# if format.get('preference') is None and format.get('ext') in ('f4f', 'f4m'): # Not supported?
# format['preference'] = -1000
+ if format.get('preference') is None and format.get('ext') == 'flv' and re.match('[hx]265|he?vc?', format.get('vcodec') or ''):
+ # HEVC-over-FLV is out-of-spec by FLV's original spec
+ # ref. https://trac.ffmpeg.org/ticket/6389
+ # ref. https://github.com/yt-dlp/yt-dlp/pull/5821
+ format['preference'] = -100
+
# Determine missing bitrates
if format.get('tbr') is None:
if format.get('vbr') is not None and format.get('abr') is not None: