Pay once, use forever

LangFast is an online LLM playground for rapid testing and evaluation of prompts. You can run prompt tests across multiple models, compare responses side-by-side, debug results, and iterate on prompts in one place with your own API keys.

Type a prompt and stream a response, then switch or compare models side-by-side; you can save/share a link, use Jinja2 templates or variables, and create as many test cases as you want.

Currently LangFast is limited to OpenAI/GPT models only. If you need access to models from other providers, just let us know, and we'll add them.

Yes. You need to provide your own API keys to run models on LangFast. API keys are sent over secure transport to the backend and stored encrypted server-side. The plaintext value is used only for provider calls and is not exposed back to your browser.

We stream tokens through a tiny proxy layer for low-latency responses with your API keys. Typical first token time is low fraction of a second. Speed varies by model/load.

Depends on the model (e.g., 8K–200K tokens). We show it next to each model.

Yes, as long as they are supported by the model itself.

Yes. You can open as many chat tabs as you want to see multiple models answer the same prompt.

Yes. Use "Share" button to manage sharing permissions. You can create public URLs or share access with specific email addresses.

We route to model providers; see the Data & Privacy page for regions and details.

Generally yes, subject to each model's terms. We link those on the model picker.

Yes, you can. Just let us know, and we'll add them to your workspace.

Yes. Reach out to us to get more information.

LangFast is point-and-click for quick evaluation, while paid LLM APIs provide programmatic control, higher throughput, predictable limits, and SLAs for production. Use LangFast to find the right prompt-to-model setup, then ship with APIs.

LangFast focuses on instant multi-provider testing with your API keys, consistent UI, side-by-side comparisons, share links, and exports in one place, offering a streamlined alternative to OpenAI Playground and Hugging Face Spaces.