Langfa.st

LangFast is a prompt playground and evaluation platform for testing LLM prompts. Users write prompts in a no code editor, run them across models, and compare outputs side by side, with token and cost statistics for each run. The playground supports dynamic variables, structured outputs, function calling, multimodal input and all model settings, and prompts can be organized in folders, versioned, forked and shared by link. It is built for product teams and subject matter experts who need to prototype and validate prompts before shipping AI features. The evaluation tools add datasets, assertions and metrics, and performance comparison across models, so a prompt can be chosen with data. Team features include role based permissions, collaboration and a safe testing environment. The product is labeled Beta. Users bring their own API keys, with more than 150 models listed. The pricing page lists OpenAI models as supported and marks Claude and Gemini models as coming soon, while the home page names GPT, Gemini and Claude. Pricing is by one time payment with no subscription, in US dollars, and excludes API costs and optional extra cloud storage. A 1 Year Pass costs $49 for 12 months of access. Lifetime Access is listed at $120, with a limited time 50 percent off offer showing $60, and includes free updates and new features. Both plans include 1 GB of storage and a 14 day money back guarantee.