The previous entry ended with the thumbnail backfill merged and covers draining out to every Short on the channel. It drained. Every send came back with YouTube confirming the thumbnail was set. And within minutes each one was gone, replaced by a frame YouTube chose itself.
The one that held
The test that settled it was the owner’s. They took the same cover file the app had sent and uploaded it by hand in YouTube Studio, on a computer, to one of the Shorts. That one stayed.
The owner had said “Let’s try this” to sending covers at publish time. After watching them disappear, the instruction changed: do the research first. The research was short. Google’s issue tracker has an open report, “YouTube v3 API Thumbnail Set on Shorts Not Working”, closed as Won’t Fix, with a staff note that custom thumbnails for Shorts are not fully supported through the API. Comments through this summer still describe the set picture falling back to a random still, and one describes a set after upload clearing a thumbnail that had been placed by hand. YouTube’s own help page says Shorts thumbnails can only be added in Studio on a computer.
So the fix from two days ago had been correct and pointless: it paced the sends perfectly into a door that does not open. Worse, leaving it running could have wiped the covers the owner set by hand.
The reversal
Shorts never reach the thumbnail call now, on any path: not the publish run, not the retries, not the waiting list, not the backlog, not the on-demand command. The backlog that was built one day earlier is gone, along with its tests. A one-time migration marks every Short’s cover as a Studio job.
What the operator sees instead is a card that reads “Cover ready: upload it in YouTube Studio”, with two links: one that opens the video in Studio, one that saves the cover as a file. The on-demand command lists Shorts as Studio uploads and counts them separately from long-form covers still waiting.
Long-form videos are untouched. The API sets their thumbnails and always did; the problem was specific to Shorts.
The shape of the lesson
The cover work now spans four entries: the editor, the covers that never arrived, the fix, and this. The first fix was built from the log alone, which showed only rate-limit errors, and the rate limit was real. The disappearing covers only showed up by looking at the channel afterward. The manager redirected the builder mid-task once the research came in, and the whole change merged the same day with one review round.