soundtrack_fade_exceeds_output: a bed fade longer than a short clip

soundtrack_fade_exceeds_output appears when soundtrack.fade_out_seconds is larger than audio.duration_seconds. The fade-out of a music bed is also capped at 10 seconds in the schema. For a short clip, choose a fade well below the length of the output; the hint is shorten_soundtrack_fade.

The error is easy to hit when a template sets a fixed fade, such as 8 seconds, and is reused on a 6 second hook clip. YouTube Shorts run from very short up to 3 minutes (YouTube Help, read 2026-10-05), so one template across lengths needs a rule that scales the fade.

Three different fade limits

There are separate fade controls in Timeline 1.0, and each has its own code. Mixing them up is the usual cause of a wrong fix.

A fade that scales with the clip

A simple rule is a fixed share of the output, clamped to the cap. For a 6 second clip a 1 second fade keeps most of the bed audible; for a 180 second Short the cap of 10 seconds is more than enough. This helper returns a value the schema accepts.

def bed_fade(duration_seconds, share=0.1):
    return round(min(10, max(0.5, duration_seconds * share)), 2)

for length in (6, 30, 180):
    print(length, bed_fade(length))

Using the value

Put the result in the render body next to the bed. Keep loop: true if the bed is shorter than the spine, so the fade lands at the end of the output and not at the end of a short file.

{
  "audio": { "mode": "silence", "duration_seconds": 6 },
  "soundtrack": {
    "url": "https://media.sume.com/artifacts/artf_demo/bed.mp3",
    "loop": true,
    "fade_out_seconds": 0.6
  },
  "video": [
    { "source_url": "https://media.sume.com/artifacts/artf_demo/a.mp4", "start": 0, "duration": 6 }
  ]
}

Why the cap exists

The 10 second ceiling and the within-output rule are both validated at admit, before a worker starts, so a failing request costs nothing and returns quickly. That is deliberate: a fade longer than the clip would otherwise render a bed that never reaches full volume, which is a quality bug that is hard to notice by ear. When you batch many clips of different lengths, run /plan on one representative of each length bucket, not on every clip.

When this is not the problem

If your fade is within range and the bed still ends abruptly, the bed may be shorter than the spine without looping. Sume reports a soft warning for that case rather than failing. Warnings appear in the render result, not in /plan, because a plan cannot predict short-source behaviour.