.mov vs .mp4 — what is actually different?
Almost nothing, and that is the joke of this whole conversion. Both are containers built on the same ISO Base Media File Format (the "atoms" spec). A modern QuickTime recording is usually H.264 video and AAC audio inside an MOV wrapper — the same codecs MP4 uses. The only thing standing between your file and broad compatibility is the container label. So in most cases this tool is a remux: we rewrite the wrapper, leave the streams untouched, and the result is bit-for-bit identical in quality but plays in places that silently refuse .mov.
When does it have to re-encode?
If your .mov was recorded with ProRes (older Final Cut exports), HEVC at high bit-rates, or an older Apple-only audio codec, a stream-copy to MP4 may produce a file that some players stumble on. We default to copy + AAC re-encode for audio because that catches the most common "won't play in PowerPoint" case. If a video stream needs full transcoding, the conversion will still work, just slower.
Where this kind of conversion comes up
Mostly: someone made a screen recording with QuickTime, they emailed it to a Windows colleague, the colleague's media player will not open it. Or they tried to upload to a CMS that only accepts MP4. Or they dropped it into a Notion page and the embed silently failed. None of those are quality problems — they are container-label problems. Hence this tool.
If you record with QuickTime a lot
QuickTime is fine, but if your end goal is "send a link my coworker can watch", Clipy skips this step entirely. You record, you get a hosted link, the recipient watches in their browser. No file format, no email attachment, no "please use Windows Media Player." Worth a look if MOV-to-MP4 is on your weekly chore list.