[Nxssie]

Summary

• A YouTube radio ("Mix") link (watch?v=…&list=RD…) now queues the radio's first 25 songs as a playlist. Before, the v= parameter made it play only the seed video.
• /play no longer runs two full yt-dlp extractions back to back. Resolution extracts once (yt-dlp -J), caches the info JSON in memory, and the stream starts from it through --load-info-json -. Measured first audio byte: 0.67 s cached vs 4.99 s fresh.

Changes

File	Change
packages/server/src/lib/youtube.ts	isRadioUrl and RADIO_TRACK_LIMIT = 25
packages/server/src/lib/infoCache.ts	Bounded TTL cache of yt-dlp info JSON (10 min, 20 entries), consumed once per stream
packages/server/src/index.ts	Radios take the playlist path with --playlist-end 25; getVideoInfo extracts with -J and caches; createAudioStream streams from the cache and falls back to a fresh extraction
README.md	Lists radio links among the supported inputs
odd/tasks/youtube-radio-fast-start.md	Feature document with specs, measurements and evidence

Design notes

• Why 25 songs: a radio is endless. Unlimited, yt-dlp returned 360 songs in 15.4 s; capped at 25 it resolves in ~1.3–1.8 s.
• Why the cache is safe: the 10-minute TTL is far below the ~6 h lifetime of YouTube's signed media URLs, so a cached entry never streams from an expired URL. A missing, expired or already-used entry falls back to today's extraction.
• Memory bound: info JSON measured 86–660 KB per video, so 20 entries cap the cache at ~13 MB.
• watch?v=X&list=PL… still plays the single video; playlist?list=RD… is rejected by YouTube ("This playlist type is unviewable"), so only the watch form counts as a radio.

Test plan

• bun test: 84 pass, 0 fail (new isRadioUrl and infoCache cases)
• bun build src/index.ts --target bun: exit 0
• Live yt-dlp: radio with --playlist-end 25 → 25 entries in 1.8 s; first audio byte 0.67 s cached vs 4.99 s fresh
• Live Discord smoke test (no interaction harness in this repo): /play with a single YouTube link, /play with a radio link

Known limitations

• The first song of a YouTube playlist or radio still extracts while starting: those entries arrivewith a title, so they skip the cached resolution. Prefetching the next song during playback would
  cover them and the gap between songs.
• A generic HLS/DASH stream with no audio-only format now shows its URL as the title; such streams
  already failed to play with the same format selector.

Stacked on fix/queue-reset-on-leave; merge that one first.