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Pricing: $0.005/megapixel. Endpoint: POST /v1/models/black-forest-labs/flux-2-klein-4b/runs.
Compact 4-billion parameter text-to-image model from Black Forest Labs. Step-distilled to 4 inference steps for sub-second generation. Unifies text-to-image and image editing in a single architecture with Apache 2.0 licensing.

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Overview

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Pricing

  • Base price: $0.005/megapixel
  • Note: Per megapixel

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FLUX.2 [klein] 4B API

Endpoint: POST /v1/models/black-forest-labs/flux-2-klein-4b/runs

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Run the model

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Python

import requests

response = requests.post(
    "https://api.runflow.io/v1/models/black-forest-labs/flux-2-klein-4b/runs",
    headers={"Authorization": "Bearer RUNFLOW_API_KEY"},
    json={
        "input": {
            "prompt": "A vintage travel poster for 'RUNFLOW AIRWAYS' featuring a retro DC-3 airplane flying over snow-capped mountains at sunset, bold art deco typography, warm orange and teal color palette, textured paper grain, 1940s airline advertising style"
        },
        "callback_url": "https://your-server.com/webhook"
    },
)

data = response.json()
print(data)

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Node.js

const response = await fetch(
  "https://api.runflow.io/v1/models/black-forest-labs/flux-2-klein-4b/runs",
  {
    method: "POST",
    headers: {
      "Authorization": "Bearer RUNFLOW_API_KEY",
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
        "input": {
            "prompt": "A vintage travel poster for 'RUNFLOW AIRWAYS' featuring a retro DC-3 airplane flying over snow-capped mountains at sunset, bold art deco typography, warm orange and teal color palette, textured paper grain, 1940s airline advertising style"
        },
        "callback_url": "https://your-server.com/webhook"
    }),
  }
);

const data = await response.json();
console.log(data);

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cURL

curl -X POST https://api.runflow.io/v1/models/black-forest-labs/flux-2-klein-4b/runs \
  -H "Authorization: Bearer $RUNFLOW_API_KEY" \
  -H "Content-Type: application/json" \
  --data-binary @- <<'JSON'
{
    "input": {
        "prompt": "A vintage travel poster for 'RUNFLOW AIRWAYS' featuring a retro DC-3 airplane flying over snow-capped mountains at sunset, bold art deco typography, warm orange and teal color palette, textured paper grain, 1940s airline advertising style"
    },
    "callback_url": "https://your-server.com/webhook"
}
JSON

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Request parameters

ParameterTypeRequiredDescription
inputobjectrequiredModel input parameters. See “Input schema” below.
callback_urlstring | nulloptionalWebhook URL - POSTed when the run reaches a terminal state.
metadataobject | nulloptionalArbitrary key-value pairs attached to the run.

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Input schema

FieldTypeRequiredAllowed valuesDescription
promptstringrequiredAnyText description of the image to generate.
image_sizestringoptionalsquare_hd, square, portrait_4_3, portrait_16_9, landscape_4_3, landscape_16_9Size preset for the generated image. Alternatively use width/height for custom dimensions.
num_imagesintegeroptionalAnyNumber of images to generate (1-4).
num_inference_stepsintegeroptionalAnyNumber of denoising steps. Default 4 for distilled variant, use higher for base variant.
seedintegeroptionalAnyRandom seed for reproducible generation. Leave empty for random.
output_formatstringoptionaljpeg, png, webpOutput image format.
enable_safety_checkerbooleanoptionalAnyEnable NSFW content filtering.
sync_modebooleanoptionalAnyWait for the result synchronously instead of returning a request ID.

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Output schema

FieldTypeDescription
outputsjsonUnified output array - one entry per generated artifact with url/type/width/height/duration/etc.
seedjsonThe seed used for generation.
model_datajsonModel-specific extra fields (prompt, description, actual_prompt, masks, etc.) keyed by field name.
nsfw_detectedjsontrue if the provider flagged output as NSFW, false if cleared, null if not checked.
timingjsonProvider timing info (inference_ms etc.), or null.

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Callback payload

When you provide a callback_url, Runflow POSTs to it once the run reaches a terminal state.
FieldTypeDescription
eventstringEvent type: “run.completed”, “run.failed”, or “run.cancelled”.
run_idstringThe unique identifier of the run.
statusstringTerminal status: “succeeded”, “failed”, or “cancelled”.
outputobject | nullThe run output. Null if the run failed or was cancelled.
duration_msnumber | nullTotal run duration in milliseconds.
created_atstring | nullISO 8601 timestamp when the run was created.
completed_atstring | nullISO 8601 timestamp when the run reached terminal state.
metadataobject | nullThe metadata object passed at run creation, if any.
  • Retries: 3 attempts with exponential backoff (1s, 2s). Retries on 5xx / network errors only.
  • Headers: Runflow-Request-Id is always sent. Runflow-Signature is sent if a signing secret is configured.

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Additional Resources

Browse all models

Browse the catalog.

Run lifecycle

Callbacks, polling, statuses.

Callbacks

Handle async results.

Pricing

How requests bill out.