Stitch 10-second AI clips into a 3-minute Short with Timeline 1.0

YouTube's help page says its generative tools make standalone clips "up to 10 seconds long" for Shorts, and the Shorts length page says a Short can be up to three minutes. Bridging those is an assembly job.

The arithmetic

A 180-second Short built from 10-second clips needs 18 slots with no overlap. Timeline 1.0 takes 1 to 200 video[] slots and an audio.duration_seconds from 1 to 1,800, so 18 slots sit well inside it.

Rules that bite

Per the Timeline docs, video[0].start must be 0, later starts must increase, and coverage can stop at most 0.5 s before the end of the spine. Above 12 slots, render.strategy: single is refused, so leave it on auto. The default output is 1080 by 1920.

Plan first

The unbilled plan call returns billable_minutes before you spend. Build the slots, then check the plan.

import json, os, urllib.request

slots = [
    {"source_url": f"https://media.sume.com/artifacts/example/clip{i}.mp4",
     "start": i * 10, "duration": 10}
    for i in range(18)
]
body = {
    "audio": {"mode": "silence", "duration_seconds": 180},
    "video": slots,
}
req = urllib.request.Request(
    "https://api.sume.com/v1/timeline-1.0/plan",
    data=json.dumps(body).encode(),
    headers={
        "Authorization": "Bearer " + os.environ["SUME_API_KEY"],
        "Content-Type": "application/json",
    },
    method="POST",
)
print(urllib.request.urlopen(req).read().decode())