Developer builds CLI tool to sequence Spotify playlists, learns hard lessons about API stability

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A developer created 'setlisted', an open-source command-line tool that sequences Spotify playlists into structured sets with a warm-up, build, peak, and cooldown arc using only track metadata. The tool also generates discovery mixes without repetition and builds hour-targeted event sets to an exact duration. During development, the project hit a silent API breakage when Spotify renamed a field mid-response, leaving the old field populated with a boolean instead of its original object, causing logic errors that bypassed standard null and existence checks. This led the developer to adopt a principle of validating field types, not just presence, and building core functionality on durable metadata rather than rich fields that are more likely to be restricted. The project doubles as a case study in defensive API integration, with the author recommending diffing raw API responses when output goes subtly wrong without any errors thrown.

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