Langfa.st

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.