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by Black Forest Labs

Full-quality FLUX 3 Klein images and edits up to 2K

black-forest-labs/flux-3-klein-base/text-to-imageFrom 6 creditsper image · $0.06
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  • Up to 2K
  • Up to 3 reference images
  • 15 aspect ratios
  • Create Image
  • Edit Image

About FLUX 3 Klein Base

Full-quality FLUX 3 Klein images and edits up to 2K.

Call it with the model name black-forest-labs/flux-3-klein-base/text-to-image on POST /jobs/createTask, from 6 credits per image ($0.06).

Introduction

FLUX 3 Klein Base Text to image draws one image from a written prompt on Black Forest Labs' FLUX 3 Image Klein Base, the undistilled 5.6B open-weights checkpoint released on 15 October 2026. Its makers run Base at 50 sampling steps against 16 for the distilled Klein, so each picture takes longer. Choose one of fifteen aspect ratios at 1k or 2k and JPEG, PNG or WebP.

Choose a mode by starting material

What you haveMode
a written descriptionFLUX 3 Klein Base Text to image
one to three images to change or combineFLUX 3 Klein Base Image editing

Key features

Full sampling, natural light

Without any shortcut in sampling, Base drew soft, believable skin and mixed light well in our tests: cold blue daylight through pine boards against an amber stove glow, wet hair, wool and linen textures. It suits stills you will look at closely rather than quick drafts, and the 2k tier gives about four megapixels to crop from.

Fifteen shapes

Every ratio from 3:1 to 1:3 is available, including 21:9, 4:5 and 2:1, and each came back at its own native size in our tests, from 1760 x 576 to 576 x 1760 at 1k. Banners, posters and phone screens come straight out of the model without cropping.

Repeatable seeds

A repeated seed with the same inputs returned identical pixels in our tests. Change one sentence at a time and every difference you see comes from that change, which makes prompt work on a slow model far less of a guess.

Request parameters

ParameterRequiredDescription
seedNoReuse the same number to get a repeatable result. ≥ 0 · ≤ 2147483647
promptYesDescribe the whole scene in full sentences: framing, subject, setting and light.
resolutionNoOutput size. Bigger sizes look sharper and cost more. 1k · 2k · Default: 1k
aspect_ratioNoThe shape of the frame: wide, square or tall. 1:1 · 16:9 · 9:16 · 4:3 · 3:4 · 3:2 · 2:3 · 4:5 · 5:4 · 21:9 · 9:21 · 2:1 · 1:2 · 3:1 · 1:3 · Default: 1:1
output_formatNoFile type of the result. jpeg · png · webp · Default: jpeg

Pricing

Prices are in credits, the same credits you use on the website (1 credit = $0.01). The hold is taken when the task is accepted and settled to the actual charge when it finishes; failed tasks are refunded.

OptionCredits per image≈ USD
resolution 1K5.4$0.054
resolution 2K21.6$0.22

Each task is rounded up to whole credits, so one run at the lowest tier costs 6 credits per image ($0.06).

For an exact number before you run, send the same body to POST /jobs/quote.

Use cases

Before you fine-tune

Base is the checkpoint Black Forest Labs publishes for fine-tuning. See how it draws your subject and style from a prompt before you decide whether a LoRA or a full fine-tune is worth training.

Prompt guide & pro tips

Write captions, not keywords

Klein is trained on long scene captions, and Black Forest Labs normally rewrites short prompts before generation. This endpoint sends your words as written, so describe framing, subject, clothing, place, light, lens and grade in full sentences.

Draft at 1k, finish at 2k

Settle the prompt and seed at 1k, then render the keeper at 2k for about four megapixels.

API quick start

Call the FLUX 3 Klein Base API in three steps

  1. Create an API key. Sign in to PopNow and create a key in the API console. Keys start with sk-popnow- and use the same credits as the website.
  2. Send the request. POST /jobs/createTask with model black-forest-labs/flux-3-klein-base/text-to-image and an input object that follows the parameter table. The response returns a task ID straight away.
  3. Get the result. Poll GET /jobs/recordInfo with the task ID, or pass a callBackUrl and we post the result to it. Download the output links when the task succeeds.
# 1. Create the task. Keep the Idempotency-Key if you retry after a network error.
export IDEMPOTENCY_KEY="$(uuidgen)"
curl -X POST https://api.popnow.ai/api/v1/jobs/createTask \
  -H "Authorization: Bearer $POPNOW_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $IDEMPOTENCY_KEY" \
  -d '{
  "model": "black-forest-labs/flux-3-klein-base/text-to-image",
  "input": {
    "prompt": "A ceramic teapot on a linen cloth, soft window light, muted palette",
    "aspect_ratio": "1:1",
    "resolution": "1k",
    "output_format": "jpeg"
  }
}'

# 2. Copy data.taskId from the response, then poll until state is succeeded, failed, canceled or expired.
TASK_ID="TASK_ID_FROM_RESPONSE"
curl "https://api.popnow.ai/api/v1/jobs/recordInfo?taskId=$TASK_ID" \
  -H "Authorization: Bearer $POPNOW_API_KEY"

Replace the prompt and any links with your own. The API tab has the response, polling and callback reference.

FLUX 3 Klein Base API questions

How much does the FLUX 3 Klein Base API cost?

FLUX 3 Klein Base starts at 6 credits ($0.06) per image with Create Image, its cheapest task. Prices by option: 1K 5.4 credits per image, 2K 21.6 credits per image. Credits are the same as on the website (1 credit = $0.01), and failed tasks are refunded.

What is the model name for FLUX 3 Klein Base in the API?

Put black-forest-labs/flux-3-klein-base/text-to-image for Create Image; black-forest-labs/flux-3-klein-base/edit for Edit Image in the model field of POST /jobs/createTask.

Do I need a long prompt?

It helps. Black Forest Labs trained Klein on full scene captions and normally rewrites short prompts before generation. This endpoint sends your prompt as written, so describe framing, subject, setting and light yourself.

How do I refer to several images in an edit?

Write image 1, image 2 and image 3 in the order you uploaded them, and say what each one brings: the woman from image 1, the dress from image 2. The first image also sets the shape when the aspect ratio is auto.

Can I load my own LoRA?

Not on this endpoint. FLUX 3 Klein Base LoRA runs the same model with up to three LoRA files per request.

What are the limits of the FLUX 3 Klein Base API?

Resolutions: 1K · 2K; Aspect ratios: 1:1 · 16:9 · 9:16 · 4:3 · 3:4 · 3:2 +9; Prompt length (characters): 5,000. Requests outside these limits are rejected before anything is charged.

Can I try FLUX 3 Klein Base without writing code?

Yes. The Playground runs FLUX 3 Klein Base on your PopNow credits, needs no API key, and shows the matching code for every run.

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