Get YouTube video transcript

Fetch a YouTube video's caption transcript (auto-generated or uploaded) with per-segment timestamps. Returns available:false when the video has no captions in any language. PROBABILISTIC — expect roughly 1 run in 3 to fail and retry rather than treating a failure as a defect: the transcript can only be read by letting YouTube's own player request the caption track (the /api/timedtext endpoint is bound to a token the player mints, so the URLs in the page's own data return an empty response), and when YouTube serves a pre-roll ad the ad occupies the player and the caption request never happens. The script already reloads and retries 3x; more attempts do not help, because an ad that holds the player holds it for every attempt in the run. Retry the call later, or on a viewing context that is not served ads. Separately, on the managed browser YouTube increasingly answers with its "Sign in to confirm you're not a bot" gate and no transcript is returned at all; the paired-extension browser is the more reliable target today.