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FLUX 3 Klein LoRA API

by Black Forest Labs

FLUX 3 Klein images and edits with up to three of your LoRAs

black-forest-labs/flux-3-klein-lora/text-to-image
  • Up to 2K
  • Up to 3 reference images
  • 15 aspect ratios
  • Loads your own LoRA
  • Create Image
  • Edit Image

Call it with the model name black-forest-labs/flux-3-klein-lora/text-to-image on POST /jobs/createTask, from $0.018.

Model nameblack-forest-labs/flux-3-klein-lora/text-to-image
EndpointPOST https://api.popnow.ai/api/v1/jobs/createTask
# 1. Create the task. Keep the Idempotency-Key if you retry after a network error.
export IDEMPOTENCY_KEY="$(uuidgen)"
curl -X POST https://api.popnow.ai/api/v1/jobs/createTask \
  -H "Authorization: Bearer $POPNOW_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $IDEMPOTENCY_KEY" \
  -d '{
  "model": "black-forest-labs/flux-3-klein-lora/text-to-image",
  "input": {
    "prompt": "A ceramic teapot on a linen cloth, soft window light, muted palette",
    "aspect_ratio": "1:1",
    "resolution": "1k",
    "output_format": "jpeg"
  }
}'

# 2. Copy data.taskId from the response, then poll until state is succeeded, failed, canceled or expired.
TASK_ID="TASK_ID_FROM_RESPONSE"
curl "https://api.popnow.ai/api/v1/jobs/recordInfo?taskId=$TASK_ID" \
  -H "Authorization: Bearer $POPNOW_API_KEY"

The input is built from the schema below, so it passes validation as written. Change the prompt and links to your own.

Price

OptionUSD per image
resolution 1K$0.018
resolution 2K$0.054

Billed in USD · Every option and its price: GET /models/black-forest-labs/flux-3-klein-lora/text-to-image

Limits

Resolutions
1K · 2K
Aspect ratios
1:1 · 16:9 · 9:16 · 4:3 · 3:4 · 3:2 +9
Prompt length (characters)
5,000

Parameters

These fields come from the same schema we check every request against. Fields that are not listed are rejected.

  • promptstringRequired

    Describe the subject, setting, light and style in plain sentences. Put a LoRA trigger word first.

    Range
    1 – 5000 chars
  • aspect_ratiostringOptional

    The shape of the frame: wide, square or tall.

    Default
    "1:1"
    Allowed values
    "1:1""16:9""9:16""4:3""3:4""3:2""2:3""4:5""5:4""21:9""9:21""2:1""1:2""3:1""1:3"
  • resolutionstringOptionalChanges the price

    Output size. Bigger sizes look sharper and cost more.

    Default
    "1k"
    Allowed values
    "1k""2k"
  • lorasarray<object>Optional

    Load your own LoRA files to lock in a style or character.

    Default
    []
    Range
    0 – 3 items
    Fields inside
    • loras[].pathstringRequired

      Direct URL of a LoRA weights file.

      Range
      1 – … chars · application/octet-stream
    • loras[].scalenumberRequired

      Weight strength from zero to four.

      Default
      1
      Range
      0 – 4
  • output_formatstringOptional

    File type of the result.

    Default
    "jpeg"
    Allowed values
    "jpeg""png""webp"
  • seedintegerOptional

    Reuse the same number to get a repeatable result.

    Range
    0 – 2147483647
Raw JSON Schema
{
  "additionalProperties": false,
  "properties": {
    "aspect_ratio": {
      "default": "1:1",
      "description": "Output aspect ratio.",
      "enum": [
        "1:1",
        "16:9",
        "9:16",
        "4:3",
        "3:4",
        "3:2",
        "2:3",
        "4:5",
        "5:4",
        "21:9",
        "9:21",
        "2:1",
        "1:2",
        "3:1",
        "1:3"
      ],
      "type": "string"
    },
    "loras": {
      "default": [],
      "description": "Up to three LoRA weight files trained for FLUX 3 Klein (the Base or the distilled build), each with its own strength.",
      "items": {
        "additionalProperties": false,
        "properties": {
          "path": {
            "contentMediaType": [
              "application/octet-stream"
            ],
            "description": "Direct URL of a LoRA weights file.",
            "format": "uri",
            "minLength": 1,
            "type": "string"
          },
          "scale": {
            "default": 1,
            "description": "Weight strength from zero to four.",
            "maximum": 4,
            "minimum": 0,
            "type": "number"
          }
        },
        "required": [
          "path",
          "scale"
        ],
        "type": "object"
      },
      "maxItems": 3,
      "type": "array"
    },
    "output_format": {
      "default": "jpeg",
      "description": "File format of the returned image.",
      "enum": [
        "jpeg",
        "png",
        "webp"
      ],
      "type": "string"
    },
    "prompt": {
      "description": "Describe the subject, setting, light and style in plain sentences. Put a LoRA trigger word first.",
      "maxLength": 5000,
      "minLength": 1,
      "type": "string"
    },
    "resolution": {
      "default": "1k",
      "description": "Output size tier: 1k is about one megapixel, 2k about four.",
      "enum": [
        "1k",
        "2k"
      ],
      "type": "string"
    },
    "seed": {
      "description": "Random seed for controlled comparisons.",
      "maximum": 2147483647,
      "minimum": 0,
      "type": "integer"
    }
  },
  "required": [
    "prompt"
  ],
  "type": "object"
}

