The value a request gets when nobody chooses
Every parameter has a value, including the ones a request leaves out. Where a page names that value, a pipeline that forgets a field produces something checkable. Where it does not, the output is whatever the vendor chose and nothing predicts it. As of 2026-09-12.
| What the page says | What an omitted field returns | Risk to a booked placement |
|---|---|---|
| A named value | That value, predictably | Low, once it is read |
| Nothing | Undocumented | Unknown until it is measured |
| That the field is required | An error | Low: the failure is immediate |
| That the model chooses | Something different each time | High: shapes vary across a batch |
Inclusion rule. Behaviours a specification page can document for an unset parameter. Behaviour observed in practice but not stated is not a documented default. Order. Safest behaviour first.
1A rejected request is the cheapest failure
Where a field is required, an omission fails at submission. That costs a retry and a line of code. Where a default fills it silently, the omission costs a batch of usable files in a shape or at a size nobody booked.
The second failure is discovered at upload, after the generation budget is spent and the schedule is committed. Preferring the noisier behaviour is the right instinct in unattended work.
2A model-chosen default is the hardest to plan around
Where a page says the model decides, nothing errors and every piece looks correct on its own. Across a campaign the shapes or sizes vary, and each surface accepts only some of them.
The fix is to set the field on every request without exception, and it has to be deliberate, because nothing about the output looks wrong until it meets a placement specification.
3Defaults are usually the cautious value, and cautious for the wrong surface
Where a default is named at all it tends to be the cheapest tier and the horizontal shape. The first is sensible; the second matches the minority of short-form placements.
So the common case in advertising is the one that has to be asked for. That is worth reading once per vendor and writing into the pipeline, rather than rediscovering it per campaign.
A craft note, not a spec. Nothing here is taken from a platform or a vendor, and nothing here should be quoted as a requirement. The sourced material is on published limits. Related: Reading a minimum, What a scene means.
4Making the ad itself
For ad work around “What a default commits to”, SceneMixer's pages explain how its ad projects are produced:
16:9 landscape (SceneMixer's default) and 9:16 vertical (the usual short-drama frame), chosen per project.
SceneMixer, Seedance 2.0 in a Real Short-Drama Pipeline — “Which aspect ratios does the pipeline generate?” (read 2026-10-13)
Typically 60–120 seconds, always 9:16 vertical, ending on a hook. SceneMixer's episode outlines are generated with that pacing, and setting a project to 9:16 makes every segment vertical (the default is 16:9).
SceneMixer, How to Make ReelShort-Style Dramas with AI — “How long is a ReelShort-style episode?” (read 2026-10-13)