How to change recording format with OBS Websocket

[Lexian-droid]

Hello, I hope this is the right place to ask — I’m currently stuck on something and could use some help.

I’m building software that uses OBS through its WebSocket server (with the obsws-python library) to automatically record videos. Everything works fine in terms of recording — the videos are captured from start to finish exactly as intended.

The issue comes afterward: my app uses FFmpeg to re-encode and trim the recordings, but I’ve noticed that FFmpeg freezes whenever it tries to process .mkv files.

Because of that, I’d like to have OBS record directly in MP4 format instead of MKV.

Here’s the relevant part of my current code (showing where I attempt to change the recording format):

    def prep(self, output: str, width: int, height: int):
        try:
            self.cl.callback.register(self.on_record_state_changed)
            self.ws.create_profile(name=f"{helpers.get_config('APP_NAME')} - Profile")
            self.ws.create_scene(name=f"{helpers.get_config('APP_NAME')} - Scene")
            if self.ws.get_studio_mode_enabled().studio_mode_enabled:
                self.ws.set_current_preview_scene(name=f"{helpers.get_config('APP_NAME')} - Scene")
            self.ws.set_current_program_scene(name=f"{helpers.get_config('APP_NAME')} - Scene")
            self.ws.set_video_settings(
                base_width=width,
                base_height=height,
                out_width=width,
                out_height=height,
                denominator=1,
                numerator=helpers.get_config("OBS_FPS")
            )
            self.ws.set_input_mute(name="Desktop Audio", muted=True)
            self.ws.set_input_mute(name="Mic/Aux", muted=True)
            self.ws.create_input(
                sceneName=f"{helpers.get_config('APP_NAME')} - Scene",
                inputName=f"{helpers.get_config('APP_NAME')} - Capture",
                inputKind="window_capture",
                inputSettings={
                    "window": "GoExport Viewer:Chrome_WidgetWin_1:chrome.exe",
                    "cursor": False,
                    "capture_audio": True,
                    "client_area": True
                },
                sceneItemEnabled=True
            )
            self.ws.set_profile_parameter(category="Output", name="RecFormat2", value="mp4")
            helpers.wait(4, "Waiting for OBS to set up the scene and sources...")
            self.prepared = True
        except Exception as e:
            logger.error(f"Failed to prepare OBS: {e}")
            self.prepared = False

As you can see at the bottom, I tried calling:

self.ws.set_profile_parameter(category="Output", name="RecFormat2", value="mp4")

…but OBS still saves the recordings as .mkv.

My question is:
👉 How do I properly configure OBS (via WebSocket and obsws-python) to record in MP4 format instead of MKV?

Any guidance would be really appreciated — thanks in advance!

[tatcheff03]

Hi the issue is that set_profile_parameter doesn’t actually change the recording format in OBS, so it keeps saving as .mkv.

The correct way is to use SetRecordSettings, for example:

await ws.set_record_settings({
"rec_format": "mp4"
})

(or the sync version with obswebsocket.requests.SetRecordSettings).

⚠️ Just note: OBS defaults to MKV because it’s safer — if OBS crashes, MKV files can be recovered, while MP4 files may be lost completely. A common solution is to record in MKV and let OBS or FFmpeg automatically remux to MP4 after recording.