Kling 4.0

Endpoint

POST https://api.roasos.com/v1/video/kling-4

Send your API key as a bearer token. The key needs the scope api:write. Every paid request also needs an Idempotency-Key header.

The envelope fields quote_only, quote_id, webhook_url and wait_timeout_seconds behave the same on each model endpoint. Refer to Jobs and Quotes and credits.

Good at

Kling 4.0 is the only Kling video model since 2026-10-29. It replaced Kling 3, Kling 3 Pro, Kling O3, Kling O3 Pro and both 4K tiers, and those endpoints are removed.

Use it to keep one or more people, or a product, the same across a clip. Send each person or product as an element: one frontal image and up to 3 more views. The prompt names each element once as @Element1, @Element2, and so on. Send avatar_id to attach a saved person as an element.

An element can also be a video of the person or object (video_url, 3 to 8 seconds, at least 700 px on each side) instead of images. Up to 3 elements can be video elements, and their seconds count in the 30-second clip total and in the price. Up to 3 elements can carry a voice_id: a Kling voice that you made with fal's create-voice endpoint. The element then speaks in that voice.

To change an existing video, use Kling 4.0 Edit.

The reference lane takes up to 10 images (references, named @Image1..N), 5 video clips (video_urls, named @Video1..N) and 7 elements. A first frame (image_url) on the reference lane is the literal opening frame, and it counts as one of the 10 images. Each reference clip must be 3 to 30 seconds long, and all clips together must be 30 seconds or less. A reference clip shows motion or style to follow. It does not edit that clip.

Use multi_prompt for a board of shots in one generation: up to 8 shots, each with its own prompt (512 characters or less) and its own length. The shot lengths must add up to duration_seconds.

Text-to-video takes 9:16, 16:9, 1:1 and 21:9. The reference lane also takes auto, which follows the references. Image-to-video takes its framing from the first frame, and end_image_url sets the last frame.

The provider documents no audio switch. The endpoint does not accept generate_audio.

Pricing

The unit is one clip. Pricing key: kling-4.

The credits come from the roasOS pricing table. No other field changes the price. To get the exact price of your request, send it with quote_only: true first.

Modes

• text-to-video
• image-to-video
• reference-to-video

roasOS selects the mode from the media fields of your request. You do not send a mode.

Provider endpoints: kling/v4/image-to-video · kling/v4/reference-to-video · kling/v4/text-to-video.

Request fields

The schema is strict. A field that is not in this table gets 400 unsupported_option. The field names are the roasOS role names. One role has one name in every family.

Media

This endpoint accepts these media roles:

• avatar_id — identity; the saved character reference sheet, with a derived portrait only where the model requires a single face or source frame
• elements — identity; one frontal image plus more views, per person or object
• references — style and appearance references
• image_url — first frame (or the single source image)
• end_image_url — last frame
• scene_id — first frame — a scene this workspace owns
• video_urls — reference videos

Every other role is refused: product_id, product_images, reference_images, scene_image, video_url, audio_urls, audio_url, voice_id. A refused role gets 400 unsupported_option.

Prompt

Limit 2,500 characters for prompt, and 512 characters for each multi_prompt shot.

• Think in shots: "Shot 1 (3s): …". Define each subject and the setting in shot 1, and keep them the same.
• In each shot, write the subject, the action and its motion path, the setting, the camera, and the mood. Use one action and one camera move for each shot.
• Write motion as physics: the main movement, then the movement of hair, fabric or particles.
• Write each spoken line in quotes with a speaker label and a voice: [Mia, warm]: "Okay, this texture."
• In image-to-video, write only what changes. The first frame sets the scene.
• Use each @Image, @Video and @Element tag once. The platform adds a missing @Image or @Video tag at the end of a single prompt. You must write each @Element tag yourself.
• End with "No text overlays, no captions." Add text in post-production.

Not exposed

These catalog entries describe omitted inputs, pinned values and source constraints. They do not imply that the provider accepts every listed field.

• video_urls — these values are not available: MOV containers.
• middle_image_urls — not accepted by this public endpoint.
• voice_ids — not accepted by this public endpoint.

Result

The result kind is video. Always present: url.

{
  "data": {
    "id": "gen_…",
    "kind": "video",
    "model": "kling-4",
    "status": "completed",
    "result": {
      "url": "https://cdn.roasos.com/workspaces/ws_01/result"
    }
  }
}

The files are immutable objects on https://cdn.roasos.com. A result URL is a valid media input for your next request.

Example

The three steps are always the same: quote, run, poll.

# 1. Quote. Nothing is spent.
curl -X POST https://api.roasos.com/v1/video/kling-4 \
  -H "Authorization: Bearer roasos_sk_live_..." \
  -H "Idempotency-Key: 0f1c9e2a-kling-4-0001" \
  -H "Content-Type: application/json" \
  -d '{"quote_only":true,"prompt":"A woman holds the bottle at eye level in a bright kitchen, morning light, static camera."}'
# 200 { "data": { "kind": "quote", "quote_id": "q_7Hk...", "credits": 9, "expires_at": "...",
#        "confirmation": { "model": "kling-4", "example": "3s_1080p" } } }

# 2. Run. Same body, same Idempotency-Key, quote_id in place of quote_only.
curl -X POST https://api.roasos.com/v1/video/kling-4 \
  -H "Authorization: Bearer roasos_sk_live_..." \
  -H "Idempotency-Key: 0f1c9e2a-kling-4-0001" \
  -H "Content-Type: application/json" \
  -d '{"quote_id":"q_7Hk...","prompt":"A woman holds the bottle at eye level in a bright kitchen, morning light, static camera."}'
# 201 { "data": { "id": "gen_...", "status": "queued", "credits_reserved": 9 } }

# 3. Poll, then read the result.
curl https://api.roasos.com/v1/jobs/gen_.../status -H "Authorization: Bearer roasos_sk_live_..."
curl https://api.roasos.com/v1/jobs/gen_.../result -H "Authorization: Bearer roasos_sk_live_..."

See also

• llms.txt for this model — the same facts as one plain-text file for an agent.
• GET https://api.roasos.com/v1/catalog/kling-4 — the machine-readable form: enums, defaults, limits and prices. Refer to the API reference.
• Jobs — how to poll a job and read its result.
• Provider endpoints: kling/v4/image-to-video · kling/v4/reference-to-video · kling/v4/text-to-video.