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Flux 2 Klein 4B Text to Image

bfl/flux-2/klein/4b

Flux 2 Klein 4b is BFL's advanced text-to-image AI model. Create photorealistic images, illustrations, and concept art from natural language descriptions with exceptional detail and prompt adherence.

Base price
$0.008USD / run
Execution
async
Model type
image
Input fields
5
Production route

Try the model

Playground

Open playground
Input
The prompt to generate an image from.
48
The number of inference steps to perform. Range: 4 to 8.
The format of the generated image. Allowed values: jpeg, png.
The aspect ratio of the generated image. Allowed values: 21:9, 16:9, 3:2, 4:3, 5:4, 1:1, 4:5, 3:4, 2:3, 9:16.
The seed to use for the generation. If not provided, a random seed will be used.
OutputExample

Examples

Select an example to load its inputs and result into the playground.

Model card

Specifications

Pricing

Base price
$0.008 / run

Context & modalities

Input
Schema-defined
Output
image

Capabilities

Chat
Not supported
Vision
Not supported
Reasoning
Not supported
Structured output
Not supported
Function calling
Not supported
Audio input
Not supported

Access

Provider
BFL
Model ID
bfl/flux-2/klein/4b
Execution
async
Base URL
https://api.sandbase.ai
API
Unified Run API
Endpoint
/v1/run

Human preference

Arena

Elo ratings from blind side-by-side votes, compared with other image and video models.

Rankings sourced from Artificial Analysis Arena

Leaderboard
Text to Image
Rank
#91 of 102
Elo
840 ±13
Gap to #1
−377

Text-to-image arena: people pick the better of two images generated from the same prompt, without seeing the model names.

  1. #86LongCat ImageMeituan862±21
  2. #87FLUX1.1 [pro] UltraBlack Forest Labs856±20
  3. #88Lucid Origin FastLeonardo.Ai851±19
  4. #89FIBOBria848±21
  5. #90Luma PhotonLuma Labs846±21
  6. #91FLUX.2 [klein] 4BBlack Forest LabsThis model840±13
  7. #92Phoenix 0.9 UltraLeonardo.Ai840±21
  8. #93Phoenix 1.0 FastLeonardo.Ai835±22
  9. #94FLUX.1 Krea [dev]Black Forest Labs828±21
  10. #95FIBO LiteBria814±17

API README

Flux 2 Klein 4B Text to Image

FLUX.2 [klein] 4B, the smallest FLUX.2 model, built for speed. Text to image.

Fields

FieldTypeDefaultNotes
promptstring, required—Text instruction.
num_inference_stepsinteger 4–84More steps add detail and time.
output_formatjpeg / pngpngOutput file format.
aspect_ratio21:9 / 16:9 / 3:2 / 4:3 / 5:4 / 1:1 / 4:5 / 3:4 / 2:3 / 9:1616:9Output shape.
seedintegerrandomFix it to compare prompt changes.

Billing

$0.008 per image.

RequestPrice
One image$0.008

Failed runs are not charged.

Prompting

  • Write subject, setting, composition, style, then light.
  • Quote any text that must appear.

Limits

  • One output image per request.

Also on SandBase

Other FLUX.2 endpoints: bfl/flux-2/max, bfl/flux-2/pro, bfl/flux-2/flex, bfl/flux-2/dev, bfl/flux-2/klein/4b/edit.

Start building

Send your first request

OpenAI-compatible endpoint with unified authentication and usage tracking.

Production API
Unified Run API endpoint
https://api.sandbase.ai/v1/run
Model ID
bfl/flux-2/klein/4b
# The quoted heredoc keeps Unicode and shell metacharacters unchanged.
result=$(curl --fail-with-body --silent \
  -X POST "https://api.sandbase.ai/v1/run" \
  -H "Authorization: Bearer $SANDBASE_API_KEY" \
  -H "Content-Type: application/json" \
  --data-binary @- <<'SANDBASE_JSON'
{
  "model": "bfl/flux-2/klein/4b",
  "prompt": "Cheetah in pre-sprint crouch, muscles tensed, gazelle visible in distance, African savanna golden grass, dust particles in air, National Geographic wildlife photography",
  "aspect_ratio": "16:9",
  "output_format": "png",
  "num_inference_steps": 4
}
SANDBASE_JSON
)
run_id=$(printf '%s' "$result" | jq -r .id)
for attempt in $(seq 1 120); do
  status=$(printf '%s' "$result" | jq -r .status)
  case "$status" in completed|failed|timeout) break ;; esac
  sleep 2
  result=$(curl --fail-with-body --silent \
    -H "Authorization: Bearer $SANDBASE_API_KEY" \
    "https://api.sandbase.ai/v1/run/$run_id")
done
status=$(printf '%s' "$result" | jq -r .status)
[ "$status" = completed ] || { echo "Generation ended: $status" >&2; exit 1; }
printf '%s\n' "$result"

Choose your model

Compare models

All image & video models
ModelTypeDefault priceReleased
BFLImage$0.01 / runJan 15, 2026
BFLImage$0.05 / runOct 1, 2026
BFLImage$0.05 / runOct 1, 2026
BFLImage$0.02 / runJan 15, 2026
BFLImage$0.02 / runJan 15, 2026
BFLImage$0.01 / runJan 15, 2026

Questions

FAQ

How do I call BFL Flux 2 Klein 4B Text to Image through SandBase?

Create a SandBase API key, then send requests with the model ID "bfl/flux-2/klein/4b" to Unified Run API (/v1/run) at https://api.sandbase.ai. The request examples on this page show the exact payload.

Do I need a separate BFL account?

No. One SandBase API key and balance gives you access to BFL Flux 2 Klein 4B Text to Image and the other models in the catalog; you do not need to sign up with BFL separately.