# Kling 4.0 Flash (`kling_v4_flash`)

> Kling 4.0 Flash by Kuaishou. Fast 720p drafts up to 20 seconds from text, a start frame or reference elements.

- Capability: Video (tool `video`) — Text to video, Image to video, Video to video
- Price: from $0.38 per run (1 credit = $0.01). Exact price for a request: POST https://api.varg.ai/v2/estimate
- Typical time: ~2 min
- JSON Schema: https://api.varg.ai/v2/tools/video?model=kling_v4_flash (public, no key)
- Try it: https://varg.ai/dashboard/tools/playground?model=kling_v4_flash

## Call it

REST: `POST https://api.varg.ai/v2/tools/video/call` with `Authorization: Bearer $VARG_API_KEY` (create a key at https://varg.ai/dashboard/api).
It answers `202` with a job right away. Poll `GET /v2/jobs/{id}` until `status` is `completed`, then read `output.outputs[0].url` — or pass `options.webhook_url` and get the finished job POSTed to you.

```bash
curl -X POST https://api.varg.ai/v2/tools/video/call \
  -H "Authorization: Bearer $VARG_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
  "model": "kling_v4_flash",
  "prompt": "A detective in a black trench coat stands on a rooftop in the rain, looking through a telescope.",
  "duration": 5
}'
```

MCP: the varg server `https://mcp.varg.ai/mcp` (OAuth sign-in, no key) runs it with `call_tool`:

```json
{
  "tool": "video",
  "input": {
    "model": "kling_v4_flash",
    "prompt": "A detective in a black trench coat stands on a rooftop in the rain, looking through a telescope.",
    "duration": 5
  },
  "wait": true
}
```

## Inputs

| field | type | required | default | allowed | description |
|---|---|---|---|---|---|
| `model` | string | yes | — | `kling_v4_flash` or a route key below | Which model runs the job. |
| `prompt` | string | no | — | — | — |
| `files` | array of { url } | no | — | — | — |
| `aspect_ratio` | string | no | — | e.g. 16:9 \| 9:16 \| 1:1 \| 4:3 \| 3:4 \| 21:9 \| auto | — |
| `resolution` | string | no | — | 480p \| 720p \| 1080p \| 4k | Quality tier. Supported by this model: 720p. Unsupported tiers fall back to the nearest available. |
| `audio` | boolean | no | — | — | Native audio track request. Not supported by this model — ignored if sent. |
| `duration` | integer | no | — | 1–120 | Length in seconds. Supported by this model's routes: fal: integer 3-20. A non-numeric value such as "auto" is routed to a route whose schema accepts it; when none does, the request is rejected. |

Media inputs are URLs. Upload a local file first with `POST /v2/files` and pass the returned URL.

## Provider options

Optional, per provider, under `provider_options.<provider>`. Only the options of the provider that runs the job apply — pin a route to choose it.

### provider_options.fal

| field | type | required | default | allowed | description |
|---|---|---|---|---|---|
| `aspect_ratio` | string | no | — | 16:9 \| 9:16 \| 1:1 \| 21:9 \| auto | — |
| `start_image_url` | string (URL) | no | — | — | Image to use as the first frame of the video. Flash does not take an end frame. Max file size: 30.0MB, Min width: 300px, Min height: 300px, Min aspect ratio: 0.40, Max aspect ratio: 2.50, Timeout: 20.0s |
| `video_url` | string (URL) | no | — | — | The video to edit (3-20s), referenced as @Video1. Max file size: 300.0MB, Min width: 700px, Min height: 700px, Max width: 4553px, Max height: 4553px, Min aspect ratio: 0.40, Max aspect ratio: 2.50, Min duration: 3.0s, Max duration: 20.05s, Min FPS: 23.9, Max FPS: 60.05, Timeout: 30.0s |

## Routes

`"model": "kling_v4_flash"` picks a route automatically (priority order, then the inputs you send). To pin one, pass its key as `model`.

| model key | provider | modality | price from | typical time |
|---|---|---|---|---|
| `fal:fal-ai/kling-video/v4/flash/text-to-video` | fal | text-to-video | $0.48 | ~2 min |
| `fal:fal-ai/kling-video/v4/flash/image-to-video` | fal | image-to-video | $0.48 | ~2 min |
| `fal:fal-ai/kling-video/v4/flash/reference-to-video` | fal | image-to-video | $0.48 | ~2 min |
| `fal:kling/v4/flash/video-to-video` | fal | video-to-video | $0.38 | ~3 min |

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Catalog version 2026-11-11T12:59:09.901887+00:00. All models: https://varg.ai/models/llms.txt