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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.

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Overview

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Pricing

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

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FLUX.2 [klein] 4B Edit API

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

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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);

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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

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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
promptstringrequiredAnyA text description of the image you want to generate.
stepsintegeroptionalAnyNumber of diffusion steps. More steps = slightly higher quality, longer generation.
seedintegeroptionalAnyRandom positive integer for reproducible results. Leave blank to randomise.
image_sizestringoptionalsquare_hd, square, portrait_4_3, portrait_16_9, landscape_4_3, landscape_16_9Output aspect ratio preset.
reference_image_1imageoptionalAnyReference image 1 for FLUX.2 multi-reference edit (1-4 images).
reference_image_2imageoptionalAnyReference image 2 for FLUX.2 multi-reference edit (1-4 images).
reference_image_3imageoptionalAnyReference image 3 for FLUX.2 multi-reference edit (1-4 images).
reference_image_4imageoptionalAnyReference image 4 for FLUX.2 multi-reference edit (1-4 images).
image_urlsimage_listoptionalAnySource image URLs for editing (1-4).

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Output schema

FieldTypeDescription
outputsjsonUnified output array - one entry per generated artifact with url/type/width/height/content_type.
seedjsonDeterministic seed used for generation, or null if not returned.
timingjsonProvider timing info (inference_ms etc.), or null.
nsfw_detectedjsontrue if flagged as NSFW, false if cleared, null if not checked.

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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.