Responses, polling and callbacks

Every model works the same way: create a task, then poll it or wait for a callback.

Price it first

cURL
curl -X POST https://api.popnow.ai/api/v1/jobs/quote \
  -H "Authorization: Bearer $POPNOW_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
  "model": "black-forest-labs/flux-3-klein-lora/text-to-image",
  "input": {
    "prompt": "A ceramic teapot on a linen cloth, soft window light, muted palette",
    "aspect_ratio": "1:1",
    "resolution": "1k",
    "output_format": "jpeg"
  }
}'

Send the same request body to POST /jobs/quote to see the estimated and maximum USD charge before running. A quote does not run the task or charge your balance.

Accepted

HTTP
HTTP/1.1 202 Accepted

{
  "code": 200,
  "msg": "success",
  "data": {
    "taskId": "task_01k3m8x9q2z4v7n5p6r8s0t1w3",
    "state": "queued"
  },
  "request_id": "req_example_create"
}

HTTP 202 means accepted: the maximum USD amount is held until settlement. When a task is accepted, we reserve the maximum quoted amount from your API balance. On completion, we charge for actual usage, never more than the hold, and release the remainder. If the task fails, the hold is released.

Check status

cURL
curl "https://api.popnow.ai/api/v1/jobs/recordInfo?taskId=$TASK_ID" \
  -H "Authorization: Bearer $POPNOW_API_KEY"

Poll every few seconds with backoff and stop once state is succeeded, failed, canceled or expired.

Finished task

JSON
{
  "code": 200,
  "msg": "success",
  "data": {
    "taskId": "task_01k3m8x9q2z4v7n5p6r8s0t1w3",
    "model": "black-forest-labs/flux-3-klein-lora/text-to-image",
    "state": "succeeded",
    "output": {
      "assets": [
        {
          "url": "https://cdn.popnow.ai/tasks/task_01k3m8x9q2z4v7n5p6r8s0t1w3/output.png?signature=FROM_RESPONSE",
          "expiresAt": "2026-10-17T12:20:00Z",
          "mime": "image/png",
          "width": 1024,
          "height": 1024
        }
      ]
    },
    "settled": true
  },
  "request_id": "req_example_record"
}

Download from url before expiresAt. Need a fresh link later? Ask POST /common/download-url.

Verify a callback

Node.js
import { createHash, createHmac, timingSafeEqual } from "node:crypto";
import express from "express";

const app = express();
// express.raw keeps the exact bytes that were signed; express.json would not match.
app.post("/hooks/popnow", express.raw({ type: "application/json" }), (req, res) => {
  const timestamp = req.header("X-Webhook-Timestamp");
  const signature = req.header("X-Webhook-Signature") ?? "";
  const body = JSON.parse(req.body.toString("utf8"));
  const taskId = body.data?.taskId ?? body.task_id;
  const digest = createHash("sha256").update(req.body).digest("hex");
  const expected = createHmac("sha256", process.env.POPNOW_WEBHOOK_SECRET)
    .update(`${taskId}.${timestamp}.${digest}`)
    .digest("base64");
  const ok = expected.length === signature.length && timingSafeEqual(Buffer.from(expected), Buffer.from(signature));
  // Check the signature first, then reject anything older than five minutes.
  if (!ok || Math.abs(Date.now() / 1000 - Number(timestamp)) > 300) return res.status(401).end();
  res.status(200).end(); // answer fast, then fetch the task and store its files
});

With callBackUrl set, we post the finished task to you. Check the signature against the raw body before you trust it.

