arkennemasis fal · Flux 3 Extend Video · $0.41-0.53/s

Here's a surprise worth knowing before you wire this up. FLUX 3's continue-a-clip endpoint is the most expensive node in the Flux 3 family: $0.41 per second at 720p, $0.53 at 1080p. Text-to-video is $0.17/s at 720p. So extending a clip costs roughly two and a half times what generating one from scratch does - which is the opposite of the intuition that "more of the same" should be cheap.

Why? Continuation is a harder ask. The model has to honour the source's final frames, its motion, its lighting and its audio, from a starting point it didn't choose. You're paying for constraint, not for novelty.

ArkFal_blackforestlabs_flux_3_extend_video is FLUX 3 Video's continuation endpoint: a source clip plus a prompt describing what happens next, out comes footage that picks up where the original stopped.

What you set

• video - required. The source clip. MP4, under 50MB, under 15 seconds. There's no client-side length check here, so an over-long clip is caught by fal after the upload rather than before - keep your sources short and trimmed.
• prompt - how it continues. The tooltip says it plainly: describe what happens next from the source's final frames.
• resolution - 720p (default) or 1080p. At these rates the difference is $0.12 per second, so nearly a dollar on an 8-second extension.
• duration - auto, or pinned. Pin it when you're assembling a timeline; auto lets the model decide how much it needs.
• generate_audio - on by default, and at these prices you should at least be getting a soundtrack for it.
• aspect_ratio - auto makes sense here, since the source has a shape already; force it and you're asking for a reframe.
• safety_tolerance - 0 to 4, default 2.

Optional and shared: max_cost_usd (default $20 - set it to $5 on this node, honestly), reuse_identical_run, extra_json, max_concurrent.

Outputs: video, video_path, seed, info.

Where extend actually earns its price

Not for "make my clip longer". For shot ends that need a landing: the character reaches the door and you need three more seconds for the dialogue to finish. Or the reverse - a clip whose final moment is perfect but whose setup drags, which is what the edit endpoint is for. Extension is a repair tool as much as a creative one, and thinking of it that way keeps the bill sane.

For iterating, the draft version of this node is $0.12/s and returns a draft cache you can finish with draft_enhance at $0.29/s. On a 10-second continuation that's $1.20 to find out whether the join works, versus $4.10 to find out the hard way.

Install

cd ComfyUI/custom_nodes
git clone https://github.com/Hishamahmer/comfyui-arkennemasis

Restart, or let ComfyUI Manager handle it - search arkennemasis. Then:

FAL_KEY=your-key-from-fal.ai/dashboard/keys

in the .env beside run_nvidia_gpu.bat (portable build) or in the ComfyUI folder. The node has no key field by design, so a key can never be saved into a workflow you share. Nothing is downloaded; FLUX 3 is fal-only at this point.

Gotchas

Your source clip is uploaded to fal's CDN. That's free and it happens before the paid call - but it does mean your footage leaves the machine, same as every other API node in this category.

Seconds come from the clip you connect or from the duration box, and the browser-side badge can't measure a clip, so it shows a rate while the real figure appears at run time. Since this is the priciest node in the family, that's the one place I'd actually set max_cost_usd low and let it argue with you.

Files are saved to output/fal/blackforestlabs_flux-3_extend-video/, every submit is logged in output/fal/_requests.jsonl, and a ComfyUI restart mid-render doesn't lose the job - fal Recover Result collects it by request id, free.