Langfa.st

LangFast is an online playground designed for fast testing and evaluation of prompts with large language models (LLMs). Users can run side-by-side comparisons across multiple GPT-based models, debug results, iterate on prompts, and manage test cases, all in one place. LangFast requires users to supply their own API keys and offers handy features like streaming token responses, chat sharing, and file/image uploads (where supported by the model). This tool is built for prompt engineers, AI researchers, and developers seeking a quick, low-friction environment for prompt iteration and model comparison.