Langfa.st
AI Prompt Testing Plarform
Overview
Langfa.st is a fast, no-signup prompt engineering playground that helps teams design, test, and optimize AI prompts with full control and minimal friction. Our platform is built for prompt engineers, developers, and product teams working with large language models (LLMs) who need to rapidly iterate and validate prompt behavior before going to production.
With Langfa.st, users can use Jinja2 templating to insert dynamic variables, test multiple prompt scenarios, and instantly view raw outputs from models without any API abstraction or noise. This allows for faster debugging, deeper understanding of model responses, and better alignment between prompt structure and expected behavior.
We designed Langfa.st to remove the biggest blockers in AI product development — slow iteration cycles, fragile testing setups, and lack of collaboration tools. Users can easily share prompts with teammates, reuse prompt components, compare outputs, and build a library of high-quality, production-ready prompts.
Whether you're building an AI writing tool, a chatbot, a content generation system, or any LLM-powered experience, Langfa.st gives you the environment to test faster, collaborate better, and build smarter.
Core Features
Prompt versioning and snapshot comparisons
Side-by-side and diff-style output testing
Flexible schema-based evaluation of structured AI output
Support for OpenAI and local models
Real-time feedback capture from team or testers
Auto-scoring with custom rules or few-shot examples
GitHub-style prompt collaboration with history and comments
HTML and JSON output rendering
Secure prompt management with role-based access
Use Cases
Product managers testing AI features before release
Prompt engineers optimizing and debugging system prompts
QA teams automating validation of AI-generated content
AI researchers comparing model responses under different prompt setups
Developers building prompt workflows with reproducible outputs
Startups validating business-critical AI features in production
Teams conducting regression testing on prompt performance
Pros & Cons
Pros
- Instant access without signup
- Dynamic and flexible prompting
- No API abstractions
- Customizable workspace for users
- Seamless collaboration features
- Pay-as-you-go pricing model
- User-friendly interface
- Real-time feedback on prompts
- Supports various use cases
- Ideal for teams and individuals
Cons
- Limited features in free version
- Requires internet access
- No mobile app available
- Learning curve for advanced features
- May not suit all AI applications
FAQs
Langfa.st Alternatives
Poper
AI-powered onsite engagement platform for popups, widgets, forms, notifications, and embeds.
Lunroo
Free AI Tools for Social Media Marketing
Cinematic
Automatically generate short-form videos to grow your business with AI
LinkDR
The Ultimate Link Building Software
Prompt Mixer
Collaborative tool to create and test AI-powered solutions.
Bigdevsoon App
"Build real-world projects, daily challenges"
FormBeat
Intuitive, AI-powered cycling analytics that help you train smarter.
🚀 Launch Your Startup in Days, Not Weeks!
Supercharge your SaaS or AI tool development with ShipFast
Key Features:
NextJS Boilerplate
Production-ready setup with essential integrations
Payment Processing
Stripe & Lemon Squeezy integration
Authentication
Google OAuth & Magic Links for secure login
Databases
MongoDB & Supabase integration
Email Integration
Mailgun setup for transactional emails
UI Components
Ready-to-use components and animations
Time Saved:
- ✅ 4 hours on email setup
- ✅ 6 hours on landing page design
- ✅ 4 hours handling Stripe webhooks
- ✅ 2 hours on SEO tag implementation
- ✅ 3 hours on DNS record configuration
🎉 Limited Time Offer: $100 off for the next 12 visionaries! Only 12 spots left!
"I shipped in 6 days as a noob coder... This is awesome!" - Happy ShipFast User
"ShipFast helped me launch my AI tool and reach $450 MRR in just 10 days!" - Christian H.
Featured
Luma AI
Dream Machine
Midjourney
Create AI generated images from a text prompt
Vercel AI SDK
The AI Toolkit for TypeScript
QuillBot
QuillBot AI
Capital Companion
Adding an AI Edge to Trading and Investing
Easy Folders
All-in-one Chrome extension for ChatGPT & Claude.
Perplexity
Where knowledge begins
Taskade
AI-Powered Productivity. A Second Brain for Teams
Kling AI
Next-Generation AI Creative Studio
AI Content Detector by Leap AI
Use our free AI Content detector to analyze text and see if it was generated by AI or not. AI Checker tool, 100% free forever.
Cursor
The AI Code Editor
Groq
A GroqLabs AI Language Interface.
Vidnoz AI
Free AI Video Generator
Hugging Face
The AI community building the future
Humanize AI
“Where AI Gets Its Human Touch.”
Lunary AI
The production platform for LLM apps.
![FLUX.1 [schnell]](https://cdn.brouseai.com/logo/gAATMVVzTB-BCugiC3hVp.png){kind=logo}
FLUX.1 [schnell]
The fastest image generation model tailored for local development and personal use
ChatPDF
Chat with any PDF - Your PDF AI to ask your PDF anything
Stability AI
Activating humanity's potential through generative AI
AI PDF Summarizer by PDF Guru
PDF Summarizer
![FLUX.1 [pro]](https://cdn.brouseai.com/logo/LjKDkTabCoBjQ9yqT998X.png){kind=logo}
FLUX.1 [pro]
State-of-the-art image generation with top of the line prompt following, visual quality, image detail and output diversity.
VEED.IO
AI Video Editor - Fast, Online, Free
Claude 3.5 Sonnet
Our most capable model yet
Runway
Tools for human imagination
Directonaut
Marketing accelerator as a software
![FLUX.1 [dev]](https://cdn.brouseai.com/logo/B6GQOp69ZOm-Zte81GQnv.png){kind=logo}
FLUX.1 [dev]
A 12 billion parameter rectified flow transformer capable of generating images from text descriptions
AI Hairstyle
AI Hairstyle
Vheer
Free-to-use image generator
Luma AI by Serviceaide
Activate AI for your Enterprise
Movavi
AI-powered video editing tool
AnotherWrapper
10+ customizable AI demo apps: pick one, make it yours, launch your startup quickly and start making money
Raycast
Your shortcut to everything
Supermaven
Free AI Code Completion
Undetectable AI
AI Detector, AI Checker, & AI Humanizer
AI Paraphrasing Tool by Leap AI
Rephrase any text in seconds with this free AI paraphrasing tool. Rewrite, edit and change the tone of sentences with ease.
Langfa.st
AI Prompt Testing Plarform
SoundHound AI
Technology for a voice-enabled world
v0.dev
Generate UI with simple text prompts. Copy, paste, ship.
AI Clothes Changer
AI Clothes Changer
Typefully
Write better content Grow your audience faster
Gemini
Chat to supercharge your ideas - Google
Midday
Run your business smarter
Video Background Remover
AI Design
