---
title: "newFlashblocks"
description: "Subscribe to the full Flashblock payload stream and receive each pre-confirmed block delta as the sequencer builds it, every ~200ms."
source: https://basehub.org/api-reference/flashblocks/newflashblocks/
---
Subscribe via `eth_subscribe` to receive full block state updates as each Flashblock is built. Each message contains the accumulated pre-confirmed state for the block in progress.

Only available on Flashblocks WebSocket endpoints: `wss://mainnet-preconf.base.org` and `wss://sepolia-preconf.base.org`.

Requires [base/base](https://github.com/base/base) minimum client version v0.3.1.

Each subscription emits one Flashblock object per WebSocket message, with events arriving roughly every 200ms. If your handler does heavy work per event, throttle or debounce it to avoid blocking.

## Parameters

| Name | Type | Required | Description |
| --- | --- | --- | --- |
| `subscriptionType` | string | Yes | Must be `"newFlashblocks"`. |

## Returns

| Field | Type | Description |
| --- | --- | --- |
| `result` | string | Hex-encoded subscription ID. Each event notification delivers a Flashblock object — not a standard block object. The payload contains `payload_id`, `index`, `diff`, and (on index 0) `base`. See the [infrastructure stream schema](/api-reference/flashblocks/overview/#flashblock-object) for the full structure. |

## Example

### Subscribe

```json
{"jsonrpc": "2.0", "id": 1, "method": "eth_subscribe", "params": ["newFlashblocks"]}
```

### Subscription ID response

```json
{"jsonrpc": "2.0", "id": 1, "result": "0x3b8cd9e5f4a7b2c1d0e3f4a5b6c7d8e9"}
```

### JavaScript

```javascript
import WebSocket from 'ws';

// Use a Flashblocks-enabled provider WSS endpoint in production
const ws = new WebSocket('wss://mainnet-preconf.base.org');

ws.on('open', () => {
  ws.send(JSON.stringify({
    jsonrpc: '2.0',
    method: 'eth_subscribe',
    params: ['newFlashblocks'],
    id: 1
  }));
});

ws.on('message', (data) => {
  const msg = JSON.parse(data.toString());
  if (msg.method === 'eth_subscription') {
    // Fires every ~200ms with the latest Flashblock state
    console.log('Flashblock update:', msg.params.result);
  }
});
```