newFlashblocks - Flashblocks

Example code for the newFlashblocks Flashblocks method. Complete guide on how to use newFlashblocks Flashblocks in GetBlock Web3 documentation.

This subscribes to the full Flashblocks payload stream as each preconfirmed sub-block is built. Each notification delivers a Flashblocks Object containing payload_id, index (0-9 on Base, 0-7 on Optimism), diff (the delta since the previous Flashblocks), and — on the first Flashblocks of a new block (index: 0) — a base field with the block's initial state. This is the lowest-latency Flashblocks subscription and the canonical way to build sub-block indexers.

WebSocket-only method. This method requires the WebSocket transport at wss://go.getblock.io/<ACCESS-TOKEN>/. It will not work via HTTP POST. Preconfirmed events arrive at the Flashblocks cadence — approximately every 200ms on Base, 250ms on Optimism.

Parameters

Parameter

Type

Required

Description

subscriptionType

string

Yes

Must be "newFlashblocks"

Request Example

# WebSocket-only. Use wscat (or similar) to connect first:
wscat -c 'wss://go.getblock.io/<ACCESS-TOKEN>/'

# Then send:
{"jsonrpc": "2.0", "method": "eth_subscribe", "params": ["newFlashblocks"], "id": "getblock.io"}

import WebSocket from 'ws';

const ws = new WebSocket('wss://go.getblock.io/<ACCESS-TOKEN>/');

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

ws.on('message', (data) => {
    const msg = JSON.parse(data.toString());
    if (msg.method === 'eth_subscription') {
        // Preconfirmed data update
        console.log(msg.params.result);
    } else {
        // Subscription ID response
        console.log('Subscribed:', msg.result);
    }
});

import asyncio
import json
import websockets

async def main():
    async with websockets.connect('wss://go.getblock.io/<ACCESS-TOKEN>/') as ws:
        await ws.send(json.dumps({
    "jsonrpc": "2.0",
    "method": "eth_subscribe",
    "params": [
        "newFlashblocks"
    ],
    "id": "getblock.io"
}))
        async for message in ws:
            msg = json.loads(message)
            if msg.get('method') == 'eth_subscription':
                # Preconfirmed data update
                print(msg['params']['result'])
            else:
                # Subscription ID response
                print('Subscribed:', msg.get('result'))

asyncio.run(main())

use tokio_tungstenite::connect_async;
use tokio_tungstenite::tungstenite::Message;
use serde_json::json;
use futures_util::{SinkExt, StreamExt};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let (mut ws_stream, _) = connect_async("wss://go.getblock.io/<ACCESS-TOKEN>/").await?;

    let payload = json!({
        "jsonrpc": "2.0",
        "method": "eth_subscribe",
        "params": [
                "newFlashblocks"
        ],
        "id": "getblock.io"
});
    ws_stream.send(Message::Text(payload.to_string())).await?;

    while let Some(msg) = ws_stream.next().await {
        println!("{:?}", msg?);
    }
    Ok(())
}

Response Example

{
    "jsonrpc": "2.0",
    "id": "getblock.io",
    "result": "0x3b8cd9e5f4a7b2c1d0e3f4a5b6c7d8e9"
}

Response Parameters

Field

Type

Description

result

string

Hex-encoded subscription ID. Each subsequent eth_subscription notification carries a Flashblocks Object payload with payload_id, index, diff, and (on index 0) base

Use Cases

Sub-block indexers — the primary streaming source for Flashblocks-native data
Real-time analytics dashboards showing block construction in progress
Trading infrastructure that needs the earliest possible view of sequencer-ordered transactions
MEV monitoring — observing preconfirmed transactions as they land

Error Handling

Status Code

Error Message

Cause

403

Forbidden

Missing or invalid <ACCESS-TOKEN>

-32602

Invalid params

Request parameters are missing or malformed

-32603

Internal error

Server-side error while processing the request

429

Too Many Requests

Rate limit exceeded for your plan

-32600

Invalid Request

newFlashblocks subscription called over HTTP; use WebSocket

SDK Integration

import { ethers } from 'ethers';

const provider = new ethers.WebSocketProvider('wss://go.getblock.io/<ACCESS-TOKEN>/');

// Flashblocks subscription types aren't first-class in ethers — use the raw send interface:
const subscriptionId = await provider.send('eth_subscribe', ["newFlashblocks"]);
console.log('Subscribed:', subscriptionId);

// Listen for incoming events on the WebSocket transport
provider.websocket.addEventListener('message', (event) => {
    const msg = JSON.parse(event.data);
    if (msg.method === 'eth_subscription' && msg.params?.subscription === subscriptionId) {
        console.log('Flashblocks update:', msg.params.result);
    }
});

import { createPublicClient, webSocket } from 'viem';

const client = createPublicClient({
    transport: webSocket('wss://go.getblock.io/<ACCESS-TOKEN>/')
});

// Flashblocks subscription types use the raw request interface:
const subscriptionId = await client.request({
    method: 'eth_subscribe',
    params: ["newFlashblocks"]
});
console.log('Subscribed:', subscriptionId);

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