sample_frames_from_url now returns (frames, reason) — reason carries the
SPECIFIC cause on failure (ffmpeg's stderr tail, e.g. "moov atom not found",
or the timeout) instead of only logging it agent-side. The worker folds it
into the failure it reports, so curator's GpuJob.error reads e.g.
no frames sampled from video — ffmpeg exit 183: moov atom not found ...
instead of the bare "(unprocessable)". The errored-jobs list becomes
self-describing: after a retry sweep, surviving errors name their real
defect without needing the agent log. Return-value plumbing (not shared
state) so concurrent downloaders stay isolated. Agent VERSION → 2026-07-02.2.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
bvandeusen
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