---
title: Update Mob Effect
description: "Play 0x84, server → client."
---

`play` · `0x84` · **clientbound** (server → client)

Applies or refreshes a status effect on an entity. `flags` control ambient, particle and icon visibility, and whether the effect is blended. The server sends it when an effect is added or changed; the client renders particles and the HUD icon.

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

## Fields

| Field | Type | Notes |
|---|---|---|
| `entityId` | VarInt |  |
| `effectId` | VarInt |  |
| `amplifier` | VarInt |  |
| `duration` | VarInt |  |
| `flags` | UByte |  |

## Send it

```ts
import { helpers } from "mc-protocol";

// Speed II for 10 seconds, with the HUD icon but no particles.
await client.sendPacket("UpdateMobEffect", {
  entityId: 42,
  effectId: 1,
  amplifier: 1,
  duration: 200,
  flags: helpers.mobEffectFlags(["showIcon"]),
});
```

`effectId` is the `minecraft:mob_effect` registry ID. `flags` is one byte: ambient `0x01`, showParticles `0x02`, showIcon `0x04`, blend `0x08`. See [Packet flags](/docs/flags#mob_effect_flags).

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

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