Mixes are different from playlists in Youtube, with the main difference that they’re tailor-made for every user and will be different for each visitor, based on their previous watch history. This means that Youtube downloading tools can’t process mixes the same way that it does playlists, so the command will fail. The best way is to read the list of videos in the list available on the side and save it to a text file, which will then be passed as parameter to youtube-dl or yt-dlp.

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