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Aspect ratios

FLUX 3 Image takes aspect_ratio as one of 15 fixed ratios, from 21:9 to 9:21, or auto: auto follows the first image in images. With no references it produces 1:1, so set the ratio yourself for any text-to-image request that is not square. See Aspect ratio for the request details. Choose the frame for the subject and where the image will be used: The ratio and the prompt should agree. If the frame is ultra-wide, say so in the prompt and give the model something to put across the width:
A vintage car with its headlights on, driving a coastal road between fog-covered cliffs and a turquoise sea at dawn.

FLUX 3 Image, aspect_ratio 21:9, resolution 4k

“Cliffs on one side, turquoise sea on the other” fills the width that 21:9 creates. The same prompt in a 9:16 frame would have to squeeze both sides into a narrow column.
For edits, auto keeps the input’s frame, which is usually what you want. Set a ratio explicitly when the edit should change the frame, for example to turn a square product shot into a 16:9 banner, and describe what fills the new space. For layout prompts, set aspect_ratio to the shape you drew the boxes for: the box grid stretches with the frame.

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Resolution

Choose an output size with resolution. Check output_mp in the submit response for the exact size in megapixels. Iterate at a low resolution, then use 2k or 4k when you need more detail. Changing the resolution makes a new generation, so the composition can differ from the draft. See Resolution.

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Working without negative prompts

FLUX 3 Image has no negative prompt field. If a negative instruction gives an unwanted result, describe the intended scene directly: “an empty promenade” instead of “no crowds”.

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The replacement strategy

When you catch yourself writing a negative phrase:
  1. Identify the unwanted element: “no crowds”
  2. Ask what would fill that space: “What would I see there?”
  3. Describe that instead: “an empty promenade” or “a single figure on the pier”
Common replacements:

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Practical examples

  • Instead of: “a street with no cars” Write: “a quiet pedestrian walkway with cobblestones”
  • Instead of: “a landscape without buildings” Write: “untouched wilderness with open natural terrain”
  • Instead of: “a room with no furniture” Write: “a spacious empty room with polished wooden floors”
  • Instead of: “a person without a hat” Write: “a person with loose hair falling to the shoulders”
  • Instead of: “a portrait with no glasses” Write: “a portrait showing clear, unobstructed eyes”
  • Instead of: “not dark or scary” Write: “a warm, welcoming atmosphere with soft golden lighting”
  • Instead of: “not too realistic” Write: “stylized illustration with simplified forms and bold color blocks”
  • Instead of: “portrait with no background distractions” Write: “portrait with a smooth gradient background from deep blue to black”

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The beach progression

Positive framing plus specific detail moves the image toward what you want. Each step adds detail to the one before:

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When positive alternatives don’t work

If an unwanted element keeps appearing:
  1. Be more specific about what you do want in that space.
  2. Describe it early: put the replacement near the start of the sentence that sets the scene, not in a trailing clause.
  3. Add detail to the positive alternative so it takes up the space: material, color, what it’s doing.
  4. Give it context so the replacement belongs in the scene: “a deserted beach at dawn, before the first swimmers arrive”.
In an edit, removing something is an instruction, not a negation. “Remove the cat from the sofa” works as an edit prompt, and Edit with boxes shows how to remove an element with a box. FLUX 3 Image has no negative_prompt or prompt_upsampling field.