BFL modelsimage generation api

bfl/flux-2/klein/4b

Name: Flux 2 Klein 4b
Author: BFL

Flux 2 Klein 4b is BFL's advanced text-to-image AI model. Create photorealistic images, illustrations, and concept art from natural language descriptions with exceptional detail and prompt adherence.

Input

Prompt(string)

The prompt to generate an image from.

Number of Inference Steps(integer)

The number of inference steps to perform. Range: 4 to 8.

Output Format(string)

The format of the generated image. Allowed values: jpeg, png.

Aspect Ratio(string)

The aspect ratio of the generated image. Allowed values: 21:9, 16:9, 3:2, 4:3, 5:4, 1:1, 4:5, 3:4, 2:3, 9:16.

Seed(integer)

The seed to use for the generation. If not provided, a random seed will be used.

Idle

Example output — click Run to generate your own

Production endpoint

Send your first request

OpenAI-compatible endpoint with unified authentication and usage tracking.

POSThttps://api.sandbase.ai/v1/run

Model IDbfl/flux-2/klein/4b

Input Schema

5 parameters · 1 required · 4 optional

Parameter	Type	Required	Description
`prompt`	string	Required	The prompt to generate an image from.
`seed`	integer	Optional	The seed to use for the generation. If not provided, a random seed will be used.
`aspect_ratio`	string	Optional	The aspect ratio of the generated image. · Options: 21:9, 16:9, 3:2, 4:3, 5:4, 1:1, 4:5, 3:4, 2:3, 9:16 · Default: "16:9" 21:916:93:24:35:41:14:53:42:39:16
`output_format`	string	Optional	The format of the generated image. · Options: jpeg, png · Default: "png" jpegpng
`num_inference_steps`	integer	Optional	The number of inference steps to perform. · Min: 4 · Max: 8 · Default: 4

Output Schema

Field	Type	Description
`id`	string	Unique identifier for the generation task
`status`	string	Task status: pending, running, completed, failed, timeout
`model`	string	Model used for the generation
`outputs`	array	Array of output items
`outputs[].url`	string	URL of the generated artifact
`outputs[].content_type`	string	MIME type (e.g. image/png, video/mp4)
`error`	object \| null	Error details if failed, null on success
`error.type`	string	Machine-readable error type code
`error.message`	string	Human-readable error description

Async Workflow

This model uses asynchronous execution. Submit a request and poll for the result.

Submit — POST to /v1/run, receive an id
Poll — GET /v1/run/{id} until status is completed, failed, or timeout
Retrieve — Read outputs from the completed response

Code Examples

Ready-to-run snippets

# Step 1: Submit
curl -X POST https://api.sandbase.ai/v1/run \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
  "model": "bfl/flux-2/klein/4b",
  "prompt": "Cheetah in pre-sprint crouch, muscles tensed, gazelle visible in distance, African savanna golden grass, dust particles in air, National Geographic wildlife photography",
  "aspect_ratio": "16:9",
  "output_format": "png",
  "num_inference_steps": 4
}'

# Step 2: Poll result (replace <id>)
curl https://api.sandbase.ai/v1/run/<id> \
  -H "Authorization: Bearer YOUR_API_KEY"

← Back to Flux 2 Klein 4b

API README

Flux 2 Klein 4b

Flux 2 Klein 4b Black Forest Labs’ smallest FLUX.2 Klein model, a compact architecture built for interactive text-to-image generation and source-aware editing. The 4B variant emphasizes accessibility and low latency while preserving photorealistic rendering, multi-reference composition, and detailed prompt understanding.

Use it for responsive design tools, local experimentation, rapid campaign variants, and high-volume image work where the 9B model’s additional capacity is unnecessary. Write detailed prompts because Klein does not depend on automatic prompt expansion; identify the role of each reference when combining visual inputs.

Highlights

Compact 4B visual intelligence. Runs generation and editing through the smallest Klein architecture for efficient inference.
Interactive image creation. Produces useful visual results at latency suitable for responsive creative applications.
Unified generation and editing. Handles prompt-only creation, source revision, and multi-reference composition in one model.
Consumer-hardware accessibility. Brings capable FLUX.2 generation to more modest local compute environments.

Pricing

Configuration	Billing unit	Price
Base generation	Per request	$0.01

When to Use

✅ Good fit	❌ Consider alternatives
The project needs this exact generation or editing route	The intended task belongs to video, audio, or 3D media
Available references match every required local field	Required source assets or rights are unavailable
The brief can specify composition, color, and finish	Output must be deterministic at pixel level
Supported dimensions and formats fit final placement	Delivery requires unsupported canvas specifications
An asynchronous generated image fits production	A live frame-synchronous response is mandatory

Prompt Guide

Write a complete visual brief: identify the subject and references, define composition and viewpoint, describe lighting, palette, materials, typography, and finish, then list exact preservation constraints. FLUX.2 Klein does not rely on prompt upsampling, so use clear descriptive sentences rather than sparse keywords.

{
  "aspect_ratio": "21:9",
  "output_format": "jpeg",
  "prompt": "A photorealistic editorial composition with precise subject placement, controlled lighting, exact colors, and clean typography",
  "seed": 1
}

Technical Specs

Spec	Value
Model ID	`bfl/flux-2/klein/4b`
Input fields	`seed` (integer)<br>`prompt` (string)<br>`aspect_ratio` (string; `21:9`, `16:9`, `3:2`, `4:3`, `5:4`, `1:1`, `4:5`, `3:4`, `2:3`, `9:16`)<br>`output_format` (string; `jpeg`, `png`)<br>`num_inference_steps` (integer; 4–8)
Required input	`prompt`
Output fields	`url`, `content_type`
Execution	Asynchronous job

Related Models

bfl/flux-2/klein/4b/base — Compare this concrete local family route.
bfl/flux-2/klein/4b/base/edit — Compare this concrete local family route.
bfl/flux-2/klein/4b/base/edit/lora — Compare this concrete local family route.