Langfa.st

Langfa.st - AI Prompt Testing Platform

Langfa.st is a next-generation AI prompt development playground that helps teams prototype, test, and ship reliable AI features up to 10x faster — without signup. Langfa.st is a lightweight and flexible prompt testing platform built to accelerate AI feature development. Created by a team with experience scaling AI-powered SaaS products to over 15M users, it eliminates friction in prompt engineering by offering instant access, dynamic Jinja2-based templating, and raw LLM output without API abstractions. Key features include a fast and intuitive prompt editor, variable support, real-time output preview, and a customizable workspace. Share and reuse prompts easily, collaborate with your team using public links, and iterate faster with no setup required. Langfa.st gives you full control over your prompt workflow, enabling faster iteration, better testing coverage, and more confident shipping — all in a clean, no-signup playground designed for builders.