What it does
LangFast is an online playground for testing and evaluating prompts. You type a prompt, stream the response, and compare how different models answer the same prompt side by side. You supply your own API keys, which the site says are stored encrypted on the server. For prompt work it offers variables and Jinja2 templates, structured outputs, function calling, multimodal input and access to all model settings. The evaluation side adds assertions and metrics, datasets, performance comparison and token and cost stats. Teams get a no-code prompt editor with version control and role-based permissions, and chats can be shared through public links or with specific email addresses. The site is inconsistent about model coverage: the benefits list mentions 150+ models, while the FAQ says it is currently limited to OpenAI models, so check which providers are available before buying.
Capabilities
- Side-by-side comparison of model answers
- Variables and Jinja2 templates
- Assertions, metrics and datasets for evaluation
- Token and cost stats
- Share links and collaboration
- Version control and role-based permissions
When to use it
- Comparing how different models respond to the same prompt
- Iterating on prompts before shipping an AI feature
- Letting non-engineers edit and test prompts without a code deployment
