SoundCloud serves a ~30s preview clip for tracks gated behind Go+
or login (extremely common for major-label uploads — what's actually
on SoundCloud is bootlegs, fan reuploads, type beats, and these
previews). yt-dlp accepts the preview as the download payload, the
post-download integrity check catches the duration mismatch and
quarantines the file, but the user only sees "all candidates failed"
with no obvious explanation.
Filter at validation time when we know expected_duration: drop
SoundCloud candidates whose duration is below half the expected
length OR within ~5s of the 30s preview boundary, gated on
expected being non-trivially long (>60s) so genuinely short tracks
still pass through.
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