pendingLogs - Flashblocks

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

This Subscribes to contract event logs emitted by preconfirmed transactions — sub-block latency for on-chain events. Filter by contract address (single or array) and event topics (same format as eth_getLogs).

This is the fastest way to react to on-chain events, up to ~1.8 seconds ahead of the standard logs subscription.

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 "pendingLogs"
filter	object	No	Filter object: address (single address or array), topics (same format as eth_getLogs)

Request Example

cURL

# 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": ["pendingLogs", {"address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", "topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]}], "id": "getblock.io"}

JavaScript (Axios)

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": [
        "pendingLogs",
        {
            "address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
            "topics": [
                "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"
            ]
        }
    ],
    "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);
    }
});

Python (Requests)

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": [
        "pendingLogs",
        {
            "address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
            "topics": [
                "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"
            ]
        }
    ],
    "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())

Rust (Reqwest)

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": [
                "pendingLogs",
                {
                        "address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
                        "topics": [
                                "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"
                        ]
                }
        ],
        "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": "0x7e2ef8b9c0d1e2f3a4b5c6d7e8f9a0b1"
}

Response Parameters

Field	Type	Description
result	string	Hex-encoded subscription ID. Each subsequent notification carries a Log object matching the filter, from a preconfirmed transaction

Use Cases

• Real-time event indexers — the lowest-latency way to react to on-chain events

• Trading bots watching for specific ERC-20 transfers, DEX swaps, or oracle updates

• Liquidation bots watching for Aave/Compound account state changes

• Social/gaming applications reacting to on-chain events with instant UX feedback

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	Subscription called over HTTP; use WebSocket
-32602	Invalid filter	Filter object malformed — check address and topics format

SDK Integration

ethers.js

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', ["pendingLogs", {"address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", "topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]}]);
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);
    }
});

viem

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: ["pendingLogs", {"address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", "topics": ["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]}]
});
console.log('Subscribed:', subscriptionId);