LangFast is an online LLM playground for rapid prompt testing, side-by-side model comparison, version control, and performance tracking.
What it is
LangFast is a playground environment built for product teams and developers to prototype, test, and evaluate AI prompts. Users connect their own API keys to run tests across various models, compare outputs side-by-side, track token usage, and refine instructions before deploying features into production environments.
Key features
- Side-by-side response comparison across multiple AI models
- Prompt version control with folders, editing, and fork capabilities
- Support for Jinja2 templates, dynamic variables, and structured outputs
- Built-in token and cost statistics tracking per prompt
- Role-based permissions and team collaboration options
- Secure server-side encryption for user-provided API keys
Who it's for
LangFast targets software engineers, product managers, and developers who build features powered by large language models and need a reliable way to debug prompts and manage model performance.
Pricing
LangFast offers paid options with no recurring subscriptions, including a 1-year pass for $49 USD and a lifetime access pass for $60 USD.
Use cases
- Compare prompt outputs side-by-side
- Track token and cost statistics
- Manage prompt versions in folders
- Test prompts with dynamic variables
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