YouTube is currently experimenting with server-side ad injection. This means that the ad is being added directly into the video stream.
This breaks sponsorblock since now all timestamps are offset by the ad times.
For now, I set up the server to detect when someone is submitting from a browser with this happening and rejecting the submission to prevent the database from getting filled with incorrect submissions.
I think the backend could just generate the ad ridden video feed for the specific user. Most probably it would be very resource intensive, but I can only hope so… but then I also don’t know much about HLS and other fragmented streams so it might not be a performance problem at all.
I think the full list of chunks is (currently) known beforehand. That’s how yt-dlp can download on multiple threads, but also how it can show the number of total fragments relatively quickly on the progress bar