FLUX.2 [klein] 4B

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.

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.

Overview

• Endpoint: https://api.runflow.io/v1/models/black-forest-labs/flux-2-klein-4b/runs
• Model ID: black-forest-labs/flux-2-klein-4b
• Provider: Black Forest Labs
• License: commercial
• Last Updated: 2026-04-06

Pricing

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

FLUX.2 [klein] 4B API

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

Run the model

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)

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);

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

Request parameters

Parameter	Type	Required	Description
input	object	required	Model input parameters. See “Input schema” below.
callback_url	string | null	optional	Webhook URL - POSTed when the run reaches a terminal state.
metadata	object | null	optional	Arbitrary key-value pairs attached to the run.

Input schema

Field	Type	Required	Allowed values	Description
prompt	string	required	Any	Text description of the image to generate.
image_size	string	optional	square_hd, square, portrait_4_3, portrait_16_9, landscape_4_3, landscape_16_9	Size preset for the generated image. Alternatively use width/height for custom dimensions.
num_images	integer	optional	Any	Number of images to generate (1-4).
num_inference_steps	integer	optional	Any	Number of denoising steps. Default 4 for distilled variant, use higher for base variant.
seed	integer	optional	Any	Random seed for reproducible generation. Leave empty for random.
output_format	string	optional	jpeg, png, webp	Output image format.
enable_safety_checker	boolean	optional	Any	Enable NSFW content filtering.
sync_mode	boolean	optional	Any	Wait for the result synchronously instead of returning a request ID.

Output schema

Field	Type	Description
outputs	json	Unified output array - one entry per generated artifact with url/type/width/height/duration/etc.
seed	json	The seed used for generation.
model_data	json	Model-specific extra fields (prompt, description, actual_prompt, masks, etc.) keyed by field name.
nsfw_detected	json	true if the provider flagged output as NSFW, false if cleared, null if not checked.
timing	json	Provider timing info (inference_ms etc.), or null.

Callback payload

When you provide a callback_url, Runflow POSTs to it once the run reaches a terminal state.

Field	Type	Description
event	string	Event type: “run.completed”, “run.failed”, or “run.cancelled”.
run_id	string	The unique identifier of the run.
status	string	Terminal status: “succeeded”, “failed”, or “cancelled”.
output	object | null	The run output. Null if the run failed or was cancelled.
duration_ms	number | null	Total run duration in milliseconds.
created_at	string | null	ISO 8601 timestamp when the run was created.
completed_at	string | null	ISO 8601 timestamp when the run reached terminal state.
metadata	object | null	The 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.

Additional Resources

• Playground
• API Reference
• Documentation
• OpenAPI

Related

Browse all models

Browse the catalog.

Run lifecycle

Callbacks, polling, statuses.

Callbacks

Handle async results.

Pricing

How requests bill out.