OBS crashed — and now the MP4 won't open

It's a common and painful sequence: you finish a long recording, OBS crashes or the machine reboots, and the resulting .mp4 is unplayable — sometimes it's the right size, sometimes it's 0 bytes, and sometimes it plays as a black screen with the wrong duration. This page explains what happened, how to recover the recording you already have, and the one setting change that stops it happening again.

Why OBS MP4 recordings break on a crash

An MP4 file stores its video in an mdat block and its index in a moov atom. OBS writes the moov when the recording is finalised — that is, when you press "Stop Recording" and OBS exits cleanly.

If OBS crashes, the encoder is killed, or the machine loses power before that point, the index is never written. The recorded video data is sitting in the file, but nothing can navigate it. That is exactly the missing moov atom problem.

Why MKV recordings usually survive a crash

Matroska (.mkv) is a streaming-oriented container. It writes its structure incrementally as the recording progresses, so a crash leaves a file that is still readable up to the last written moment. That is why OBS's own documentation has long recommended recording to MKV (or FLV) and remuxing to MP4 afterwards, rather than recording straight to MP4.

Recovering the MP4 you already have

Before trying anything, check which of these three situations you're in — they have different answers:

1. The file is truly 0 bytes. Nothing was written. No container repair can recover this; the media never reached disk.
2. The file has real size but won't open. This is the recoverable case. The media data is there and only the index is missing.
3. The file plays but is truncated or shows a black frame. Often a partially written index — also worth attempting.

Step by step for case 2 and 3

• Copy the file first. Never work directly on the only copy, and stop writing to that drive or card.
• Record a short reference clip in OBS with the exact same settings — same resolution, frame rate, encoder, and bitrate. The reference gives the repair tool a healthy index to use as a template. Details: how to prepare a reference clip.
• Run the repair locally in the FixFile repair tool. Add the damaged recording, add the reference clip, and repair. Your file is processed in your browser and never uploaded — useful if the recording contains anything private.
• Check the free preview. If the footage appears, pay the one-time $4.99 to download the full HD file. If it doesn't, you pay nothing.

Prevention: the setting that actually fixes this

This is the part worth doing today, before your next recording:

• Record to MKV, then remux. In OBS, set Settings → Output → Recording → Recording Format to Matroska (.mkv). After recording, use File → Remux Recordings to convert the MKV to MP4 losslessly — it takes seconds and does not re-encode.
• Check your OBS version's MP4 options. Newer OBS releases have added MP4 recording modes designed to leave a usable file after an interruption. Check your version's output settings before assuming plain MP4 is your only option.
• Keep the machine stable. Long recordings plus a crash-prone encoder setting are the main cause. Lowering encoder load (hardware encoder, or a lower preset) reduces the risk.
• Stop recordings cleanly. Where possible, stop the recording before shutting anything down.

What this tool does and does not do

• Does: rebuild MP4 and MOV containers whose index is missing or truncated.
• Does not: recover a genuinely 0-byte file, recover media overwritten by later recordings, or repair non-MP4/MOV containers. A crashed .mkv is best handled by OBS's own remux feature, not by this tool.

FAQ