
BLACKFORESTLABS FLUX 3 Extend Video API by Black Forest Labs
Extend a video, continuing from the source clip final frames, using FLUX 3.
BLACKFORESTLABS FLUX 3 Extend Video は Black Forest Labs が開発したモデルです。Atlas Cloud(運営:Atlas Cloud AI LLC)は本モデルへのアクセスを提供するものであり、その所有者ではありません。すべての商標は各所有者に帰属します。
FLUX 3 Extend Video
FLUX 3 Extend Video continues an existing video clip from where it left off. Provide a source video and a prompt describing how the action continues, and the model generates a seamless new segment that flows naturally from the final frames — with optional native audio.
Why Choose This?
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Seamless continuation The model reads the source clip's final frames and continues the motion naturally.
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Native audio generation Produce the extension and its synchronized audio in a single request.
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Explicit duration required Duration must be set explicitly to validate keyframe positions.
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Flexible aspect ratios
auto,21:9,2:1,16:9,4:3,1:1,3:4, or9:16. -
High resolution output Generate the extension in 720p or 1080p.
Parameters
| Parameter | Required | Description |
|---|---|---|
| video_url | Yes | URL of the input video to extend. MP4, under 50 MB and under 15 seconds |
| prompt | No | Text description of how the video continues from the source clip's final frames |
| aspect_ratio | No | Aspect ratio of the generated extension. Default: auto. Options: auto, 21:9, 2:1, 16:9, 4:3, 1:1, 3:4, 9:16 |
| resolution | No | Video resolution. Default: 720p. Options: 720p, 1080p |
| duration | No | Duration in seconds. Default: 5. Options: 5–20. Cannot be auto — required to validate keyframe positions |
| generate_audio | No | Whether to generate audio. Default: true |
| safety_tolerance | No | Safety level 0 (strictest) to 4 (most permissive). Default: 2 |
| seed | No | Random seed for reproducibility |
How to Use
- Provide your source video — must be MP4, under 50 MB, and under 15 seconds.
- Write your prompt — describe what happens next, continuing from where the clip ends.
- Choose resolution — match the source resolution for a consistent result.
- Set duration — use
autoor specify how many additional seconds to generate. - Run — submit and download your extended video.
Pricing
Billed per second of generated video, by resolution:
| Resolution | Cost per second |
|---|---|
| 1080p | $0.583 |
| 720p | $0.451 |
Best Use Cases
- Long-form Content — Chain multiple extensions to build longer sequences.
- B-roll Extension — Stretch a short clip to fill a longer edit slot.
- Iterative Storytelling — Keep a narrative going beyond a single generation.
- Loop Building — Extend a clip and then join the end back to the start.
Pro Tips
- Describe the continuation as if you're directing the next shot: "the camera follows the subject as they turn left".
- Use
aspect_ratio: "auto"to preserve the source video's proportions. - Keep the prompt consistent in style and tone with the source clip for the smoothest result.
- Match the resolution to the source video to avoid quality mismatches.
Notes
video_urlis the only required field;promptis optional but improves directional control.- Source video constraints: MP4 format, under 50 MB, under 15 seconds.
durationcontrols the extension length, not the total output duration.- Generation is asynchronous — submit, then poll for the finished video.
Related Models
- FLUX 3 Text-to-Video — Generate video from a text prompt alone.
- FLUX 3 Image-to-Video — Animate a starting image.
- FLUX 3 First & Last Frame to Video — Interpolate between two images.
- FLUX 3 Keyframes to Video — Pin multiple images to exact frame positions.


















