status } = useAgentChat({
    agent
    // resume: true is the default - streams automatically resume on reconnect
    // cancelOnClientAbort: false is the default - browser cleanup does not
    // cancel the server turn
  });

  // ... render your chat UI
}
```

## Under the Hood

### Server-side (`AIChatAgent`)

- Creates SQLite tables for stream chunks and metadata on construction
- Each stream gets a unique ID and tracks chunk indices
- Chunks are batched (every 10 chunks) and flushed to SQLite for performance
- When a client sends `CF_AGENT_STREAM_RESUME_REQUEST`, the server checks for active streams and responds with `CF_AGENT_STREAM_RESUMING`
- Stale streams (older than 5 minutes) are cleaned up on restore
- Stream buffers are garbage collected from a scheduled alarm: completed or errored streams are retained for 10 minutes (a brief reconnect-and-replay grace; the assistant message itself is persisted separately), and abandoned in-flight streams are retained for 1 hour after their last chunk before being reclaimed

### Client-side (`useAgentChat`)

- After the message handler is registered in `useEffect`, sends `CF_AGENT_STREAM_RESUME_REQUEST` to the server
- This avoids a race condition where the server's `onConnect` notification could arrive before the client's handler is ready
- On receiving `CF_AGENT_STREAM_RESUMING`, sends `CF_AGENT_STREAM_RESUME_ACK`
- Receives all buffered chunks with `replay: true` flag and applies them in a single batch
- Continues receiving live chunks as they arrive from the ongoing stream

### The `replay` flag

Replayed chunks include `replay: true` to distinguish them from live chunks. The client uses this to batch-apply all replayed chunks before rendering, which prevents intermediate states (like reasoning "Thinking..." indicators) from flashing briefly during replay. During a live stream, chunks arrive gradually and React renders each intermediate state naturally.

## Durable client cleanup

`resume: true` controls whether the client tries to reconnect to an active stream. `cancelOnClientAbort: false` is the default cancellation behavior: generic client stream abort/cleanup is local-only, while explicit `stop()` still cancels the server turn.

```tsx
const { messages, stop } = useAgentChat({
  agent
});
```

If your app intentionally wants client lifecycle to own server lifecycle, opt in:

```tsx
const { messages } = useAgentChat({
  agent,
  cancelOnClientAbort: true
});
```

Use this for request-lifetime or token-saving flows. Explicit `stop()` is always
server-side cancellation regardless of `cancelOnClientAbort`.

## Disabling Resume

If you do not want automatic resume (for example, for short responses), disable it:

```tsx
const { messages } = useAgentChat({
  agent,
  resume: false // Disable automatic stream resumption
});
```

## Try It

See [examples/resumable-stream-chat](https://github.com/cloudflare/agents/tree/main/examples/resumable-stream-chat) for a complete working example. Start a long response, refresh the page mid-stream, and watch it resume automatically.

## Related Docs

- [Chat Agents](./chat-agents.md) — Full `AIChatAgent` and `useAgentChat` reference
- [Client Tools Continuation](./client-tools-continuation.md) — Client-side tool execution and auto-continuation