FLUX 3 reference image aspect ratio: auto follows the first image

In FLUX 3 Image, aspect_ratio defaults to auto: the output follows the first reference image's aspect ratio, or is 1:1 when you send no reference. On Sume the safer habit for edits is to send aspect_ratio: "auto" explicitly, because the docs say omitting the field is not the same as auto.

BFL facts are from its FLUX 3 Image reference; Sume facts from the Image API, read 2026-10-01.

What values does FLUX 3 Image accept?

The enum lists auto plus 15 ratios, from 21:9 to 9:21: 21:9, 2:1, 16:9, 3:2, 7:5, 4:3, 5:4, 1:1, 4:5, 3:4, 5:7, 2:3, 9:16, 1:2 and 9:21. Because the default is auto, the order of your images list matters: the first image decides the shape, even if a later one is the main subject.

How does Sume treat auto?

The Image API docs say to use "auto" for provider choice, and that on edit and image-to-image calls you should prefer aspect_ratio: "auto" to match the reference. They add that omitting the field is not the same as auto. So on Sume the match-the-reference behavior is something you ask for, not something the default gives you.

What should I send on an edit?

For a Sume edit, send your references in input_references and set aspect_ratio to auto. Then read the model's supported_parameters on GET /v1/images/models, since a model only accepts the values its catalog descriptors list. Put the picture whose shape you want to keep first, as a habit that also satisfies FLUX 3's rule if you later move a request between the two.

If you need a fixed output shape instead, name the ratio. The sizes and ratios Sume lists are in image API aspect ratios and custom sizes.

Why does my output come out square?

On FLUX 3 Image, no reference means 1:1 under the default. On Sume, a missing aspect_ratio on an edit can give a different shape than the reference, per the docs' warning. In both cases the fix is to name the ratio, or to send auto with a reference in place.