How to Combine Multiple AI-Generated Video Clips Without It Falling Apart

If you’re generating video with AI tools clip-by-clip and then trying to join them into one longer piece, there are two very different ways to combine them — and the wrong one has burned us in a real project before. Here’s a working example, generated fresh for this piece, of the safer approach.

Two clips, generated independently

These two clips share nothing — different subjects, different generation calls, submitted separately. That’s deliberate: it’s exactly the situation that causes real problems when clips get combined carelessly.

The pitfall: stream-copying clips from different generation batches

The fast way to join two video files with ffmpeg is -c copy — it doesn’t re-encode anything, just repackages the existing streams, so it’s close to instant. We’ve hit a real, previously-documented failure with exactly this approach: concatenating clips generated in separate sessions produced non-monotonic DTS (decode timestamp) errors — the container’s internal timing got confused about what order frames belonged in, because the two source files didn’t share a consistent timestamp baseline. It’s the kind of bug that doesn’t show up in a quick preview and then causes playback glitches or outright failures downstream.

Worth being direct about what happened when we tried to reproduce that specific error for this piece: with the two clips above, a plain -c copy concat completed cleanly, no warning. That’s not a contradiction — it’s a reminder that this failure depends on the two files’ internal timestamp bases actually mismatching, which isn’t guaranteed on every pair of clips. It’s real, it’s happened, and it’s cheap enough to guard against that there’s no good reason to gamble on whether today’s pair of clips will be the ones that trigger it.

The reliable fix: re-encode, don’t stream-copy

Swap -c copy for an actual re-encode when combining clips from separate generation calls:

ffmpeg -f concat -safe 0 -i list.txt -c:v libx264 -crf 18 -preset fast -c:a aac output.mp4

This costs you encode time (seconds, not minutes, for short clips) and normalizes timestamps across every source file as it goes, regardless of whether the source clips happen to line up. Here’s the actual combined result from the two clips above, built this way:

An even safer default: don’t try to make one continuous shot

The most reliable fix found in practice wasn’t a better concat command at all — it was avoiding continuous single-take stitching in favor of a multi-cut montage: several independently-generated shots edited together as visibly separate cuts, the way a real film edits scenes, rather than pretending several AI generations are one unbroken take. Independently generated shots share no frame to drift across and no continuity to maintain between them, which sidesteps the timestamp mismatch risk (and a whole separate class of scene-drift problems) entirely. If what you’re building doesn’t strictly need one continuous take, cut between shots instead of trying to stitch them into one.

The practical takeaway

Never stream-copy (-c copy) when concatenating video clips from separate AI generation calls — always re-encode. And where your project allows it, prefer editing independently-generated shots as a montage over trying to force them into one continuous take; it’s both safer and, in our experience, the better-looking default anyway.

Both source clips above were generated fresh for this piece on fal.ai, the same platform behind our portrait-realism study and our voice-quality piece.