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FLUX 3 Extend Video: Video-to-Video Clip Continuation on Models and API | RunComfy

blackforestlabs/flux-3/extend-video

FLUX 3 Extend Video lengthens a source clip from its final frames with prompt-guided motion and optional native audio at 720p or 1080p.

The text prompt describing how the video continues from the source clip's final frames.
URL of the input video. MP4, under 50 MB and under 15 seconds.
Aspect ratio of the generated video. auto lets the model choose.
Resolution of the generated video.
Duration of the generated video in seconds. auto lets the model choose.
Whether to generate audio for the video.
The safety tolerance level for the generated video. 0 is the strictest and 4 is the most permissive.
Idle
The rate is $0.41 per second at 720p, and $0.53 per second at 1080p.

Introduction To FLUX 3 Extend Video

Black Forest Labs' FLUX 3 Extend Video takes a source clip and a continuation prompt, then generates the next stretch of footage with optional native audio at 720p or 1080p, for auto or 5–20 seconds per pass.
Trading cut-and-paste stitch edits and mismatched motion for prompt-guided continuation from the last frames, FLUX 3 Extend Video helps editors, marketers, and previz teams grow short takes into longer shots.
For developers, FLUX 3 Extend Video on RunComfy can be used both in the browser and via an HTTP API, so you don't need to host or scale the model yourself.
Ideal for: Shot Lengthening | Multi-Hop Sequence Building | Scene-Consistent Follow-Through

Black Forest Labs / FLUX 3 Extend Video#


Upload a short source clip, describe what should happen next, and get a continuation that picks up from the final frames. On RunComfy you set the Input in the model UI, then reuse the same fields over HTTP API.


Output format: Video / source clip under 15 s / extension auto or 5–20 s / 720p or 1080p / optional native audio


Highlights#


  • Continue from the end: FLUX 3 Extend Video reads the trailing frames of your clip and invents the next stretch of action instead of restarting from a blank slate.
  • Prompt-guided follow-through: Spell out camera moves, subject action, and mood so the extension matches the intent you write when you use FLUX 3 Extend Video.
  • Motion and scene carryover: The model aims to keep pace, framing, and setting aligned with the source so the join feels like one take.
  • Sound in the same pass: Leave Generate Audio on for speech, ambience, and effects timed to the new footage, or turn it off for silent output.
  • Flexible length: Use auto duration or lock any whole-second length from 5 to 20 for the generated segment.
  • Framing options: Aspect ratios from ultrawide 21:9 to vertical 9:16, plus auto when framing can stay flexible.
  • Two delivery tiers: Iterate at 720p, then step up to 1080p when you need sharper finals.
  • Safety dial: Set safety tolerance from 0 (strictest) to 4 (most permissive).

Parameters#


ParameterRequiredTypeDefaultRange / OptionsDescription
prompt*Yes (*)string——Text describing how the video continues from the source clip's final frames.
video_url*Yes (*)string (video_uri)—MP4, under 50 MB and under 15 secondsURL of the input video to extend.
aspect_ratioNostringautoauto, 21:9, 2:1, 16:9, 4:3, 1:1, 3:4, 9:16Output frame shape; auto lets the model choose.
resolutionNostring720p720p, 1080pOutput resolution.
durationNostringautoauto, 5–20Length of the generated segment in seconds; auto lets the model choose.
generate_audioNobooleantruetrue, falseWhether to generate synchronized audio.
safety_toleranceNointeger20–40 is strictest; 4 is most permissive.

Pricing#


Billing for FLUX 3 Extend Video is based on the length of the generated segment and the selected resolution.


ResolutionRate
720p$0.41 per second
1080p$0.53 per second

Estimated cost examples (720p)


DurationApprox. cost
5 s~$2.05
10 s~$4.10
20 s~$8.20

Estimated cost examples (1080p)


DurationApprox. cost
5 s~$2.65
10 s~$5.30
20 s~$10.60

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Frequently Asked Questions

What is FLUX 3 Extend Video used for in video-to-video workflows?

FLUX 3 Extend Video continues an existing clip past its final frames using a written prompt. It helps editors, marketers, and previz teams lengthen a locked opening take without rebuilding the whole shot from scratch.

What inputs does FLUX 3 Extend Video require?

FLUX 3 Extend Video requires a prompt and a video_url. The source video should be MP4, under 50 MB, and under 15 seconds. Optional fields include aspect_ratio, resolution, duration, generate_audio, and safety_tolerance.

How long can a FLUX 3 Extend Video extension be?

You can set duration to auto or lock any whole-second length from 5 to 20 for the generated segment. Auto lets the model choose; fixed lengths give predictable pacing when you chain multiple FLUX 3 Extend Video passes.

What resolution and aspect ratio options does FLUX 3 Extend Video support?

Resolution options for FLUX 3 Extend Video are 720p (default) and 1080p. Aspect ratio defaults to auto, or you can lock 21:9, 2:1, 16:9, 4:3, 1:1, 3:4, or 9:16 for known delivery formats.

Does FLUX 3 Extend Video generate audio with the extension?

Yes. Generate Audio is on by default in FLUX 3 Extend Video, so the model can output synchronized speech, SFX, and ambience with the new footage. Turn it off when you need silent video.

How does FLUX 3 Extend Video differ from FLUX 3 text-to-video?

Text-to-video invents a full clip from a prompt alone. FLUX 3 Extend Video is video-to-video: it conditions on your source clip’s trailing frames and generates the next stretch so motion and scene can stay aligned with what you already shot.

How do I move from testing FLUX 3 Extend Video in the browser to production API integration?

Prototype FLUX 3 Extend Video in the RunComfy model UI, then call the same template through the RunComfy API with identical Input fields (prompt, video_url, aspect_ratio, resolution, duration, generate_audio, safety_tolerance). Validate prompts and joins in the UI first, then use your account API key and credits for automated jobs.

How much does FLUX 3 Extend Video cost on RunComfy?

Billing for FLUX 3 Extend Video is based on generated video duration and resolution: $0.41 per second at 720p, and $0.53 per second at 1080p. See the pricing shown on this page for the latest rates.

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Examples Of FLUX 3 Extend Video

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