Three image edits at once or one by one? Cost and drift on Sume

Black Forest Labs' FLUX 3 Image docs describe element kinds (new, move, anchor) with source and target boxes, so several elements change in one edit, and they warn that pixels outside the boxes only usually stay the same: shadows, reflections or nearby lighting may still change (Black Forest Labs docs, read 2026-10-04). The Sume Image API has no box field. The buyer question is the same, though: should three changes go in one request or three?

The cost side is simple

Each request is billed per image, so chaining multiplies cost. At Nano Banana 2's 1K rate of about $0.10 per image (catalog list $0.08 × 1.25), one call that asks for three changes is about $0.10, and three chained calls are about $0.30. A failed generation is not billed, so a retry costs only when it succeeds (Sume Image API).

The drift side needs a test

Neither approach is better by rule. A single pass leaves fewer chances to shift lighting; a chain gives each change its own full attention. Test it on ten of your own photos, count how many results kept all three changes, and diff the untouched areas. If you chain, save PNG between passes so JPEG artifacts do not stack.

A single-call prompt

Number each change and end with a keep-the-rest sentence. If a change goes missing, re-run only that change on the result.

Make exactly three changes to the reference photo:
1. Change the left cushion to mustard yellow.
2. Remove the cable on the floor.
3. Replace the picture on the wall with a plain white frame.
Keep everything else identical: layout, lighting, shadows, perspective.
Do not add any text or markings.