[ffmpeg] mp4: track 1: codec frame size is not set

mpv Information

mpv v0.41.0-690-g1ac687d79 Copyright © 2000-2026 mpv/MPlayer/mplayer2 projects
 built on May 26 2026 13:45:37
libplacebo version: v7.364.0 (v7.360.0-57-g051cc36-dirty)
FFmpeg version: N-124653-g0ac3b00a1
FFmpeg library versions:
   libavcodec      62.34.102
   libavdevice     62.4.100
   libavfilter     11.17.100
   libavformat     62.18.100
   libavutil       60.31.100
   libswresample   6.4.100
   libswscale      9.7.100

Other Information

- Windows version: Windows 10 IoT Entreprise LTSC 21H2
- GPU model, driver and version: AMD Radeon RX 9070 XT - 32.0.31007.5012 - 26.5.2
- Source of mpv: zhongfly/mpv-winbuild
- Latest known working version: Idk, issue still occurs with oldest build available : mpv-x86_64-v3-20260428-git-a42b1b9103
- Issue started after the following happened: N/A

Reproduction Steps

1. Open an Youtube video with lastest zhongfly/mpv-winbuild
2. Dump-cache

Also, same happen with Kick live stream but with this error (red) :

[ffmpeg] mp4: sample rate not set
[recorder] Writing header failed.
Cache dumping stopped due to error.

I don't know if it's the same issue but the result is the same (1MiB outfile not playable)

FYI, same happen with build shinchiro/mpv-winbuild-cmake since build mpv-x86_64-v3-20260517-git-059bc70 : https://github.com/shinchiro/mpv-winbuild-cmake/issues/828

Expected Behavior

Example from shinchiro/mpv-winbuild-cmake mpv-x86_64-v3-20260416-git-d20d108 :
mpv https://youtu.be/hrk8ZhjoB-Q

[auto_profiles] Applying auto profile: youtube
[osd/libass] fontselect: Using default font family: (mpv-osd-symbols, 400, 0) -> ArialMT, 0, ArialMT
Not seekable, but enabling seeking on user request.
● Video --vid=1 (h264 1920x1080 59.9401 fps 3298 kbps) [default]
● Audio --aid=1 --alang=eng (aac 2ch 44100 Hz 128 kbps) [default]
File tags:
Date: 20191121
Uploader: Danucd
Channel_URL: https://www.youtube.com/channel/UCsIiIEGRoBILi8LSkWNPqXA
AO: [wasapi] 192000Hz stereo 2ch float
VO: [gpu-next] 1920x1080 yuv420p
[modernz] URL detected.
[modernz] Approximating file size...
[modernz] Executing: yt-dlp -f (bv*[vcodec~='^(av(0|c)1)']+ba[acodec^=mp4a])/b --no-download -O %(filesize,filesize_approx)s https://youtu.be/hrk8ZhjoB-Q
[osd/libass] fontselect: Using default font family: (mpv-osd-symbols, 400, 0) -> ArialMT, 0, ArialMT
[modernz] Download size: 31.91 MiB
Cache dumping started.
[ffmpeg] mp4: track 0: codec frame size is not set
[recorder] This is an experimental feature. Output files might be broken or not play correctly with various players (including mpv itself).
[recorder] Discontinuity at timestamp 0.000000.
Cache dumping successfully ended.
AV: 00:00:11 / 00:01:16 (16%) A-V: 0.000 Cache: 64s/30MB
Exiting... (Quit)

Outfile 32MiB playable
warning [ffmpeg] mp4: track 0: codec frame size is not set

Actual Behavior

mpv https://youtu.be/hrk8ZhjoB-Q

[auto_profiles] Applying auto profile: youtube
[osd/libass] fontselect: Using default font family: (mpv-osd-symbols, 400, 0) -> ArialMT, 0, ArialMT
Not seekable, but enabling seeking on user request.
● --edition=0 '299 - 1920x1080 (1080p60) (h264 60 fps 3353 kbps)'
○ --edition=1 '301 - 1920x1080 muxed-14 (h264 60 fps) (aac)'
○ --edition=2 '298 - 1280x720 (720p60) (h264 60 fps 1605 kbps)'
○ --edition=3 '300 - 1280x720 muxed-12 (h264 60 fps) (aac)'
○ --edition=4 '18 - 640x360 (360p) muxed-11 (h264 30 fps) (aac 44100 Hz)'
○ --edition=5 '134 - 640x360 (360p) (h264 30 fps 281 kbps)'
○ --edition=6 '93 - 640x360 muxed-9 (h264 30 fps) (aac)'
○ --edition=7 '160 - 256x144 (144p) (h264 30 fps 111 kbps)'
○ --edition=8 '91 - 256x144 muxed-7 (h264 30 fps) (aac)'
● Video --vid=1 '299 - 1920x1080 (1080p60)' (h264 1920x1080 60 fps 3353 kbps) [default]
○ Audio --aid=6 '251 - audio only (medium)' (opus 48000 Hz 130 kbps)
● Audio --aid=7 '140 - audio only (medium)' (aac 44100 Hz 129 kbps) [default]
○ Audio --aid=8 '139 - audio only (low)' (aac 22050 Hz 48 kbps)
File tags:
Date: 20191121
Title: NEW SONG BACK2BACK | DANUCD SKINS | Danucd
Uploader: Danucd
Channel_URL: https://www.youtube.com/channel/UCsIiIEGRoBILi8LSkWNPqXA
[modernz] URL detected.
[modernz] Approximating file size...
[modernz] Executing: yt-dlp -f (bv*[vcodec~='^(av(0|c)1)']+ba[acodec^=mp4a])/b --no-download -O %(filesize,filesize_approx)s https://youtu.be/hrk8ZhjoB-Q
AO: [wasapi] 48000Hz stereo 2ch float
VO: [gpu-next] 1920x1080 yuv420p
[osd/libass] fontselect: Using default font family: (mpv-osd-symbols, 400, 0) -> ArialMT, 0, ArialMT
Cache dumping started.
[ffmpeg] mp4: track 1: codec frame size is not set
[recorder] This is an experimental feature. Output files might be broken or not play correctly with various players (including mpv itself).
[recorder] Discontinuity at timestamp 0.000000.
Cache dumping successfully ended.
[modernz] Download size: 31.91 MiB
Exiting... (Quit)

Output file 1MiB and not playable
warning [ffmpeg] mp4: track 1: codec frame size is not set

Log File

output.txt

Sample Files

No response

I carefully read all instruction and confirm that I did the following:

• I tested and confirmed that the issue exists with the latest release version or newer.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Start by reproducing cache dumping with the YouTube URL and reviewing the attached output.txt, focusing on the recorder entry point and the FFmpeg MP4 warnings for codec frame size and sample rate. Done means cache dumping produces a playable MP4 without these errors for both video and Kick stream cases.