FLUX.3 Klein API

FLUX.3 Klein (Oct 2026), Black Forest Labs' fast image model — about 1-megapixel images in any of five shapes, with built-in prompt expansion. 1¢ an image on Varosity Credits (fal's $0.009 per image at 1K).

$0.01/img on Varosity credits ($0.009/img with your own key).

Use FLUX.3 Klein with your agent Get an API key

no fal.ai account needed · provider's list price; credits round each call up to the cent · failed renders never billed

Best for

Black Forest Labs' newest image model
fast and cheap
built-in prompt expansion

Trade-offs

text-to-image only via Varosity
1K (long side 1024 px) via Varosity

How to get the best result

Fast, cheap images from Black Forest Labs' newest model: good photoreal and illustration at 1¢, with the prompt expanded for you. Pick instead: gpt-image-2 or nano-banana for exact text in the image; flux-2-pro for hex-color control; krea-3 for a stylized look.

Prompting

Describe framing, subject, background and lighting: the model is trained on detailed captions.
Short prompts work too; prompt expansion is on by default (providerOptions.expandPrompt: false turns it off).
There is no negative prompt: say what you want instead.

Settings on Varosity

Always 1K (long side 1024): 1024×576 (16:9), 576×1024 (9:16), 1024×1024 (1:1), 816×1024 (4:5), 1024×432 (21:9).
1¢ an image on Credits (fal's $0.009 per image at 1K, rounded up to the whole cent).
Text-to-image only via Varosity; referenceImageUrl is refused (400 reference_not_supported, unbilled).

Avoid

Expecting 2K or 4K output; Varosity renders at 1K.

Example prompt

A red fox standing in fresh snow at sunrise, its breath visible in the cold air, soft golden side light, pine forest blurred behind, telephoto lens at eye level.

From fal.ai, checked against what Varosity sends.

Call it from your code

One Varosity key works for every model.

curl -X POST https://varosity.ai/api/v1/images \
  -H "Authorization: Bearer $VAROSITY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "prompt": "studio product shot of a matte black water bottle, soft rim light",
    "modelId": "flux-3-klein",
    "aspectRatio": "1:1"
  }'

Or ask your agent

Connect Claude, Cursor, or any MCP client to https://varosity.ai/api/mcp with your key, then say:

"Use Varosity with flux-3-klein to generate an image of …"

Agent setup guide