FLUX.2 Klein 4B API

About FLUX.2 Klein 4B

The FLUX.2 Klein 4B API is a low-cost FLUX endpoint for drafts and background edits. Text to image costs about the same at 1K and 2K, and edits take up to three references with typical latency around 30 seconds. It is a good free-tier or preview model in front of a premium finisher.

Call it with the model name black-forest-labs/flux-2-klein-4b/text-to-image on POST /jobs/createTask, from 2 credits per image ($0.02). New accounts get 30 free credits to try it in the Playground first.

Introduction

FLUX.2 [klein] 4B Text to image makes one image from a written prompt on Black Forest Labs' smallest FLUX.2 model, which is open source under Apache 2.0. Choose one of nine aspect ratios at about one megapixel (1k) or up to 2048 pixels on the long side (2k) and get a JPEG back in roughly 15 to 35 seconds.

Choose a mode by starting material

Key features

Small and quick

With 4 billion parameters this is the lightest FLUX.2 model. It suits drafts, mockups and trying many ideas in a row.

Quoted text and hex colors

Short headlines in quotation marks and hex color codes are understood. Results are decent, not guaranteed: lettering can be misspelled and a color may show up on a different object than you intended.

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

Draft before you commit

Try a dozen prompts and seeds quickly, then take the best idea to a larger model if you need more detail.

Posters and neon scenes

Event posters, ad mockups and night scenes with signs and colored light work well within this model's range.

Moody portraits

Single subjects with one clear light source come out cleanly, especially with the hands covered or out of frame.

Prompt guide & pro tips

Describe the outfit yourself

This model adds clothing on its own. Write exactly what each person wears, piece by piece, and it will follow that.

Quote text, keep it short

One or two short words in quotes are safest. If lettering looks wrong, change only the seed and run again.

Keep two-person hands simple

Hands are the first thing to fail with two people in frame. Put them on shoulders, on a railing or in gloves, or run several seeds and pick the best.

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/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-2-klein-4b/text-to-image",
  "input": {
    "prompt": "A ceramic teapot on a linen cloth, soft window light, muted palette",
    "aspect_ratio": "1:1",
    "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.

What to build with the FLUX.2 Klein 4B API

• Free-tier image generation with low cost per image.
• Background and mood swap tools for portraits.
• Draft stages before FLUX.2 Pro or Max renders.

FLUX.2 Klein 4B parameter notes

• resolution 1k or 2k, priced nearly the same.
• image_urls on edit takes up to three references.
• seed supported; no negative prompt or steps.
• Hex colour codes in the prompt steer hue approximately.

How FLUX.2 Klein 4B API pricing adds up

Very low per-image prices; 2K costs only marginally more than 1K, and edits have their own flat price.

• resolution 1k: 2 credits ($0.02) per run, 200 credits ($2.00) for 100 runs
• resolution 2k: 2 credits ($0.02) per run, 200 credits ($2.00) for 100 runs

Async jobs, webhooks and rate limits

Every FLUX.2 Klein 4B request is asynchronous: createTask returns a taskId straight away, and the result arrives when the job finishes. Poll GET /jobs/recordInfo every few seconds, or pass a public https callBackUrl and verify the X-Webhook-Signature header (HMAC-SHA256 over taskId, timestamp and the body hash) before trusting the payload. Each account gets 1,000 write requests per 10 seconds; a 429 carries Retry-After. Send an Idempotency-Key with createTask so a retried request for black-forest-labs/flux-2-klein-4b/text-to-image never runs or bills twice.

Common FLUX.2 Klein 4B API errors

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 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

More about FLUX.2 Klein 4B

Try it

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

Endpoints

FLUX.2 Klein 4B · Edit Image

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