> ## Documentation Index
> Fetch the complete documentation index at: https://heygen-1fa696a7.mintlify.app/llms.txt
> Use this file to discover all available pages before exploring further.

# Models

> Browse every HeyGen AI model behind the API - avatar models, voice models, lipsync engines, and video generation backends.

<video controls muted playsInline className="w-full aspect-video rounded-xl" src="https://mintcdn.com/heygen-1fa696a7/xiWXoDBsEfd3CTgQ/images/videos/avatar-models.mp4?fit=max&auto=format&n=xiWXoDBsEfd3CTgQ&q=85&s=6d3d157629e0e8f62b00cab49b1440ed" title="Avatar rendering engines in 30 seconds — Avatar III vs IV vs V and how to choose" data-path="images/videos/avatar-models.mp4" />

# HeyGen Avatar Rendering Engines

HeyGen has three generations of rendering engines. New integrations should use **Avatar IV** or **Avatar V** via `POST /v3/videos`.

***

## Avatar III (Legacy)

Only available to existing users through the **v1/v2 legacy endpoints**. Not offered to new users and not accessible via `POST /v3/videos`.

***

## Avatar IV

The **default engine** on the v3 API. Omitting the `engine` field automatically uses Avatar IV.

**Supported avatar types:** `studio_avatar`, `digital_twin`, `photo_avatar`, `image` (arbitrary), `prompt`

**Exclusive features:**

* `motion_prompt` — natural-language string to control body motion and hand gestures (e.g. `"walk towards the camera slowly"`). Photo avatars and arbitrary images only.
* `expressiveness` — controls energy and range of movement: `high`, `medium`, or `low`. Photo avatars and arbitrary images only. Defaults to `low`.
* **Arbitrary image support** — animate any image by setting `type: "image"` without a registered avatar.

**Example:**

```json theme={null}
{
  "type": "avatar",
  "avatar_id": "YOUR_LOOK_ID",
  "script": "Hello from Avatar IV.",
  "voice_id": "YOUR_VOICE_ID",
  "resolution": "1080p"
}
```

> `engine` is omitted — Avatar IV is selected automatically.

***

## Avatar V

The **latest engine**, offering more natural motion and lip-sync through cross-reference-driven animation. Must be **explicitly opted into** and is only available for avatar looks that support it.

**Supported avatar types:** `studio_avatar`, `digital_twin`, `photo_avatar`, `prompt` (all subject to eligibility) **Not supported:** arbitrary image input (`type: "image"`)

**Supported parameters:**

* `motion_prompt` — natural-language control of body motion and hand gestures.

**Not supported (Avatar IV only):**

* `expressiveness`

### Checking Eligibility

Not every avatar look supports Avatar V. Before using it, fetch the look and check `supported_api_engines`:

```bash theme={null}
GET /v3/avatars/looks/{look_id}
```

```json theme={null}
{
  "id": "lk_abc123",
  "name": "My Digital Twin",
  "avatar_type": "digital_twin",
  "supported_api_engines": ["avatar_iv", "avatar_v"]
}
```

Avatar V is only available if `"avatar_v"` appears in `supported_api_engines`. Requesting it for an ineligible look returns a `400` error.

**Example:**

```json theme={null}
{
  "type": "avatar",
  "avatar_id": "YOUR_LOOK_ID",
  "script": "Hello from Avatar V.",
  "voice_id": "YOUR_VOICE_ID",
  "resolution": "1080p",
  "engine": { "type": "avatar_v" }
}
```

***

## Comparison

| Feature                          | Avatar III     | Avatar IV | Avatar V            |
| -------------------------------- | -------------- | --------- | ------------------- |
| API version                      | v1/v2 (legacy) | v3        | v3                  |
| Available to new users           | ✗              | ✓         | ✓                   |
| Default engine                   | ✗              | ✓         | ✗ (explicit opt-in) |
| Arbitrary image input            | ✗              | ✓         | ✗                   |
| `motion_prompt`                  | ✗              | ✓         | ✓                   |
| `expressiveness`                 | ✗              | ✓         | ✗                   |
| Cross-reference animation        | ✗              | ✗         | ✓                   |
| Better motion & lip-sync quality | ✗              | ✗         | ✓                   |
| Eligibility check required       | ✗              | ✗         | ✓                   |