---
title: Boss Event
description: "Play 0x09, server → client."
---

`play` · `0x09` · **clientbound** (server → client)

Creates, updates, or removes a boss bar identified by `entityUUID`. The `action` selects add, remove, update health, update title, update style (color and dividers), or update flags (darken sky, play boss music, create fog). The client shows or updates the bar at the top of the screen.

Sent by the server, read by the client. Listen for it with `client.on("BossEvent", (data) => …)` on the receiving side.

## Fields

| Field | Type | Notes |
|---|---|---|
| `entityUUID` | UUID |  |
| `action` | VarInt |  |
| `title` | Struct |  |
| `health` | Struct |  |
| `color` | Struct |  |
| `dividers` | Struct |  |
| `flags` | Struct |  |

## Send it

```ts
await client.sendPacket("BossEvent", {
  entityUUID: /* UUID */,
  action: /* VarInt */,
  title: /* Struct */,
  health: /* Struct */,
  color: /* Struct */,
  dividers: /* Struct */,
  flags: /* Struct */,
});
```

Field aliases listed under Notes are accepted as alternative keys when writing.

Source: [`src/Core/Versions/776/Packets/play/clientbound/BossEvent.ts`](https://github.com/GDLev/mc-protocol/blob/master/src/Core/Versions/776/Packets/play/clientbound/BossEvent.ts)