> For the complete documentation index, see [llms.txt](https://docs.fastnode.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fastnode.io/real-time/websocket-api.md).

# WebSocket (WSS) API

The WebSocket API lets you subscribe to real-time blockchain events — new blocks, pending transactions, and log events — over a single persistent connection, instead of polling.

## Endpoint format

```
wss://<network>.fastnode.cc/YOUR-API-KEY
```

For example, BNB Smart Chain:

```
wss://bsc.fastnode.cc/YOUR-API-KEY
```

## Example: subscribe to new blocks (EVM)

```javascript
const ws = new WebSocket("wss://bsc.fastnode.cc/YOUR-API-KEY");

ws.onopen = () => {
  ws.send(JSON.stringify({
    jsonrpc: "2.0",
    id: 1,
    method: "eth_subscribe",
    params: ["newHeads"],
  }));
};

ws.onmessage = (event) => {
  const msg = JSON.parse(event.data);
  console.log("New block:", msg.params?.result?.number);
};
```

## Subscription types (EVM)

| Subscription             | Description                                         |
| ------------------------ | --------------------------------------------------- |
| `newHeads`               | Fires on every new block header.                    |
| `logs`                   | Fires on logs matching a filter (address / topics). |
| `newPendingTransactions` | Fires on new pending transaction hashes.            |

To unsubscribe, call `eth_unsubscribe` with the subscription ID returned when you subscribed.

## Bitcoin / UTXO chains

UTXO chains use the **Blockbook WebSocket** interface with its own subscription methods (e.g. `subscribeNewBlock`, `subscribeAddresses`). See the [Bitcoin](/bitcoin-like-utxo/bitcoin.md) page for the endpoint.

{% hint style="warning" %}
WebSocket connections may be closed after a period of inactivity. Implement a reconnect-with-backoff strategy and re-establish subscriptions on reconnect.
{% endhint %}

## Connection limits

The number of concurrent WebSocket connections depends on your plan. Track active connections in the [Monitoring](/advanced/monitoring.md) dashboard.


---

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