Kwaivgi Kling V4.0 Video Extend API Documentation

Kwaivgi Kling V4.0 Video Extend API Documentation

Playground

Kling V4.0 Video Extend continues a video from its last frame with a new 3 to 30 second segment guided by your prompt, at 720p or 1080p with synchronized stereo audio, and returns the original and the new segment joined. Ready-to-use REST inference API, best performance, no coldstarts, affordable pricing.

Features

Kling V4.0 Video Extend continues an existing video from its last frame. Describe what should happen next and generate a new segment from 3 to 30 seconds at 720p or 1080p, with synchronized audio.

The generated segment is appended to the original video to create one continuous clip.

Why Choose This?

• Continue existing videos — Extend a clip naturally from its final frame.
• Flexible extension length — Generate an additional 3–30 seconds.
• 720p and 1080p output — Choose the resolution that fits your workflow.
• Synchronized audio — Generate sound together with the new video segment.
• Optional ending frame — Provide a last_image to guide the continuation toward a specific final frame.

Parameters

How to Use

1. Upload a video — Provide the clip you want to continue.
2. Describe what happens next — Write a prompt for the new action, camera movement, and sound.
3. Set the duration — Choose an extension length from 3 to 30 seconds.
4. Choose resolution — Select 720p or 1080p.
5. Add a last image optional — Provide a target ending frame when you want more control over how the continuation finishes.
6. Submit — Generate and retrieve the extended video.

Pricing

Only the newly generated segment is charged.

For example, extending a video by 5 seconds costs $0.785 at 720p or $1.175 at 1080p.

last_image does not add a separate charge.

Best Use Cases

• Story continuation — Extend an existing scene with new action or events.
• Longer cinematic sequences — Continue shots without starting a new generation from scratch.
• Camera movement extensions — Carry an existing camera motion into a longer sequence.
• Dialogue and performance — Continue character actions with synchronized speech or sound.
• Scene transitions — Use a target ending image to guide the clip toward a specific visual destination.

Pro Tips

• Describe what should happen next rather than repeating everything already visible in the source video.
• Keep the continuation consistent with the final moment of the source clip.
• Match the extension length to the amount of action in the prompt.
• Describe important ambience, music, dialogue, or sound effects when audio matters.
• Use last_image when you want the extended scene to finish at a specific visual state.

Notes

• video and prompt are required.
• duration supports 3–30 seconds and defaults to 5.
• resolution defaults to 720p.
• The extension begins from the source video’s last frame.
• The original video and generated segment are returned as one continuous clip.
• The source video’s existing audio is retained, while the new segment includes generated synchronized audio.

Related Models

• Kling V4.0 Text-to-Video — Generate videos directly from text prompts.
• Kling V4.0 Image-to-Video — Animate a reference image with Kling V4.0.

Authentication

For authentication details, please refer to the Authentication Guide.

API Endpoints

Submit Task & Query Result

set -euo pipefail

export WAVESPEED_API_KEY="your-api-key"

REQUEST_BODY=$(cat <<'JSON'
{
  "prompt": "A cinematic ocean wave at sunrise, highly detailed",
  "video": "https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4",
  "resolution": "720p",
  "duration": 5
}
JSON
)

# 1. Submit the prediction once. Never retry this request automatically:
# a retry after an unclear failure can create a second, billed task.
CHECK='The task may already exist. Check History or the predictions list before submitting again.'
if ! SUBMIT_RESPONSE=$(curl --silent --show-error --fail-with-body \
  --connect-timeout 10 --max-time 60 \
  -X POST "https://api.wavespeed.ai/api/v3/kwaivgi/kling-v4.0/video-extend" \
  -H "Authorization: Bearer ${WAVESPEED_API_KEY}" \
  -H "Content-Type: application/json" \
  -d "${REQUEST_BODY}"); then
  printf 'Submit failed. %s\n' "${CHECK}" >&2
  exit 1
fi

TASK=$(printf '%s' "${SUBMIT_RESPONSE}" | jq 'if type == "object" and has("data") then .data else . end')
PREDICTION_ID=$(printf '%s' "${TASK}" | jq -r '.id // empty')
if [ -z "${PREDICTION_ID}" ]; then
  printf 'Submission response did not contain a prediction id. %s\n' "${CHECK}" >&2
  exit 1
fi
RESULT_URL="https://api.wavespeed.ai/api/v3/predictions/${PREDICTION_ID}/result"

# 2. Poll until the prediction reaches a final status.
BODY_FILE=$(mktemp)
trap 'rm -f "${BODY_FILE}"' EXIT
while true; do
  # Transient errors (429, 5xx) are retried, honoring Retry-After.
  HTTP_CODE=$(curl --silent --show-error --fail-with-body \
    --connect-timeout 10 --max-time 60 --retry 5 --retry-delay 2 \
    --output "${BODY_FILE}" --write-out '%{http_code}' \
    "${RESULT_URL}" \
    -H "Authorization: Bearer ${WAVESPEED_API_KEY}" || true)
  case "${HTTP_CODE}" in
    2??) ;;
    000|429|5??)  # dropped connection or temporary error: keep polling
      printf 'Task %s: HTTP %s, polling again in 10s\n' "${PREDICTION_ID}" "${HTTP_CODE}" >&2
      sleep 10; continue ;;
    *) printf 'Task %s: HTTP %s\n' "${PREDICTION_ID}" "${HTTP_CODE}" >&2
      cat "${BODY_FILE}" >&2; exit 1 ;;
  esac
  RESULT=$(jq 'if type == "object" and has("data") then .data else . end' "${BODY_FILE}")
  STATUS=$(printf '%s' "${RESULT}" | jq -r '.status // empty')

  case "${STATUS}" in
    completed) printf '%s\n' "${RESULT}" | jq '.outputs'; break ;;
    failed|cancelled|timeout|deleted)
      printf 'Task %s ended with status %s\n' "${PREDICTION_ID}" "${STATUS}" >&2
      printf '%s\n' "${RESULT}" | jq . >&2; exit 1 ;;
    *) sleep 2 ;;
  esac
done

Parameters

Task Submission Parameters

Request Parameters

Every model also accepts the global request options enable_sync_mode and enable_base64_output; see their pages for details.

Response Parameters

Result Request Parameters

Result Response Parameters