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
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Key Features:
NextJS Boilerplate
Production-ready setup with essential integrations
Payment Processing
Stripe & Lemon Squeezy integration
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MongoDB & Supabase integration
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Mailgun setup for transactional emails
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