The duration your video API accepts is not the duration it renders

A sequence I cut to a music bed was three frames out at the first transition, nine at the second, and by the sixth segment nothing lined up with anything. I had asked every generation for ten seconds. Every generation had returned a file that was not ten seconds. Nothing in the API said so. The request took duration: 10 , returned 200 , and produced an MP4 whose container duration was 8.708 . No warning field, no note in the response body, and — the part that actually cost me the afternoon — no mention of it on the docs page I had read three times. This is a general property of latent video models rather than a bug in one provider, and once you know the shape of it you can handle it in about twenty lines. Here is the shape. Seconds are the wrong unit A video diffusion model does not work on frames. It works on a compressed latent tensor, and the compression is temporal as well as spatial: a causal 3D autoencoder folds a run of input frames into a single latent frame. Because the encoder is causal, the first frame is kept whole and everything after it is compressed in groups. With a temporal stride of s , a clip of F frames becomes latent_frames = ( F - 1 ) / s + 1 which only divides evenly when F ≡ 1 (mod s) . Frame counts that miss that condition get padded or truncated, so implementations pick the nearest legal count and render that instead. Stack a second constraint on top — many of these models generate in fixed blocks of latent frames rather than one at a time — and the set of renderable lengths collapses into a short arithmetic progression: F = head + block · n n ∈ ℕ Every legal duration is one of those F values divided by the frame rate. Nothing between them is reachable. duration: 10 is not a request. It is a hint that gets snapped to a grid you were never shown. What the snapping does to users Three separate problems, and only the first is obvious. The output is not the length you promised. Your UI said 10s, the file is 8.708s, so your UI lied. Not by much,