> ## Documentation Index
> Fetch the complete documentation index at: https://docs.runflow.io/llms.txt
> Use this file to discover all available pages before exploring further.

# 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.

<Info>
  Pricing: **\$0.005/megapixel**. Endpoint: `POST /v1/models/black-forest-labs/flux-2-klein-4b/runs`.
</Info>

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](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

```python theme={"dark"}
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

```javascript theme={"dark"}
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

```bash theme={"dark"}
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](https://app.runflow.io/models/black-forest-labs/flux-2-klein-4b)
* [API Reference](https://app.runflow.io/models/black-forest-labs/flux-2-klein-4b?tab=api)
* [Documentation](https://docs.runflow.io)
* [OpenAPI](https://docs.runflow.io/api/openapi.public.json)

## Related

<CardGroup cols={2}>
  <Card title="Browse all models" icon="grid" href="/models">Browse the catalog.</Card>
  <Card title="Run lifecycle" icon="play" href="/concepts/runs">Callbacks, polling, statuses.</Card>
  <Card title="Callbacks" icon="webhook" href="/concepts/callbacks">Handle async results.</Card>
  <Card title="Pricing" icon="coin" href="/concepts/pricing">How requests bill out.</Card>
</CardGroup>