preconfSubscribe

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "preconfSubscribe",
  "params": [
    {
      "failed": false,
      "regionInclude": ["ewr", "fra"],
      "accountInclude": ["9WzDXwBbmkg8ZTbNMqUxvQRAyrZzDsGYdLVL9zYtAWWM"]
    }
  ]
}

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "preconfSubscribe",
  "params": [{ "includeBam": false }]
}

const WebSocket = require('ws');

const ws = new WebSocket('wss://beta.helius-rpc.com/?api-key=<API_KEY>');

ws.on('open', () => {
  ws.send(JSON.stringify({
    jsonrpc: '2.0',
    id: 1,
    method: 'preconfSubscribe' // Helius and BAM preconfirmations by default
    // Optional filter:
    // params: [{ failed: false, accountInclude: ['9WzDXwBbmkg8ZTbNMqUxvQRAyrZzDsGYdLVL9zYtAWWM'] }]
    // Helius preconfirmations only:
    // params: [{ includeBam: false }]
  }));

  setInterval(() => ws.ping(), 30_000);
});

ws.on('message', (data, isBinary) => {
  // The subscribe acknowledgement arrives as a JSON text frame
  if (!isBinary) {
    const msg = JSON.parse(data.toString());
    if (msg.id === 1) console.log('Subscribed, ID:', msg.result);
    return;
  }

  // Notifications arrive as binary frames:
  // version (u8) | slot (u64 LE) | tx_index (u64 LE) | status (u8) | transaction bytes
  const buf = Buffer.from(data);
  const version = buf.readUInt8(0);
  if (version !== 1) return; // unknown schema version; update your decoder
  const slot = buf.readBigUInt64LE(1);
  const txIndex = buf.readBigUInt64LE(9); // always 0 for BAM preconfirmations
  const status = buf.readUInt8(17); // 0 = failed, 1 = success, 2 = unknown
  const txBytes = buf.subarray(18); // transaction in Solana wire format (legacy, v0, or v1)

  console.log('Preconfirmation:', { slot, txIndex, status, bytes: txBytes.length });
});

{ "jsonrpc": "2.0", "id": 1, "result": 24040 }

version   u8              payload schema version, currently 1
slot      u64 (LE)        slot the transaction is scheduled in
tx_index  u64 (LE)        index of the transaction within the slot (always 0 for BAM)
status    u8              0 = failed, 1 = success, 2 = unknown
tx        bytes           transaction in Solana wire format (legacy, v0, or v1)