Container Swap

Sometimes you have a video file that won't play on a certain device — not because there's anything wrong with the video itself, but because the device doesn't recognize the file format. Container swap (also called "remuxing") fixes this by changing the wrapper without touching the actual video.

Containers vs Codecs

Think of a container as a box that holds your video and audio. The video inside might be perfectly compatible with your device, but if the box is the wrong shape, the device refuses to open it.

Codecs (like H.264 or H.265) determine how the video is compressed. Containers (like MP4 or MKV) determine how the video is packaged.

Same video, different container = same quality, different compatibility.

Why Container Swap is Great

Because you're not re-encoding the video, container swaps are:

• Instant — A 10 GB file converts in under a minute
• Lossless — Zero quality loss, it's the exact same video
• Efficient — Output file is the same size as the input

Compare this to transcoding, which can take hours and always involves some quality trade-off.

Common Containers

MP4 is the go-to for compatibility. If a device or app plays video at all, it almost certainly plays MP4.

MKV is more flexible — it can hold multiple audio tracks, multiple subtitle tracks, and preserves more metadata. Great for archiving, but some devices won't play it.

When to Use Container Swap

MKV won't play on your TV? Swap to MP4. The TV probably supports the video inside, just not the MKV container.

Need multiple audio tracks? Keep the MKV. MP4 has limited support for multiple tracks, and a container change in Fetchii keeps only one audio track (see below).

Sharing with someone? MP4 is safest. Everyone can play it.

Preserving subtitles? MKV keeps them embedded. MP4 might lose some subtitle formats.

How to Swap Containers

In your download list, right-click a downloaded video and choose Convert > Change Container. Pick the target format under Container (MP4, MKV, WebM or MOV) and click Start. Done in seconds.

In a hurry? Select the video and choose Convert > Quick Remux in the menu bar, then to MP4, to MKV, to WebM or to MOV — it starts right away, no sheet.

The new file is saved next to the original and appears under its source video in History. Fetchii converts one downloaded video at a time; it can't convert files it didn't download. You can start another conversion while one is running.

Compatibility Notes

Not every codec fits in every container. Here's what works:

If the video's codec doesn't fit the container you picked, the conversion fails and its row shows Conversion failed. You'd need to transcode in that case.

MP4 vs MKV: How to Choose

Choose MP4 when:

• You need maximum compatibility
• Sharing files with others
• Streaming to devices like Roku, Apple TV, or Chromecast
• File will be played on phones or tablets

Choose MKV when:

• Video has multiple audio tracks (like different languages)
• Video has embedded subtitles you want to keep
• You're archiving and want to preserve everything
• Playing on a computer or Plex/Jellyfin server

What Might Not Survive

When converting from MKV to MP4, you might lose:

• Some subtitle formats (especially styled/formatted subtitles)
• Extra audio and subtitle tracks beyond the first (Fetchii's container change keeps one video, one audio and one subtitle track, whichever container you pick)
• Chapter markers (depends on the player)

If these matter to you, keep the MKV version or keep a backup.

Speed Expectations

Container swap is fast — it's mostly just copying data:

Speed depends mainly on your disk. SSDs are much faster than spinning hard drives.

Troubleshooting

"Incompatible codec" error: The video inside uses a codec that doesn't fit in your target container. You'll need to transcode instead.

Audio missing after swap: Some audio codecs (like Opus) don't work in all containers. Try MKV, which supports everything, or convert the audio separately.

Subtitles disappeared: MP4 doesn't support all subtitle formats. Keep MKV if you need subtitles, or extract them as a separate .srt file.