FFmpeg can also remux MKV while creating cues, but it’s less flexible than mkvmerge:
ffmpeg -i input.mkv -c copy -movflags +faststart output.mkv
Note: -movflags +faststart is actually for MP4’s "moov atom," but it works for MKV by shifting the index forward. For MKV-specific cues, prefer mkvmerge.
Some converters (especially command-line FFmpeg with default settings) omit the Cues entirely or place them at the end of the file. This is "streaming-friendly" but terrible for seeking. The player must download the entire file just to read the index at the end. mkv index
Before attempting repairs, diagnose the state of your MKV index. Here are three reliable methods:
Download MediaInfo (GUI or CLI). Open your MKV and look under the "Menu" section. If you see "Seek head" list and "Cues" with a count >0, you have an index. If the "Menu" section is completely absent, you have no index. FFmpeg can also remux MKV while creating cues,
Some files have an index, but it’s "sparse" (e.g., only one cue point every 10 seconds). For video editing or precise seeking (like frame-by-frame analysis), this is insufficient.
The MKV index is a perfect example of invisible infrastructure. When it works, you never think about it. When it breaks, your video becomes a linear, un-skippable relic of the VHS era. Note: -movflags +faststart is actually for MP4’s "moov
Whether you are archiving a media library, streaming over a network, or just trying to skip the intro of your favorite show, respecting and maintaining the index is key. Tools like MKVToolNix and ffmpeg are your best friends for inspecting and repairing this hidden map. Next time a video refuses to seek, you’ll know exactly why: the map is missing.
Some players (e.g., VLC, MPV) implement a fallback: