fix(yt): pick subtitle matching the main audio language and bump to 1.3.13

Previously the topmost track in captionTracks was always selected, which
often differed from the voice language. Now resolves the caption via
audioTracks[defaultAudioTrackIndex].captionTrackIndices, preferring
manual over auto-generated (asr), with a vssId-based fallback for older
API responses.

Author: ysm-dev

lib/yt/getVideoSubtitle.ts

@@ -2,11 +2,72 @@ import { memoize } from "@fxts/core"
 import { ofetch } from "ofetch"
 import { getVideoInfo } from "@/lib/yt/getVideoInfo"
 
+type CaptionTrackLike = {
+  baseUrl?: string
+  vssId?: string
+  kind?: string
+  languageCode?: string
+}
+
+const isManual = (t: CaptionTrackLike) => t.kind !== "asr"
+
+// YouTube vssId convention:
+//   ".{lang}"  = original manually authored caption (e.g. ".en")
+//   "a.{lang}" = original auto-generated caption    (e.g. "a.en")
+const isOriginalManual = (t: CaptionTrackLike) =>
+  Boolean(t.vssId?.startsWith(".")) && isManual(t)
+const isOriginalAsr = (t: CaptionTrackLike) =>
+  Boolean(t.vssId?.startsWith("a."))
+
 export const getVideoSubtitle = memoize(async (videoId: string) => {
   const { r, pot } = await getVideoInfo(videoId)
 
-  const url =
-    r.captions.playerCaptionsTracklistRenderer.captionTracks.at(0)?.baseUrl
+  const tracklist = r.captions?.playerCaptionsTracklistRenderer
+  const captionTracks: CaptionTrackLike[] = tracklist?.captionTracks ?? []
+
+  if (captionTracks.length === 0) {
+    return null
+  }
+
+  // Pick the caption track that matches the main audio language.
+  // Priority:
+  //   1. Manual caption in default audio's language       (kind !== "asr")
+  //   2. Auto-generated caption in default audio's lang   (kind === "asr")
+  //   3. vssId-based fallback: original manual, then original asr
+  //   4. captionTracks[0] as a last resort
+  //
+  // `captionTracks[0]` alone is unreliable: it's just whatever YouTube lists
+  // first (often a translated track), so it can mismatch the voice.
+  let captionTrack: CaptionTrackLike | undefined
+
+  // Strategy 1: audioTracks -> captionTrackIndices (most reliable on modern API)
+  const audioTracks = tracklist?.audioTracks
+  const defaultAudioTrackIndex = tracklist?.defaultAudioTrackIndex
+  const defaultAudioTrack =
+    typeof defaultAudioTrackIndex === "number"
+      ? audioTracks?.[defaultAudioTrackIndex]
+      : undefined
+
+  const audioCaptionIndices: number[] =
+    defaultAudioTrack?.captionTrackIndices ?? []
+  const audioCaptionCandidates = audioCaptionIndices
+    .map((i: number) => captionTracks[i])
+    .filter((t): t is CaptionTrackLike => Boolean(t))
+
+  if (audioCaptionCandidates.length > 0) {
+    captionTrack =
+      audioCaptionCandidates.find(isManual) ?? audioCaptionCandidates[0]
+  }
+
+  // Strategy 2: vssId-based fallback when audioTracks data is unavailable.
+  if (!captionTrack) {
+    captionTrack =
+      captionTracks.find(isOriginalManual) ??
+      captionTracks.find(isOriginalAsr) ??
+      captionTracks[0]
+  }
+
+  const url = captionTrack?.baseUrl
 
   if (!url) {
     return null

package.json

@@ -1,7 +1,7 @@
 {
   "name": "cpdown",
   "description": "Copy any webpage as clean markdown",
-  "version": "1.3.12",
+  "version": "1.3.13",
   "type": "module",
   "scripts": {
     "dev": "wxt",