> ## Documentation Index
> Fetch the complete documentation index at: https://docs.unifically.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Kling 4.0 Flash

> Generate 720p videos with Kling 4.0 Flash, the fast Kling 4.0 model. See supported inputs, parameters, pricing, response fields, and API examples.

Generate 720p AI videos with Kling 4.0 Flash, the faster, lower-cost model in Kuaishou's Kling 4.0 lineup. It takes the same request as Kling 4.0: text-to-video, up to 10 raw reference images, persistent subject/object elements, a start frame, video reference, and multi-shot generation. Videos run 3–20 seconds at 720p with native stereo audio on by default, in five aspect ratios including 21:9.

More on this model: [Kling 4.0 Flash API page](https://unifically.com/models/kling-4.0-flash).

## Model

```
kuaishou/kling-4.0-flash-video
```

## Parameters

| Parameter | Type | Required | Default | Description |
| - | - | - | - | - |
| `video_mode` | string | No | - | Input mode. Allowed: `"elements"`, `"start_end_frame"`, `"transform"`, `"video_reference"`. Omit for text-to-video |
| `prompt` | string | Conditional | - | Text prompt. Use `@Image1`, `@Element1` to reference inputs. Mutually exclusive with `multi_shots` |
| `mode` | string | No | `"std"` | `"std"` (720p) only. `"pro"` and `"4k"` are rejected |
| `duration` | integer | No | `3` | Video length in seconds (3–20). Ignored when `video_url` is set (the source video's length is used) |
| `aspect_ratio` | string | No | - | `"auto"`, `"1:1"`, `"9:16"`, `"16:9"`, or `"21:9"`. Must be `"auto"` when `video_url` is set. Omit to let the server choose (`"auto"` if `video_url` or `start_end_frame`, else `"16:9"`) |
| `native_audio` | boolean | No | `true` | Generate AI audio (two-channel stereo). On by default |
| `keep_audio` | boolean | No | `true` | Preserve original audio from the source video (`transform` / `video_reference` modes) |
| `image_urls` | string\[] | No | - | Raw reference image URLs uploaded per request (max 10). Use `@Image1`, `@Image2` in `prompt` |
| `start_image_index` | integer | No | - | 1-based index into `image_urls` marking which reference image anchors the first frame |
| `elements` | ElementInput30\[] | No | - | Persistent subject/object assets (max 7, of which at most 3 `type: "video"`). Auto-created on Kling and auto-deleted if submit fails. Use `@Element1`, `@Element2` in `prompt` |
| `start_frame_url` | string | No | - | First frame image URL (`start_end_frame` mode) |
| `video_url` | string | No | - | Source video URL (`transform` / `video_reference` modes). Billed per second of the source video at the with-video rate |
| `multi_shots` | array | No | - | Multi-shot sequence (2–6 shots, each with `prompt` and `duration`, total 3–20 s). Mutually exclusive with `prompt`. Not available in `start_end_frame` mode |
| `nsfw_check` | boolean | No | `false` | Check the prompt and input images for NSFW content before generating. Off unless set to `true` |

## Modes

| Mode | Required inputs | Optional inputs | Notes |
| - | - | - | - |
| *omitted* (text-to-video) | `prompt` or `multi_shots` | — | Rejects every input field including `elements`, `image_urls`, `start_frame_url`, `video_url` |
| `elements` | At least one of `image_urls`, `elements` | — | Combined `image_urls` + `elements` cap of **15** (`image_urls` ≤ 10, `elements` ≤ 7). `video_url` rejected |
| `start_end_frame` | `start_frame_url` | — | Image-to-video from a first frame. `end_frame_url` and `keyframe_urls` are rejected: last and middle frames are on [Kling 4.0](https://docs.unifically.com/models/video/kling/kling-4.0-video.md) only. `aspect_ratio` is ignored (the frames set it). `elements` and `multi_shots` rejected |
| `transform` | `video_url` | `image_urls`, `elements` | Combined `image_urls` + `elements` cap of **14** (the source video is the 15th reference). `aspect_ratio` must be `"auto"` |
| `video_reference` | `video_url` | `image_urls`, `elements` | Combined `image_urls` + `elements` cap of **14**. `aspect_ratio` must be `"auto"` |

> [!NOTE]
>
> **Kling 4.0 Flash vs Kling 4.0** — Flash outputs 720p only and runs up to 20 seconds. For 1080p, videos up to 30 seconds, or last and middle keyframes, use [Kling 4.0](https://docs.unifically.com/models/video/kling/kling-4.0-video.md).

## Pricing

Billed per second of output video. With a source video (`transform` / `video_reference` modes) or a video-type element, the with-video rate applies; with a source video it is billed per second of the input video.

| Mode | Resolution | Per second | With source video |
| - | - | - | - |
| `std` | 720p | \$0.0756 | \$0.1008 |

## Example - Text-to-Video

```bash theme={null}
curl -X POST https://api.unifically.com/v1/tasks \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "kuaishou/kling-4.0-flash-video",
    "input": {
      "prompt": "A street food vendor flips noodles in a flaming wok at night, sizzling oil and crowd chatter",
      "duration": 5,
      "aspect_ratio": "16:9"
    }
  }'
```

## Example - Reference Images

```bash theme={null}
curl -X POST https://api.unifically.com/v1/tasks \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "kuaishou/kling-4.0-flash-video",
    "input": {
      "video_mode": "elements",
      "prompt": "@Image1 and @Image2 meet on the cliff edge in @Image3, laughing",
      "image_urls": [
        "https://example.com/man.jpg",
        "https://example.com/woman.jpg",
        "https://example.com/cliff.jpg"
      ],
      "duration": 10
    }
  }'
```

## Example - Image-to-Video

```bash theme={null}
curl -X POST https://api.unifically.com/v1/tasks \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "kuaishou/kling-4.0-flash-video",
    "input": {
      "video_mode": "start_end_frame",
      "prompt": "The camera slowly pushes in as the lights come on",
      "start_frame_url": "https://example.com/street.jpg",
      "duration": 5
    }
  }'
```

## Example - Video Reference

```bash theme={null}
curl -X POST https://api.unifically.com/v1/tasks \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "model": "kuaishou/kling-4.0-flash-video",
    "input": {
      "video_mode": "video_reference",
      "prompt": "Same camera move, but in a snowy forest",
      "video_url": "https://example.com/source.mp4"
    }
  }'
```

## Response

```json theme={null}
{
  "code": 200,
  "success": true,
  "data": {
    "task_id": "abc123def456",
    "status": "processing"
  }
}
```

> [!NOTE]
>
> **Volume discounts** are available for almost all models. Reach out at [contact@unifically.com](mailto:contact@unifically.com) to discuss custom pricing.


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