libmpv ignores `--o` property for encoding

mpv Information

[   0.007][v][cplayer] mpv v0.40.0-133-g47bc7b2a0 Copyright © 2000-2025 mpv/MPlayer/mplayer2 projects
[   0.007][v][cplayer]  built on Jun 11 2025 00:07:46
[   0.007][v][cplayer] libplacebo version: v7.351.0 (v7.351.0-dirty)
[   0.007][v][cplayer] FFmpeg version: N-119877-gf019dd69f
[   0.007][v][cplayer] FFmpeg library versions:
[   0.007][v][cplayer]    libavcodec      62.3.101
[   0.007][v][cplayer]    libavdevice     62.0.100
[   0.007][v][cplayer]    libavfilter     11.0.100
[   0.007][v][cplayer]    libavformat     62.1.100
[   0.007][v][cplayer]    libavutil       60.3.100
[   0.007][v][cplayer]    libswresample   6.0.100
[   0.007][v][cplayer]    libswscale      9.0.100

Important Information

- Platform version: Windows
- Source of mpv: github releases

Reproduction Steps

• Initialize libmpv in your favorite language
• set the properties o, vo, ao, ovc, oac, of,
• add video filter realtime
• start playing video file with loadfile command

Values used as per the output log:

[   0.007][v][cplayer] Set property: log-file="D:\\log.txt" -> 1
[   0.007][v][cplayer] Set property: o="rtmp://127.0.0.1:1935/live/abc123" -> 1
[   0.007][v][cplayer] Set property: vo="null" -> 1
[   0.007][v][cplayer] Set property: ao="null" -> 1
[   0.007][v][cplayer] Set property: ovc="libx264" -> 1
[   0.007][v][cplayer] Set property: oac="aac" -> 1
[   0.007][v][cplayer] Set property: of="flv" -> 1
[   0.014][d][cplayer] Run command: vf, flags=64, args=[operation="add", value="realtime"]
[   0.015][d][cplayer] Run command: loadfile, flags=64, args=[url="D:\\test.mp4", flags="replace", index="-1", options=""]

The video does "play" as I get the event for the file ending after the same duration as the video should take.
But no output stream or file is produced.

Doing the same thing with mpv over the commandline works!
mpv -o=rtmp://192.168.0.16:1935/live/abc123 --of=flv --ovc=libx264 --oac=aac -vo=null -ao=null --vf-add=realtime "D:\test.mp4"

Expected Behavior

I expect the o property for the encoder output to work, or at the very least to get an error if this is not supported.

Actual Behavior

The o property gets silently ignored.

Log File

log.txt

Sample Files

No response

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

• I tested with the latest mpv version to validate that the issue is not already fixed.
• 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 with the libmpv property-handling path for the encoding output property o, then compare it with the command-line option path shown in the report. Use the supplied Windows reproduction, log file, and loadfile sequence to trace why no output is produced. Done means libmpv honors the output destination like the command line, or reports an explicit unsupported-property error.