FLUX.2 Klein 4B API

Introduction

FLUX.2 [klein] 4B Image edit changes or combines up to three source images with a written instruction, on Black Forest Labs' smallest FLUX.2 model, which is open source under Apache 2.0. Keep the shape of your first image with auto, or choose a ratio and a 1k or 2k size tier.

Choose a mode by starting material

Key features

Up to three references

Combine two portraits and a location, or restyle a single photo.

Edits that keep people

Ask for a new background, light or weather and keep faces, clothes and pose. With one source this works cleanly; with three, faces can drift slightly.

Request parameters

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.

Each task is rounded up to whole credits, so one run at the lowest tier costs 2 credits per image ($0.02).

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

Use cases

Background and mood swaps

Move a couple from a sunny street to a rainy neon night in one pass.

Bringing people to a place

Place two portraits in a location photo for storyboards, covers and character sheets.

Quick restyles

Change light, season or palette while the composition stays put.

Prompt guide & pro tips

Protect first, change second

Open with what must remain the same, then describe only the change. Short, concrete instructions work best.

Give each source a role

With several images, say what each one contributes: this person, that person, this place.

Check the details

With two or three sources, look at hands, hair and accessories; a small extra item can appear. Run another seed if something is off.

API quick start

Call the FLUX.2 Klein 4B 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-2-klein-4b/edit 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-2-klein-4b/edit",
  "input": {
    "prompt": "A ceramic teapot on a linen cloth, soft window light, muted palette",
    "image_urls": [
      "https://media.popnow.ai/influencer/examples/marco.webp"
    ],
    "aspect_ratio": "auto",
    "resolution": "1k"
  }
}'

# 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.2 Klein 4B API questions

All FLUX models

FLUX.2 Klein 9B is one size up, with steadier faces and hands.

• FLUX.2 Max APITop-tier FLUX: photoreal light and exact brand colorsFrom 11 credits per image
• FLUX 3 APIVideo with synced sound and a quick draft tierFrom 45 credits for a 5-second clip
• FLUX.2 Klein 9B LoRA APIFast FLUX.2 Klein 9B that loads up to three of your LoRAsFrom 3 credits per image
• FLUX.2 Klein Base 9B LoRA APIUndistilled Klein 9B for varied looks with your LoRAsFrom 4 credits per image
• FLUX.2 Klein 4B LoRA APISmall, quick FLUX.2 Klein 4B with your own LoRAsFrom 3 credits per image
• FLUX.2 Klein Base 4B LoRA APIUndistilled Klein 4B that loads your own LoRAsFrom 3 credits per image
• FLUX.2 Klein 9B APIFast FLUX edits that keep faces and poses intactFrom 2 credits per image
• FLUX.2 Klein 4B APIThe smallest, quickest FLUX for rapid draftsFrom 2 credits per image
• FLUX.2 Dev LoRA APIFLUX.2 Dev images and edits with your own LoRAsFrom 4 credits per image
• FLUX.2 Pro APIPhotoreal images with readable text and hex color controlFrom 5 credits per image
• FLUX.2 Flex APISpells headlines and labels exactly as writtenFrom 9 credits per image
• FLUX.2 Dev APIOpen-weights FLUX that can merge people into one sceneFrom 2 credits per image
• FLUX.1 Krea Dev LoRA APIPhotographic FLUX.1 Krea with your own LoRAsFrom 4 credits per image
• FLUX.1 Dev ControlNet LoRA APIPose, depth or edge control on FLUX.1 with your LoRAsFrom 6 credits per image
• FLUX.1 Fill Dev LoRA APIMasked inpainting on FLUX.1 Fill with your own LoRAsFrom 7 credits per image
• FLUX.1 Dev LoRA APIClassic FLUX.1 Dev with up to three of your LoRAsFrom 3 credits per image
• FLUX.1 Schnell LoRA APIFast FLUX.1 Schnell that loads your own LoRAsFrom 1 credit per image

Other image APIs

• Nano Banana 2.1 APIGoogle's newest Nano Banana, sharp text and 4K outputFrom 8 credits per image
• Nano Banana Pro APIPlans the layout first: dense text, diagrams and 4K editsFrom 17 credits per image
• GPT Image 2.5 Sunburst APIOpenAI's quality-first model, up to 4K with clean textFrom 1 credit per image
• Seedream 5.0 Pro APIHigh-detail images and edits from up to ten referencesFrom 6 credits per image
• Qwen Image 2.1 Pro APIUp to six detailed 2K images from one promptFrom 6 credits per image
• Qwen Image 2.1 Turbo APIFast 8-step renders, up to six per promptFrom 3 credits per image

More about FLUX.2 Klein 4B

Try it

FLUX.2 Klein 4B AI Image Generator onlineCreate images with FLUX.2 Klein 4B in PopNow

Docs

FLUX.2 Klein 4B model ID and input schema in the API docsPopNow API quickstart: your first request in five minutesImage generation guide for the PopNow APIWebhooks: get FLUX.2 Klein 4B results without polling