TikTok hook in 6 seconds, offer in 3: recut the opening with source_in

TikTok's performance-ad page says to put the hook in the first 6 seconds and the content proposition in the first 3. If your best moment sits at second 9 of the raw clip, pull it to the front: Timeline's source_in starts a slot inside the source, and video-trim cuts a new file from a start and an end or duration.

What TikTok's page says

"Creative best practices for performance ads" (read 2026-10-05) lists a hook in the first 6 seconds, the proposition in the first 3 seconds, sound or music, a DIY rather than overly polished style, captions or text overlays, and transitions, stickers and graphics. It does not tell you how to cut; that part is your edit.

Find the moment first

Use video frames to pull a contact sheet. With fps up to 2 or up to 24 at[] values you see the candidate seconds as stills, then pick the one where the product or result is clearest.

Move it to the front

In Timeline 1.0, set the first slot's source_in to the second you picked and give it a short duration, then follow with the rest of the clip. The first slot's start is 0, as the docs require.

If you want a standalone file, use POST /v1/video-trim with start and one of end or duration (0.2 to 900 seconds). The exact precision re-encodes with libx264 yuv420p and AAC audio; keyframe is faster but cuts on keyframes. The docs list $0.02 per job.

{
  "audio": { "url": "https://media.sume.com/artifacts/artf_demo/vo.wav", "duration_seconds": 20 },
  "video": [
    { "source_url": "https://media.sume.com/artifacts/artf_demo/raw.mp4",
      "start": 0, "duration": 4, "source_in": 9 },
    { "source_url": "https://media.sume.com/artifacts/artf_demo/raw.mp4",
      "start": 4, "duration": 16,
      "transition": { "type": "fade", "duration": 0.25 } }
  ]
}

Check the result

Pull stills at 0.5, 2.5 and 5.5 seconds and look at them: is the offer legible in the first three, is the hook clear by six? That is a visual check, not a TikTok measure. Use the plan route first to confirm the timeline compiles without cost.

• Do not start with a logo card; it spends the first seconds.
• Keep text readable for the time it is shown.
• Rerun the cut after you adjust captions.