Crash when recording: Task mux-video failed: Error queueing video frame

[dylanjha]

Description

After about 1 - 15 minutes, it randomly crashes with this error:

Task mux-video failed: Error queueing video frame:
send_video_frame/WriterFailed/The operation could not be completed

This happens reliably.

Reproduction

Recording with:

• External Dell Display
• Instant mode
• HD Pro Webcap
• USB Mic
• No System Audio

Additional Context

• Cap version: 0.4.3
• Operating system, version: MacOS Tahoe 26.2

[linear]

CAP-615

[dangngocbinh]

same with me :(

[JoshRozin]

Arguably the most annoying bug ever. Lost several hours due to this bug.

[gthay]

I was experiencing the same issue:

Task mux-video failed: Error queueing video frame:
Failed to encode audio frame: WriterFailed
The operation could not be completed

After quite a bit of testing, I was able to narrow it down to a combination of display scaling and audio input device.

---

My setup

• MacBook Air M2 (8GB)
• External Dell monitor
• BetterDisplay with custom scaled resolution
• Instant Mode enabled
• 1080p recording resolution
• 3.5mm Apple EarPods (TRRS) with inline microphone

---

What I found

1. Using a custom scaled resolution (e.g. 2038×1152 via BetterDisplay) caused frequent WriterFailed crashes.

2. Switching the recording resolution to a standard 1080p significantly improved stability and fixed the initial video encode failures.

3. However, even at 1080p, crashes still occurred when selecting the TRRS EarPods microphone (Apple wired headphones) as the audio input device.

4. When switching the input device to MacBook Air Microphone (built-in mic) and keeping the headphones only as output, the issue stopped completely.

---

Conclusion

The issue appears to be triggered by a combination of:

• Non-standard scaled display resolutions (BetterDisplay)
• Certain audio input devices (specifically TRRS analog combo-jack microphones)

My suspicion is that this leads to:

• Timestamp drift or buffer instability from the external mic
• AVAssetWriter entering a failed state due to audio stream timing mismatch

The built-in microphone works reliably, and I haven’t experienced any crashes since switching.

Hope this helps narrow it down.

[chindris-mihai-alexandru]

I recently encountered the "WriterFailed" error too, and while investigating, I found another issue that makes it worse. After dismissing the error dialog, the app appears frozen. I think that the root cause is a deadlock in the error handler that prevents the app from starting a new recording after an error occurs. I've submitted a fix that restructures the error handler to release the lock before showing the error message, allowing users to immediately retry after an error.

[judekim0507]

Encountered the same error the moment I started recording camera only using the studio mode. FaceTime Camera and mic, no BetterDisplay or weird scaling.

[gethubb]

Encountered the same error the moment I started recording camera only using the studio mode. FaceTime Camera and mic, no BetterDisplay or weird scaling.

Exactly the same here in studio mode with the same setup. Other modes, besides camera only, work.

Thank you!

[asolanki]

Still a problem.

[dangngocbinh]

It still crashes after a while, which wastes a lot of time and affects my work quite a bit. I hope the team can fix this issue soon.

[richiemcilroy]

Hey everyone! This is really frustrating :( but we're dedicating all resources to this as a priority. We're hoping it should be fixed in 0.4.83, but if anyone can replicate it and send logs over, I'll send you $20. If you can fix it and send a PR, I'll send you $200!

[dylanjha]

@richiemcilroy thank you for the update! I was still seeing this last week. I'll try again and try to pull logs next time.

FWIW, the setup I was seeing this consistently:

• DELL external monitor
• External USB-C microphone
• Recording using "Area" on the external monitor
• Instant mode

Thank you!

[gethubb]

Hey everyone! This is really frustrating :( but we're dedicating all resources to this as a priority. We're hoping it should be fixed in 0.4.83, but if anyone can replicate it and send logs over, I'll send you $20. If you can fix it and send a PR, I'll send you $200!

Hope I can help a bit on replication--the bug seems to have shifted in a positive direction!

--

I uninstalled Cap and then reinstalled it using the DMG. It has not crashed on me, although the preview froze at one point until I toggled it. The bigger problem is that camera only mode results in a video file with no audio. I compared by testing area selection recording with camera simultaneously recording, outputting to the bottom right corner; audio worked there. Logs submitted.

In a little more detail, now-

Reproduce:

1. Uninstall older Cap
2. Download DMG, install, verify permissions
3. Make a recording while speaking to camera in camera-only mode in Studio Mode using stock MacBook Pro mic
4. Export file; no sound plays
5. Make three more similar recordings; on one test, tried an external TRRS EarPods mic
6. Export and play back each recording to find each of three new files similarly plays no sound
7. Switch to area selection mode (still in Studio Mode)
8. Make recording using area selection with camera enabled and export to find microphone audio was properly recorded
9. Switch back to camera-only mode in Studio Mode
10. Make final recording speaking to camera, but using camera-only mode in Studio Mode, and after exporting, find file plays no audio
11. Upload logs (2026-04-10 4:46pm Pacific) [1]
12. Screenshot working-audio area+camera recording vs. no-audio camera-only recording files [2]

Note: At some point during step 5, note preview window froze (showing my still face); toggled preview window to quickly restore live feed.

Really hope you can nail something that’s been bugging you!

Screenshots:

[1] Upload Logs

[2] Compare Files

[richiemcilroy]

We fixed the instant mode muxer along with mic recovery handling, which addressed this WriterFailed crash (a7b2f14). This should be in the latest release, please reopen if it still crashes for you.