Instant LLM playground to test and compare prompts across 50+ models
LangFast is a lightweight prompt testing platform designed for product teams, prompt engineers, and developers working with LLMs. It provides instant access to a customizable prompt playground with no signup required, allowing users to build, test, and share prompt templates using Jinja2 syntax with real-time raw outputs directly from the LLM.
Test multiple prompt variations at once against different models to quickly identify the most effective combination of wording and parameters
Immediate access to models from OpenAI, Anthropic, Google, and more without needing to manage your own API keys
Start testing prompts instantly without creating an account or configuring API credentials
Build reusable prompt templates using Jinja2 syntax for dynamic variable injection and prompt composition
Support for image inputs via Vision APIs and function definitions that enable LLMs to interact with internal systems
Enforce structured output by defining JSON schemas, ensuring LLM responses consistently match expected data structures
Build comprehensive test cases and datasets with AI-powered evaluations and pre-defined success criteria
Product teams can quickly prototype and test prompt variations across multiple LLMs to find the best approach for their AI features
Developers can compare responses from OpenAI, Anthropic, Google, and other providers side by side to select the optimal model for their use case
Teams building AI-powered APIs can enforce JSON schema output to ensure consistent, parseable responses from LLMs
Engineering teams can share and iterate on prompt templates together, building a library of tested prompts for production use

AI-powered code integrity platform for automated testing and code review

Independent AI model benchmarking for transparent, cost-optimized decisions

An open-source framework for augmenting humans using AI

Turn any Git repo into a prompt-friendly text digest for LLMs
Share prompt templates and results with team members for collaborative prompt development and iteration

Open-source AI image model arena by fal.ai