Check whether an MP4 is arranged for progressive web playback. Your file stays on this device.
Faststart moves the moov metadata box ahead of the first mdat media-data box. That order lets a conventional web player read essential timing and track metadata without waiting for the end of a large file. It does not change the encoded audio or video streams.
moov appears before the first mdat.moof signals a fragmented MP4 structure.The checker uses the browser File and Blob APIs. It reads only the small header range needed at each top-level offset, then jumps over the declared payload. It never sends the selected file, its name, its box list, its size, or the result to the event endpoint.
The first movie metadata box is available before the first media-data box. This is the usual arrangement for progressive playback of a non-fragmented MP4. A pass is a byte-order observation, not a codec or playback guarantee. Test the actual file in every player you support.
The first media-data box comes before movie metadata. A player may need to fetch much more of the file before it knows tracks, durations, and sample tables. The displayed ffmpeg command creates a new output and asks ffmpeg to relocate metadata without re-encoding streams. Keep the source until you verify the output.
A moof box means the file uses movie fragments. Fragmented MP4 has different delivery patterns, so classic “moov before mdat” advice is incomplete. This checker reports the structure but does not certify DASH, HLS, MSE, CMAF, or streaming conformance.
The checker uses this state when a required pair is absent, a size is unsafe, a header is truncated, a box reaches beyond the file, the deadline expires, or you cancel. It deliberately avoids guessing. A file may still be valid in a format or edge case that this small top-level parser does not understand.
No. The checker only reads byte ranges. The ffmpeg command is shown as text for you to run separately, and it writes to a new output path.
Top-level boxes declare their sizes. The parser can read a header, validate the declared range, and jump to the next header. That keeps memory use small even when the media payload is large.
The exact search demand and business value have not been validated. The tool is usable, but it is not a standards validator, a playback laboratory, or a promise that a given file will work everywhere.
Only four coarse events may be counted: tool opened, file selected, analysis completed, and command copied. The endpoint rejects extra fields and separates production from preview traffic. No file-derived value is included.