40-clip montage Reel: how Timeline chunks a render past 12 slots

A fast-cut Reel with 40 clips renders in one Timeline 1.0 job, which accepts 1 to 200 video[] slots. With render.strategy left at auto, Sume chunks the render once it passes 12 segments, and single is refused above 12 slots with render_strategy_unsafe. The price does not depend on the slot count: it is $0.10 per output minute, rounded up.

Why slot count matters on a Reel

The Reels guide (read 2026-10-03) puts quick transitions and trends in the 3 to 15 second range, where cuts come fast; and Metricool (read 2026-10-03) reports Reels can now reach 3 minutes. A 90-second montage at one cut per 2.25 seconds is 40 slots, well past the point where one ffmpeg graph is safe.

Strategy values

The render.strategy field is the only knob.

Build the slot list

The slot list below makes 40 equal slots on a 90-second spine. video[0].start must be 0, later starts must increase, and coverage may trail the spine by at most 0.5 seconds. Call the unbilled plan endpoint first to read billable_minutes (2 for 90 seconds, so $0.20).

def slots(urls, total_s):
    step = total_s / len(urls)
    return [
        {
            "source_url": u,
            "start": round(i * step, 3),
            "duration": round(step, 3),
        }
        for i, u in enumerate(urls)
    ]

urls = ["https://media.sume.com/artifacts/artf_demo/c%02d.mp4" % i for i in range(40)]
video = slots(urls, 90)
print(len(video), video[0], video[-1])

Transitions and limits

Transitions go on slots after the first, last at most 1 second and no more than 50 percent of the shorter neighbour. More than 8 adjacent fades fails with too_many_chained_transitions; put a hard cut in between. Warnings such as padded or looped short sources are not failures, but read them in the result.

What Sume does not do

Sume does not choose clip order or pick beat-matched cut points, and chunking is not something you pay extra for or can inspect in the price. Every URL must already be on your media host, so import the 40 clips first.