newFlashblocks.mdx

title	newFlashblocks
sidebarTitle	newFlashblocks
description	Subscribe to receive full Flashblock payload stream as each pre-confirmed block is built. Only available on Flashblocks WebSocket endpoints.

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**. Events arrive approximately every 200ms. If your handler performs heavy processing per event, throttle or debounce it to avoid blocking.

Parameters

Must be `"newFlashblocks"`.

Returns

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](/base-chain/api-reference/flashblocks-api/flashblocks-api-overview#flashblock-object) for the full structure.

Example

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

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

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);
  }
});