chain_subscribeFinalizedHeads - Bittensor

Example code for the chain_subscribeFinalizedHeads JSON-RPC method. Complete guide on how to use chain_subscribeFinalizedHeads JSON-RPC in GetBlock Web3 documentation.

Subscribes to new finalized block headers. Slower than chain_subscribeNewHeads (finalized blocks lag the chain tip), but reorg-safe — these blocks will never be reverted.

Substrate native JSON-RPC method. Call against a GetBlock endpoint configured for the Substrate interface. For typed SDK access, use Polkadot.js (TypeScript) or substrateinterface (Python).

WebSocket-only method. This method requires the WebSocket transport at wss://go.getblock.io/<ACCESS-TOKEN>/. It will not work via HTTP POST.

Parameters

• None

Request Example

cURL

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

# Then send:
{"jsonrpc": "2.0", "method": "chain_subscribeFinalizedHeads", "params": [], "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": "chain_subscribeFinalizedHeads",
    "params": [],
    "id": "getblock.io"
}));
});

ws.on('message', (data) => {
    console.log(JSON.parse(data.toString()));
});

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": "chain_subscribeFinalizedHeads",
    "params": [],
    "id": "getblock.io"
}))
        async for message in ws:
            print(json.loads(message))

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": "chain_subscribeFinalizedHeads",
        "params": [],
        "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": "A1cB2dE3fG4hI5jK6lM7nO8pQ"
}

Response Parameters

Field	Type	Description
result	string	Subscription ID for finalized head notifications

Use Cases

• Building safe state machines that progress on finality, not chain-tip

• Bridge relayers that need finality guarantees before forwarding messages

• Withdrawal/payout pipelines that should never process a reorged block

Error Handling

Status Code	Error Message	Cause
403	Forbidden	Missing or invalid <ACCESS-TOKEN>
-32602	Invalid params	Request parameters are missing or malformed
-32601	Method not found	Method does not exist on this interface — check whether you're calling a Substrate method on an EVM endpoint or vice versa
-32603	Internal error	Server-side error while processing the request
429	Too Many Requests	Rate limit exceeded for your plan

SDK Integration

Polkadot.js (TypeScript)

import { ApiPromise, WsProvider } from '@polkadot/api';

const provider = new WsProvider('wss://go.getblock.io/<ACCESS-TOKEN>/');
const api = await ApiPromise.create({ provider });

// Typed wrapper: api.rpc.chain.subscribeFinalizedHeads(...)
const result = await api.rpc.chain.subscribeFinalizedHeads();
console.log(result.toHuman());

// Or use the raw RPC interface for any method:
// const raw = await api.rpc.send({ method: 'chain_subscribeFinalizedHeads', params: [] });

substrateinterface (Python)

from substrateinterface import SubstrateInterface

substrate = SubstrateInterface(url="wss://go.getblock.io/<ACCESS-TOKEN>/")

# Generic RPC call:
result = substrate.rpc_request("chain_subscribeFinalizedHeads", [])
print(result)