> ## 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 Edit

> Fast image-edit endpoint backed by FLUX.2 [klein] 4B. Pass a base64 source image with the prompt for low-latency edits.

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

Fast image-edit endpoint backed by FLUX.2 \[klein] 4B. Pass a base64 source image with the prompt for low-latency edits.

## Overview

* **Endpoint**: [https://api.runflow.io/v1/models/black-forest-labs/flux-2-klein-4b/edit/runs](https://api.runflow.io/v1/models/black-forest-labs/flux-2-klein-4b/edit/runs)
* **Model ID**: black-forest-labs/flux-2-klein-4b/edit
* **Provider**: Black Forest Labs
* **License**: commercial
* **Last Updated**: 2026-05-28

## Pricing

* **Base price**: \$0.005/image
* **Note**: per image, 1024x1024, up to 25 steps

# FLUX.2 \[klein] 4B Edit API

**Endpoint:** `POST /v1/models/black-forest-labs/flux-2-klein-4b/edit/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/edit/runs",
    headers={"Authorization": "Bearer RUNFLOW_API_KEY"},
    json={
        "input": {
            "steps": 4,
            "prompt": "Replace the computer monitors with a large glass aquarium full of colorful tropical fish and coral, the keyboard sits on a bed of white sand, small hermit crabs crawl on the mousepad",
            "image_urls": [
                "https://public.runflow.io/images/models/_shared/base-engineer.png"
            ]
        },
        "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/edit/runs",
  {
    method: "POST",
    headers: {
      "Authorization": "Bearer RUNFLOW_API_KEY",
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
        "input": {
            "steps": 4,
            "prompt": "Replace the computer monitors with a large glass aquarium full of colorful tropical fish and coral, the keyboard sits on a bed of white sand, small hermit crabs crawl on the mousepad",
            "image_urls": [
                "https://public.runflow.io/images/models/_shared/base-engineer.png"
            ]
        },
        "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/edit/runs \
  -H "Authorization: Bearer $RUNFLOW_API_KEY" \
  -H "Content-Type: application/json" \
  --data-binary @- <<'JSON'
{
    "input": {
        "steps": 4,
        "prompt": "Replace the computer monitors with a large glass aquarium full of colorful tropical fish and coral, the keyboard sits on a bed of white sand, small hermit crabs crawl on the mousepad",
        "image_urls": [
            "https://public.runflow.io/images/models/_shared/base-engineer.png"
        ]
    },
    "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                                                                                       | A text description of the image you want to generate.                               |
| `steps`             | integer     | optional | Any                                                                                       | Number of diffusion steps. More steps = slightly higher quality, longer generation. |
| `seed`              | integer     | optional | Any                                                                                       | Random positive integer for reproducible results. Leave blank to randomise.         |
| `image_size`        | string      | optional | `square_hd`, `square`, `portrait_4_3`, `portrait_16_9`, `landscape_4_3`, `landscape_16_9` | Output aspect ratio preset.                                                         |
| `reference_image_1` | image       | optional | Any                                                                                       | Reference image 1 for FLUX.2 multi-reference edit (1-4 images).                     |
| `reference_image_2` | image       | optional | Any                                                                                       | Reference image 2 for FLUX.2 multi-reference edit (1-4 images).                     |
| `reference_image_3` | image       | optional | Any                                                                                       | Reference image 3 for FLUX.2 multi-reference edit (1-4 images).                     |
| `reference_image_4` | image       | optional | Any                                                                                       | Reference image 4 for FLUX.2 multi-reference edit (1-4 images).                     |
| `image_urls`        | image\_list | optional | Any                                                                                       | Source image URLs for editing (1-4).                                                |

## Output schema

| Field           | Type | Description                                                                                       |
| --------------- | ---- | ------------------------------------------------------------------------------------------------- |
| `outputs`       | json | Unified output array - one entry per generated artifact with url/type/width/height/content\_type. |
| `seed`          | json | Deterministic seed used for generation, or null if not returned.                                  |
| `timing`        | json | Provider timing info (inference\_ms etc.), or null.                                               |
| `nsfw_detected` | json | true if flagged as NSFW, false if cleared, null if not checked.                                   |

## 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/edit)
* [API Reference](https://app.runflow.io/models/black-forest-labs/flux-2-klein-4b/edit?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>