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.

This subscription needs a WebSocket-enabled endpoint. The public Base endpoints (mainnet.base.org, sepolia.base.org) are HTTP only, so connect through a node provider such as Alchemy or QuickNode that offers WebSocket support for Base.

Requires 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 for the full structure.

Example

Subscribe

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

Subscription ID response

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

JavaScript

import WebSocket from 'ws';

// Use a WebSocket-enabled node provider endpoint
const ws = new WebSocket('wss://your-provider-endpoint');

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);
  }
});