Send a local file

cURL
curl -X POST https://api.popnow.ai/api/v1/common/upload-url \
  -H "Authorization: Bearer $POPNOW_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"contentType": "image/png", "bytes": 482113}'

# PUT the file bytes to data.uploadUrl with every header in data.headers, then commit it:
curl -X POST https://api.popnow.ai/api/v1/files/$FILE_ID/commit \
  -H "Authorization: Bearer $POPNOW_API_KEY"
# The commit returns a popnow://f/... reference. Put it in any image, video or
# audio field of the task input instead of a public https URL.

Public https links work as they are. Local files go through upload-url and commit first.

About FLUX 3 Klein LoRA

FLUX 3 Klein images and edits with up to three of your LoRAs.

Call it with the model name black-forest-labs/flux-3-klein-lora/text-to-image on POST /jobs/createTask, from $0.018.

Request parameters

ParameterRequiredDescription
aspect_ratioNoThe shape of the frame: wide, square or tall. 1:1 · 16:9 · 9:16 · 4:3 · 3:4 · 3:2 · 2:3 · 4:5 · 5:4 · 21:9 · 9:21 · 2:1 · 1:2 · 3:1 · 1:3 · Default: 1:1
lorasNoLoad your own LoRA files to lock in a style or character. Default: []
output_formatNoFile type of the result. jpeg · png · webp · Default: jpeg
promptYesDescribe the subject, setting, light and style in plain sentences. Put a LoRA trigger word first.
resolutionNoOutput size. Bigger sizes look sharper and cost more. 1k · 2k · Default: 1k
seedNoReuse the same number to get a repeatable result. ≥ 0 · ≤ 2147483647

Pricing

Prices are in USD. The API uses a separate USD balance. Website credits cannot be used or transferred to it, and API funds cannot be transferred to website credits. When a task is accepted, we reserve the maximum quoted amount from your API balance. On completion, we charge for actual usage, never more than the hold, and release the remainder. If the task fails, the hold is released.

OptionUSD per image
resolution 1K$0.018
resolution 2K$0.054

Send the same request body to POST /jobs/quote to see the estimated and maximum USD charge before running. A quote does not run the task or charge your balance.

API quick start

Call the FLUX 3 Klein LoRA API in three steps

  1. Create an API key. Sign in to PopNow and create a key in the API console. Keys start with sk-popnow-. API calls use your separate USD balance.
  2. Send the request. POST /jobs/createTask with model black-forest-labs/flux-3-klein-lora/text-to-image and an input object that follows the parameter table. The response returns a task ID straight away.
  3. Get the result. Poll GET /jobs/recordInfo with the task ID, or pass a callBackUrl and we post the result to it. Download the output links when the task succeeds.
# 1. Create the task. Keep the Idempotency-Key if you retry after a network error.
export IDEMPOTENCY_KEY="$(uuidgen)"
curl -X POST https://api.popnow.ai/api/v1/jobs/createTask \
  -H "Authorization: Bearer $POPNOW_API_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $IDEMPOTENCY_KEY" \
  -d '{
  "model": "black-forest-labs/flux-3-klein-lora/text-to-image",
  "input": {
    "prompt": "A ceramic teapot on a linen cloth, soft window light, muted palette",
    "aspect_ratio": "1:1",
    "resolution": "1k",
    "output_format": "jpeg"
  }
}'

# 2. Copy data.taskId from the response, then poll until state is succeeded, failed, canceled or expired.
TASK_ID="TASK_ID_FROM_RESPONSE"
curl "https://api.popnow.ai/api/v1/jobs/recordInfo?taskId=$TASK_ID" \
  -H "Authorization: Bearer $POPNOW_API_KEY"

Replace the prompt and any links with your own. The API tab has the response, polling and callback reference.

FLUX 3 Klein LoRA API questions

How much does the FLUX 3 Klein LoRA API cost?

FLUX 3 Klein LoRA starts at $0.018 per image with Create Image, its cheapest task. Prices by option: 1K $0.018 per image, 2K $0.054 per image. The API uses a separate USD balance. Website credits cannot be used or transferred to it, and API funds cannot be transferred to website credits. When a task is accepted, we reserve the maximum quoted amount from your API balance. On completion, we charge for actual usage, never more than the hold, and release the remainder. If the task fails, the hold is released.

What is the model name for FLUX 3 Klein LoRA in the API?

Put black-forest-labs/flux-3-klein-lora/text-to-image for Create Image; black-forest-labs/flux-3-klein-lora/edit for Edit Image in the model field of POST /jobs/createTask.

What are the limits of the FLUX 3 Klein LoRA API?

Resolutions: 1K · 2K; Aspect ratios: 1:1 · 16:9 · 9:16 · 4:3 · 3:4 · 3:2 +9; Prompt length (characters): 5,000. Requests outside these limits are rejected before anything is charged.

More about FLUX 3 Klein LoRA