---
title: '0.18'
description: An agent's loop is its harness, and the AI SDK loop's options move into it.
---

## Move loop options into `harness`

```ts server/assistant.ts
import { agent } from 'experimental-a2/ai/server'

export const assistant = agent({
  name: 'support-agent',
  model: 'openai/gpt-5.6-terra',
  instructions: 'Help the customer. Be concise.',
  harness: {
    type: 'ai-sdk',
    // generation, generate, compaction, and messageMetadata go here:
    compaction: { thresholdTokens: 80_000 },
  },
})
```

`agent()` takes its loop as `harness`. The AI SDK's loop is the default,
`{ type: 'ai-sdk' }`, and its options move into it: `generation`, `generate`,
`compaction`, and `messageMetadata` are no longer top-level `agent()` options,
and passing one there throws a `TypeError`. See
[Harnesses](/agents/harnesses).

## Models are AI Gateway IDs

`model` is an AI Gateway model ID, or `ModelSettings` with one, under every
harness. `agent()` no longer takes AI SDK provider model objects, and
`HarnessModelSettings` is gone: use `ModelSettings`.