Pricing: $0.005/image. Endpoint: POST /v1/models/black-forest-labs/flux-2-klein-4b/edit/runs.

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
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

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

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

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

Browse all models

Browse the catalog.

Run lifecycle

Callbacks, polling, statuses.

Callbacks

Handle async results.

Pricing

How requests bill out.

​Overview

​Pricing

​FLUX.2 [klein] 4B Edit API

​Run the model

​Python

​Node.js

​cURL

​Request parameters

​Input schema

​Output schema

​Callback payload

​Additional Resources

​Related

Browse all models

Run lifecycle

Callbacks

Pricing

Overview

Pricing

FLUX.2 [klein] 4B Edit API

Run the model

Python

Node.js

cURL

Request parameters

Input schema

Output schema

Callback payload

Additional Resources

Related