FLUX.3 Extend Video by Black Forest Labs continues an existing clip from its final frames, generating a seamless continuation plus matching audio from your prompt. Feed a source video under 15 seconds and under 50 MB, describe what happens next and how it should sound, and add 5 to 20 seconds at 720p or 1080p in aspect ratios from 21:9 to 9:16. Motion, style and subjects carry over from the source. Ideal for lengthening shots, adding a payoff beat, looping ideas and building longer sequences from short clips.