kling-4-0-flash

Quick example

curl https://reapi.ai/api/v1/videos/generations \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "kling-4-0-flash",
    "prompt": "A paper boat drifts down a rain-filled city gutter at dusk, the camera skimming the water beside it past glowing shop windows, raindrops ticking on the surface. No text, no logos.",
    "duration": 5,
    "aspect_ratio": "16:9"
  }'

import requests

response = requests.post(
    "https://reapi.ai/api/v1/videos/generations",
    headers={"Authorization": "Bearer YOUR_API_KEY"},
    json={
        "model": "kling-4-0-flash",
        "prompt": "A paper boat drifts down a rain-filled city gutter at dusk, the camera skimming the water beside it past glowing shop windows, raindrops ticking on the surface. No text, no logos.",
            "duration": 5,
        "aspect_ratio": "16:9",
    },
    timeout=60,
)
response.raise_for_status()
task_id = response.json()["id"]

const response = await fetch('https://reapi.ai/api/v1/videos/generations', {
  method: 'POST',
  headers: {
    Authorization: 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    model: 'kling-4-0-flash',
    prompt:
      'A paper boat drifts down a rain-filled city gutter at dusk, the camera skimming the water beside it past glowing shop windows, raindrops ticking on the surface. No text, no logos.',
    duration: 5,
    aspect_ratio: '16:9',
  }),
});
if (!response.ok) throw new Error(`Request failed: ${response.status}`);
const { id: taskId } = await response.json();

package main

import (
    "encoding/json"
    "fmt"
    "net/http"
    "strings"
    "time"
)

func main() {
    body := strings.NewReader(`{
      "model": "kling-4-0-flash",
      "prompt": "A paper boat drifts down a rain-filled city gutter at dusk, the camera skimming the water beside it past glowing shop windows, raindrops ticking on the surface. No text, no logos.",
        "duration": 5,
      "aspect_ratio": "16:9"
    }`)
    req, err := http.NewRequest("POST", "https://reapi.ai/api/v1/videos/generations", body)
    if err != nil { panic(err) }
    req.Header.Set("Authorization", "Bearer YOUR_API_KEY")
    req.Header.Set("Content-Type", "application/json")
    client := &http.Client{Timeout: 60 * time.Second}
    res, err := client.Do(req)
    if err != nil { panic(err) }
    defer res.Body.Close()
    var task struct { ID string `json:"id"` }
    if err := json.NewDecoder(res.Body).Decode(&task); err != nil { panic(err) }
    fmt.Println(task.ID)
}

Authentication

Every call needs a Bearer token. Generate keys at reapi.ai/settings/apikeys.

Authorization: Bearer YOUR_API_KEY

Endpoint

POST /api/v1/videos/generations
GET  /api/v1/tasks/{id}

Submission is async. The POST returns a task id right away; poll the task endpoint until status is completed or failed. Polling does not consume credits.

Request body

Every clip comes with sound; there is no sound switch. There are no first, middle or last frame fields, no negative_prompt, watermark or standalone voices field: bind a voice to an element with voice_url. Unknown fields are refused rather than ignored.

Checked by reAPI before anything is charged:

• every media field, including each element's image_urls, video_url and voice_url, is a public HTTP(S) URL — base64 and data: URIs are rejected
• resolution is 720p, and duration is a positive whole number
• every reference video and video element can be measured (its length sets the bill — see Pricing)

Every other rule on this page — counts, element shapes, names, mentions, input length — is checked by the model when the task starts. A request that breaks one ends as a failed task with code 80007 and the exact reason, and is refunded in full.

Mentions

Every material is optional; with none, the request is plain text to video. Once you send materials, point at them from the prompt:

Every @ followed by a letter, digit, underscore or Chinese character must name a material of this request, or the request fails. Names are matched longest first and without regard to case. A name that ends in a letter, digit or underscore cannot run straight into another one: @image_12 with a single image fails; write @image_1 2 or @image_1,.

Reference videos plus video elements are at most 5, and images, videos and elements together at most 15.

{
  "model": "kling-4-0-flash",
  "prompt": "The dancer from @image_1 performs the routine of @video_1 on a rooftop at dusk",
  "image_urls": ["https://example.com/dancer.png"],
  "video_urls": ["https://example.com/routine.mp4"],
  "keep_original_sound": false,
  "duration": 8
}

Elements

An element is a named subject you can reuse in the prompt. There are two kinds:

Names are 1–20 letters, digits, underscores or Chinese characters, unique without regard to case. image_N, video_N, keyframe_N, object_N, audio_N, first_frame and last_frame are reserved.

{
  "model": "kling-4-0-flash",
  "prompt": "@Mira opens the shop door and greets the morning crowd",
  "elements": [
    {
      "name": "Mira",
      "image_urls": ["https://example.com/mira-front.png", "https://example.com/mira-side.png"],
      "tag": "character",
      "description": "baker in a flour-dusted apron"
    }
  ],
  "aspect_ratio": "9:16",
  "duration": 6
}

Reference videos and video elements together must be 2–20 seconds long.

Troubleshooting

Response envelope

{
  "id": "task_018f5a3a1b6e7d9f8c2b4d6e8f0a2c4e",
  "model": "kling-4-0-flash",
  "status": "completed",
  "created_at": 1789689600,
  "output": {
    "video_urls": ["https://cdn.reapi.ai/media/tasks/.../0.mp4"]
  },
  "error": null
}

Poll GET /api/v1/tasks/{id} (see the Tasks reference) until status === "completed", then read output.video_urls. A failed task carries error.code and error.message; see the errors catalog.

Pricing

Kling 4.0 Flash bills per second of 720p video, at a rate set by whether the request has input videos:

credits = ceil(per_second_rate × billable_seconds × 1000)     1 credit = $0.001

input is the total length of the reference videos and video elements, measured by reAPI when the request is accepted and rounded up to whole seconds. output is duration (default 5). A 5-second clip guided by a 7.2-second reference video bills 5 + 8 = 13 seconds; with a 3-second reference video it bills 5 + 5 = 10 seconds. Failed tasks are refunded in full.

Live per-second rates for both rates are on the model page.

Tips

• Describe subject, place, camera move, light and sound; the clip's audio follows the prompt.
• Use Flash to try prompts, ratios and references, then move the shot that works to Kling 4.0 when you need 1080p, keyframes, editing or extension.
• Give a recurring character an element with two or three clear views instead of one reference image.
• Add no text, no logos to keep invented marks out.

Related

• Kling 4.0 Flash playground and pricing
• Kling 4.0
• Kling 3.0 Turbo
• Errors catalog
• Authentication
• Quickstart