Black Forest Labs FLUX.2 [klein]
The fastest model in the FLUX.2 family, optimized for sub-second image generation with support for up to 4 reference images.
Key capabilities
- Sub-second generation: Fastest model in the FLUX.2 family for real-time applications
- Multi-image reference: Up to 4 reference images (
input_image,input_image_2,input_image_3,input_image_4) for style/subject transfer - Flexible aspect ratios: 10 preset ratios including square, widescreen, portrait, and social media formats
- Resolution options: Choose between 1k (standard) or 2k (high resolution, capped at 2048px)
- Safety control: Adjustable tolerance level from 0 (most strict) to 5 (least strict)
- Output formats: PNG (lossless) or JPEG (compressed)
- Async processing: Webhook notifications or polling for task completion
Use cases
- Real-time applications: Chatbots and interactive tools requiring instant image generation
- Style transfer: Apply visual styles from reference images to new generations
- High-volume workflows: Generate thousands of images efficiently
- Rapid prototyping: Quick iteration on creative concepts and designs
- E-commerce: Generate product variations and lifestyle imagery at scale
Generate images with FLUX.2 Klein
Create images by submitting a request to the API. The service returns a task ID for async polling or webhook notification.Parameters
| Parameter | Type | Required | Default | Description |
|---|---|---|---|---|
prompt | string | Yes | - | Text description of the image to generate |
aspect_ratio | string | No | square_1_1 | Image aspect ratio (see table below) |
resolution | string | No | 1k | Resolution: 1k (standard) or 2k (high resolution) |
seed | integer | No | random | Seed for reproducible generation (0-4,294,967,295) |
input_image | string | No | - | Base64-encoded reference image for style transfer |
input_image_2 | string | No | - | Second reference image |
input_image_3 | string | No | - | Third reference image |
input_image_4 | string | No | - | Fourth reference image |
safety_tolerance | integer | No | 2 | Content moderation: 0 (strict) to 5 (lenient) |
output_format | string | No | - | Output format: png or jpeg |
webhook_url | string | No | - | URL for completion notification |
Aspect ratios
| Aspect Ratio | Base Dimensions (1k) |
|---|---|
square_1_1 | 1024 × 1024 |
widescreen_16_9 | 1344 × 768 |
social_story_9_16 | 768 × 1344 |
portrait_2_3 | 832 × 1216 |
traditional_3_4 | 960 × 1280 |
vertical_1_2 | 704 × 1408 |
horizontal_2_1 | 1408 × 704 |
social_post_4_5 | 896 × 1152 |
standard_3_2 | 1216 × 832 |
classic_4_3 | 1280 × 960 |
Frequently Asked Questions
What is FLUX.2 Klein and how does it work?
What is FLUX.2 Klein and how does it work?
FLUX.2 Klein is the fastest model in the FLUX.2 family by Black Forest Labs, optimized for sub-second image generation. You submit a text prompt via the API, optionally include up to 4 reference images, receive a task ID immediately, then poll for results or receive a webhook notification when processing completes.
How do reference images work in FLUX.2 Klein?
How do reference images work in FLUX.2 Klein?
You can provide up to 4 Base64-encoded reference images using
input_image, input_image_2, input_image_3, and input_image_4 parameters. The model uses these images to guide style, composition, and subject elements in the generated output while still following your text prompt.What image dimensions does FLUX.2 Klein support?
What image dimensions does FLUX.2 Klein support?
FLUX.2 Klein uses preset aspect ratios (square_1_1, widescreen_16_9, portrait_2_3, etc.) combined with resolution settings (1k or 2k). The 2k resolution doubles the base dimensions but caps at 2048px per side.
How fast is FLUX.2 Klein compared to other Flux models?
How fast is FLUX.2 Klein compared to other Flux models?
FLUX.2 Klein is the fastest model in the FLUX.2 family, achieving sub-second generation times. This makes it ideal for real-time applications where speed is critical, though FLUX.2 Pro offers higher quality for less time-sensitive use cases.
What are the rate limits for FLUX.2 Klein?
What are the rate limits for FLUX.2 Klein?
See Rate Limits for current limits by subscription tier.
How much does FLUX.2 Klein cost?
How much does FLUX.2 Klein cost?
See Pricing for detailed rates and free tier credits.
What is the difference between FLUX.2 Klein and FLUX.2 Pro?
What is the difference between FLUX.2 Klein and FLUX.2 Pro?
FLUX.2 Klein is optimized for speed with sub-second generation, ideal for real-time applications. FLUX.2 Pro offers higher quality output for premium results. Choose FLUX.2 Klein when speed is critical; choose FLUX.2 Pro when quality is the priority.
Best practices
- Reference images: Use high-quality reference images with clear style elements for best transfer results
- Prompt writing: Be specific about subjects, scenes, and visual details even when using reference images
- Aspect ratio selection: Choose the appropriate preset aspect ratio for your use case
- Resolution choice: Use 1k for speed, 2k for higher detail (capped at 2048px)
- Production integration: Use webhooks instead of polling for scalable applications
- Error handling: Implement retry logic with exponential backoff for 503 errors
Related APIs
- FLUX.2 Pro: Higher quality generation for premium results
- FLUX.2 Turbo: Balance of speed and quality
- Flux Kontext Pro: Context-aware generation with image guidance