Flux 3 Image accepts a structured JSON array of scene elements where each element has a unique identifier, a natural-language description, and coordinates mapping to a normalized 0-to-1000 grid.

From the articleBlack Forest Labs launches Flux 3 Image with multi-step editing that leaves the rest of your picture aloneAlso in Black Forest Labs Launches Flux 3 Image: JSON Bounding Boxes Lock Unchanged Pixels Numerically