fix: support separate yt-dlp video and audio streams

src/app/core/download/downloader.py

@@ -42,9 +42,9 @@ class Downloader:
 
         cap = parse_height_cap(max_download_quality)
         if cap is not None:
-            #if the requested cap isn't available, we still download the best mp4.
-            return f"best[ext=mp4][acodec!=none][vcodec!=none][height<={cap}]/best[ext=mp4][height<={cap}]/best[ext=mp4]"
-        return "best[ext=mp4][acodec!=none][vcodec!=none]/best[ext=mp4]"
+            # Prefer separate MP4 video and audio streams; yt-dlp merges them with ffmpeg.
+            return f"bestvideo[ext=mp4][height<={cap}]+bestaudio[ext=m4a]/bestvideo[ext=mp4][height<={cap}]+bestaudio/bestvideo[ext=mp4][height<={cap}]/best[ext=mp4]"
+        return "bestvideo[ext=mp4]+bestaudio[ext=m4a]/bestvideo[ext=mp4]+bestaudio/bestvideo[ext=mp4]/best[ext=mp4]"
 
     async def handle_job(self, job: DownloadJob):
         try:
@@ -97,10 +97,8 @@ class Downloader:
 
             fmt = self.build_format(getattr(job, "max_download_quality", None))
 
-            # All modes download a single muxed mp4 when possible.
-            # This avoids any ffmpeg-driven merging during the download step, satisfying:
-            #   - video: "original file, no processing"
-            #   - audio/both: extraction is done separately after download
+            # Prefer MP4 video with a separate audio stream; yt-dlp merges them when needed.
+            # Audio extraction for audio/both modes happens separately after download.
             ydl_opts = {
                 "format": fmt,
                 "outtmpl": outtmpl,

tests/test_download_quality_format.py

@@ -6,10 +6,12 @@ from app.core.download.downloader import Downloader
 def test_build_format_defaults_to_best_mp4():
     fmt = Downloader.build_format(None)
     assert "height<=1080" in fmt
+    assert "bestvideo[ext=mp4][height<=1080]+bestaudio" in fmt
     assert fmt.endswith("/best[ext=mp4]")
 
 
 def test_build_format_applies_height_cap():
     fmt = Downloader.build_format("720p")
+    assert "bestvideo[ext=mp4]" in fmt
     assert "height<=720" in fmt